4/21/07 FCST: OK / KS / TX / NE

[Billy Griffin]

Models now showing a possible severe weather event for this upcoming weekend. All are surprisingly consistent in pointing out Saturday as a potential widespread event over the central / southern Plains, although locations tend to differ slightly (no big surprise).

I don't think the GOF will be shut down too long this time, and with plenty of time to recover, by the time this weekend rolls around, this could be a good chase day for most everyone! It certainly bears watching nevertheless.

[Michael O'Keeffe]

I think this day may have potential only if we can get the Gulf to bring back the moisture after this recent cold spell I feel it will be tough for the tds to reach anything significant, but I do see at least low 60s across much of the S Plains. I agree that will may see an active severe weather day across the plains from NE into TX. If this pans out, then I'm there!

[SteveCarter]

I'm not sure why I'm even posting this far in advance...WAY to early to start this thread, but......this set-up has the potential to be a pretty big event. Moisture? Don't see any problems with that...at all. But...still to early to know anything yet. I'll post about Wednesday or Thursday.

[Billy Griffin]

The reason I have had my eye on this one is I'm seeing an all-too-familiar pattern setting up over the Gulf of Alaska. Ridging over the Pacific NW has held the past couple of days and shows a breakdown by mid-week, allowing quite a trough to dig in.

You're right - way too early to start trying to pinpoint things, and I was reluctant to even start this thread, but it should make for some good conversation/forecasts until the event arrives. The long wave trough should establish itself over the western US by mid-week. But at this point, I'm relatively confident we're looking at a significant event for the period of Friday through Sunday of this next week for the central and southern Plains.

Being in Seattle one day, Norman the next, throughout the week, it helps in seeing this pattern evolve, and kinda fun too!

[Brian Stertz]

Both Sat. and Sun look interesting...if the ext. model trends are correct...Sat. should be the dryline setup day from W/C Kansas southward into W.Oklahoma and the Texas Panhandle/Caprock areas. Sunday should be the more widespread event (NC/NE Kansas, E.Nebraska, W.Iowa) as the fairly deep surface low takes off to the northeast from NC Kansas Sunday afternoon to NC Iowa by early Monday morning. I am sure more changes are forthcoming with respect to this system's ejection, but at this point this is how things were shaping up. Moisture does not appear to be a problem as there should be a good sustained southerly flow off the Gulf developing by Friday and continuing into the weekend. Cap may be of a problem for Saturday though...unless there can be some minor wave getting kicked out ahead of the main wave which will be about to turn the corner by late Saturday to lift out towards the Plains and MO Valley.

[Angie Norris]

I'm not impressed yet...I'm seeing very slow progression of that trough, looks like the becoming typical overnight convective event Saturday night into Sunday. It's all wishcasting at this point...we've got 5 days of runs to go. Worth keeping an eye on, though.

[Ray Walker]

I'm just glad to see that the models are starting to show moisture return sticking around for a while after this front moves through the plains tuesday. It looks like a steady SW to westerly flow across the plains from friday all the way into next week and beyond. All I know is that there should be at least one pretty nice setup between friday 4/20/07 and wednesday 4/25/07.
Hopefully Oklahoma will see a good chase day soon.

[Jeff Miller]

If the pattern verifies, I am believing Sunday, not Saturday, will be the severe day for the Plains. Certainly a long way out, and just beyond the available forecast discussion guidelines for this thread (Day 8), but I too will be keeping a close eye out on the models.

If it verifies for this next Sunday, I will be chasing. I'll be watching this closely all week. Glad we have something to concentrate on as we head into the heart of the chase season.

[Brandon Sullivan]

I dont see moisture as a problem.. Even the timing looks favorable at this time... While we all know things will change, it looks nice.. Two shortwaves to move across central IL on Sunday Afternoon and Evening.. This would provide a nice cap breaker and focus point.. I am going to be watching this system!

[Kenny Drake]

What i've noticed recently has been the GFS slowing this system down considerably. The 12Z GFS run sfc forecast for Friday basically looks like the current (00Z) GFS run for Saturday. I also don't think we will see a surface low tracking over the Rockies as the landscape would not support it. If the current GFS verifies on general location the best dynamics appear to be near the warm front in NC KS/SC NE near the 500mb jet max exit region. It will be interesting to see if the GFS progged 60F Tds can make it to the KS/NE border in time. Farther south, higher temps and less cloud-cover would likely present an LCL and capping problem given low 60Tds to the coast. However, I find it hard to believe mid 60Tds would not find there way into Oklahoma. I guess this is highly contingent on the characteristics and location of the air mass that will be advected in behind the system pushing through the southern plains on Tuesday.

I probably went into way too much detail with this forecast because im sure the models will change significantly before this weekend. However, its still good to do these forecasts just to get it into your head in case of a future system that verifies very similar to what the current model is forecasting.

[Billy Griffin]

SPC and local AFDs for the region still painting an ominous forecast for this upcoming weekend! Interesting wording by SPC for this far out also!

[Alex Lamers]

Well I don't know who said they've been skunked thus far in 2007 but you are not alone. I couldn't get out to the TX Panhandle on March 28th which I wish I could have because it seems I would have stood a good chance of seeing something! But this is really my first year of chasing regularly...I've been more accustomed to the nowcasting end of things.

That being said, there seems to be a good chance of some tornadoes in the Midwest this weekend. Pretty strong trough reaches the Pac Coast by 00z Sat and then proceeds to eject enewd through the Intermountain West becoming negatively tilted as it spills out into the Plains. Should be some fairly strong cyclogenesis and for this reason I tend to like a solution closer to the ECMWF. Moisture return begins on Thursday with the GoM opening up and as southerly flow expands northward through the Plains...60s dewpoints should be spreading pretty far north by the weekend. Given our recent rainfall in this area (OK/N TX) and another deluge expected on Tuesday...with green vegetation, I wouldn't be surprised to see some of our dewpoints pushing into the upper 60s especially in the morning hours this weekend. With strong warm/moist air advection ahead of the trough, strong height falls spreading east over the weekend and vigorous S/SW jet along nearly the entire frontal boundary, it wouldn't be a shock to see some fairly widespread severe weather barring capping issues yadda yadda yadda. The setup is there.

Beyond that still looking like an active period as another strong shortwave rounds the base of the large Wrn trough and will likely plow into the Four Corners early in the next work week (Mon/Tue). Expect this would make the dryline retreat and induce some more low-level cyclogenesis in the High Plains.

Could be the upcoming 2 weeks when I bag my first tornado!

AJL

[nickgrillo]

Nice looking day progged by the models... Strong surface cyclogenesis with robust boundary layer moisture being advected northward into much of the warm sector, which will yield plenty of instability for severe storms. Nice shear throughout all of the troposphere with strong low-level veering, which should yield strong boundary layer SRH supportive of tornadic supercells. Too bad I won't be able to chase until the first week of May...

[Donald Giuliano]

Though I'm not usually one to get too excited about systems 5 days out, the strong agreement between the GFS, ECMWF, Canadian, and even NOGAPS (geez, been awhile since I checked that one) has really caught my attention. They all seem to be pointing towards a likely outbreak in western NE/KS/OK/TX, and on a Saturday no less. 40-50kt 850mb winds out of the south over a large area with good moisture return and a jet nose rounding the base of the trough and coming over a strong dryline? Count me in! If this verifies we could easily see a repeat of March 28, except with more daylight (sweet!).

[Chris Wilburn]

Just like Donald said above I don't think moisture will be a problem. I too noticed the 40-50kt 850mb winds that should draw up enough moisture. Directional shear as well as speed shear look very good. I will leave it at that for now, but agree with the mentioned targets of western Nebraska/Kansas/Oklahoma and the Texas panhandle. Low 60 td's somewhere along the dryline would be very nice. That combined with nice directional and speed shear could yield a very nice chase day on Saturday.

[Ray Walker]

This system looks very interesting, but of more importance is the weather pattern change. Starting around Thursday Moisture return will comince across the plains. With 60's dewpoints all the way into Nebraska by Saturday. This sytstem looks like a classic, eastern TX panhandle, Western Oklahoma isolated supercell event. This event sure does look good right now but alot could change. Also I really like the 997mb low that the GFS is showing in NE colorado by saturday afternoon.

[Billy Griffin]

:eek: Yes, I know, still days & days out, and this will change, but talk about a well-defined and sharp dryline bulge!
GFS models 144hr TDs is looking good!

[Brian Stertz]

E.Texas Panhandle..W.Oklahoma...and SW Kansas look very primed for Saturday's event. On 00z GFS...excellent upper divergence...neg tilted 500mb trough at 00z Sat over the dryline and double pt.....deep shear of 45-55 kts ...decent instability (2000 j/kg + CAPES) and well backed winds ahead of the dryline spells bigtime trouble late Saturday afternoon until well into the nighttime as the action spreas into C/S Kansas and C. Oklahoma. Strong moderate risk type setup if the forecast trends continue for SC Kansas...W/C Oklahoma... E.Texas Panhandle/NE Caprock...and possibly extreme NW/NC Texas.

[Ray Walker]

Brian I agree. The gulf is already getting juiced up and once the system today moves through a strong moisture return should comince late wednesday into thursday and last well into next week possibly setting the stage for another strong event early next week. However this system screams Western Oklahoma for tornadoes saturday.

[matt patterson]

Here we go again.... Ok is still looking for her first big event! Saturday per the GFS looks gangbusters with just the right amount of shear and moisture present. This is more of the classic type setup that we are used to in OK. Also to note, Friday doesn't look half bad if you target the western high plains, but that belongs in a different thread which has not been created. Anyway, I cant wait to see the 12z...... Cross your fingers Oklahoma, this may be the ONE....

[Karen Politte]

Does everybody think the atmosphere can do it as far as some spectacular moisture return?

Tds in the mid-Gulf were in the lower 50s yesterday although this morning they are in the lower 60s. I am always the pessimist - but we are going to have to see some great recovery for Saturday to really be the world-ending event that a lot of folk think it's going to be.

KL

[Jim Leonard]

Right now Im not too optomistic about moisture return for Saturday. The uppper system looks great but the GOM is too hosed for now. The trajectory of the return flow is from the NE over Florida then east over most of the Gulf by this weekend. IMHO early next week should be prime for severe storms as the next upper low drops in toward the four corners, by that time there should be good moisture return. As it looks now Moday or Tuesday could be really interesting.

[Donald Giuliano]

With 500mb temperatures forecast to be in the -15C range, we don't need spectacular moisture return for an outbreak to occur. If these mid-upper-level temps verify, the low-60 dewpoints forecast by the GFS will work just fine, especially over the higher terrain, and seem easily doable to me.

As of the 06Z GFS, things are still looking on track for an interesting Saturday, and the run-to-run and inter-model consistency displayed so far has been nothing short of remarkable. The ensemble mean is also pegged right on the same solution, which I guess should be unsurprising given the previous sentence. I usually don't have much confidence more than a couple days in advance at the most, but to me this just looks like a clear synoptic signal for an outbreak setup that's very likely to verify. There are always things that can go wrong, but right now I'm already of the opinion that there is a greater than 50% chance of having a significant outbreak this Saturday.

[SteveCarter]

We started a thread in "weather and chasing" about the entire period from 4/20 - 4/26. Seems like there will be a LOT going on over this period. Two possible set-ups. Even THIS first set-up mentioned here is still to far out to even TRY to pin-point. One thing tho...IMHO..think moisture will NOT be a problem, but like I said, kinda far out yet.

[Brandon Sullivan]

Im gonna go with steve.. Dont think moisture is a problem.

[Glen Romine]

I'm with Jim on the potential for this weekend - looks like a good primer event to help set things up for what has the potential to be a better event sometime, somewhere next week. While the GFS miraculously develops a thin tendril of ok moisture return by 00Z Sunday, the precip is rather spartan given the strength of the shortwave emerging. Making the reasonable assumption said shortwave is poorly handled by the model in terms of strength, timing and placement this far out (and generously assuming it materializes at all), certainly with favorable timing storms will be possible Saturday on the western central plains, but unless a target materializes in deep west Texas or the high plains - weak moisture will probably be the killer for significant tornado prospects. Freak things can happen, so maybe things find a way to come together for Saturday, but I'd be very, very surprised if a tornado outbreak were to materialize that day.

[Ray Walker]

Given the strength of this system and a few days of moisture return I really dont think moisture will be a big problem with this event. The gulf is already begining to moisten back up and by thursday I think 60's dewpoints will be at least into the northern gulf and possibly into parts of south texas. Ya, we wont have the 70's dewpoints that we would LOVE to see with these big systems however we do not need them. Low 60's will work just fine. However I do agree with Jim that early next week moisture will defanatly NOT be a problem as dewpoints should easily be in the mid to upper 60's.

[Donald Giuliano]

I agree that we won't be seeing really rich Gulf moisture by Saturday, but in my opinion we don't need it, and mediocre Gulf moisture will be sufficient.

One thing to remember about any potential setup this year is that there has been a tremendous amount of rain over the Southern Plains of late. Once the sun comes out later this week, there's going to be strong latent heat and moisture flux in the boundary layer due to evapotranspiration. Now generating large area of 70+ dewpoints in just a few days from that moisture source alone is unrealistic, but generating a large area of 60+ dewpoints is quite feasible. Given that 500mb temperatures are forecast to be around -15C, ~60F dewpoints yield more than sufficient instability in western NE/KS/OK/TX given the altitude (3000 CAPE near DDC by Friday evening if you believe the 12Z NAM). Anything beyond that is gravy.

[mcrowther]

I am going into the pessimistic camp with this one. Despite strong cyclogenesis over the Plains and good southerly flow, there will be a persistent ridge of high pressure extending well down into the central and eastern Gulf and northern Caribbean sea because of the omega blocking pattern that will be in its last throes. Therefore the general trajectory of the flow will be from the ENE across the Gulf- so that it seems to me that dewpoints in the upper 50s to MAYBE low 60s will be all that we will be able to manage on Saturday, the best looking day synoptically for the High Plains this weekend. So right now I am thinking that severe storms are almost a given on that day and supercells are likely as well, but a lot of tornadoes..probably not. Worth chasing if you are in the area, but I am not anticipating an outbreak. Sunday the low pressure is likely to be pretty far north in the Upper Midwest- perhaps some moderate potential up in that region. I hope I am wrong since I could come out for a spot chase, but right now it does not look like it will be worth the time and expense for me.

The real action to me looks to be for the middle portion of next week- the Gulf by that time should be providing a much richer flow of juice, and another trough will be slowly moving in from the west. This trough is likely to be slower than the OP GFS is showing- the ECMWF, SEF and ensemble mean are all suggesting a more closed off slower solution- which would suggest that the best days may be next Tuesday and Wednesday.
How good? Way too early to tell of course, but that is my best guess at this juncture. One factor in its favor- I cannot chase those days :)

[Jay McCoy]

Dewpoints back on 3-28 werent even as high as they are expected to be this saturday and you guys still question the lack of moisture?? This isnt nebraska or Missouri. You dont have to have 70's to get tornados. low 60s will be just fine for tornados. You get too high you start looking at HP's anyway. I prefer a nice 65 DP along the dryline. With DP's in the teen west of the dryline it will be quite sharp. There is ample ground moisture to help keep mixing to a minimum and LCL's should be low enough for some nice tornadics cells in west Oklahoma. We have had a number of tornado days in the panhandle this year with less than 60 DP's. I dont think I have seen above 65 yet and have 10 tornados so far..hmmmm

If you dont feel there will be enough moisture then dont chase and we will send ya some pics afterward. Will help hold down traffic congestion anyway. With it being saturday it will be a zoo as is. Pessimism only hurts the pessimist.

Now in Late May its a different stroy and low 70s are quite nice.. But its mid-April!! Temps wont be much above 80 at most so the DP depression wont be extreme. You have to consider time of year and location and not just focus on the DP itself.

[Brian Stertz]

Excellent point Jay...first off we are not yet in the "wet" season where Td's are often in the mid 60's to mid 70's...and in the High Plains a mid 50 Td is often sufficient to bring some big game to chasers. The Texas Panhandle is very notorious for this. Saturday the main wave makes a direct strike on the High Plains strongly convergent dryline from the DDC area down towards the AMA area. I am in the camp that things may be just the right balance of moderate instability and decently strong shear...plus there may be the added focus of a dryline/warm front intersection in this zone as well. The March 28th case was a bit more extreme speed shear wise, but still this system forecast shows a good balance of 30-50 kt. 850 jet and 40-55 kt. 500 jet...and a criss cross somewhere in the SW Kansas to C. Texas Panhandle area. While maybe this is not an outbreak scenario...scattered supercells may make things fun chase-wise. I am liking Monday too for a nice chase setting across S.Kansas and N.Oklahoma :)

[Craig Maire II]

On 03/28/07 DPs were in the mid to upper 50s in southwest Nebraska and I had an EXCELLENT chase! Saw one large cone tornado and had a large and powerful tornado just miss the town I was in (Grant Nebraska). So I'm not worried about the DPs for Saturday esp. with a strong dryline, plenty of windshear, instability etc. in place. All it takes is ONE good supercell to make for an excellent chase ;) :D

[Joey Ketcham]

I don't think moisture will be a problem at all, I think if things pan out we could see some supercells and tornadoes. I wouldn't mind camping out along the dryline, but have to see how things pan out.

[Michael O'Keeffe]

I agree with everyone about the former moisture concerns. If I remember correctly on February 28 the tds were in the mid-upper 50s! I think that we will see at least some nice classic dryline supercells with tornadoes across the High Plains from W NE into the TX pan and W OK with the amount of instability across this area. Though not extreme enough to produce a huge outbreak like we saw on March 28th I still think we may see a handful of decent tornadoes with maybe 1 or 2 that are pretty strong. All I can say is I'm chasing this setup...I'm keeping my fingers crossed!

[Robert Edmonds]

I'm going to wait before I start having concerns about moisture with Sat. since it is still a little ways out. Most of the moisture from what I see shouldn't start its way up for atleast a day or two. With that note I do plan on chasing this day unless this day really falls apart. So far though from watching the forecasts from day to day it seems northern OK, KS, and southern NE will atleast be seeing something this day. However, I'm going to wait before I start narrowing my target.

[Mikey Gribble]

I am worried about moisture. You have great dynamics with this system, but t-td are around 20 degrees. That is definitely doable for tornadoes, but I like to see a little less of a spread in the Kansas-Oklahoma area where the mid-level jet streak crosses the LLJ (and you get the best 850-500mb crossover). It definitely looks like a good severe weather day, but I wouldn't go as far as to call it a tornado outbreak yet. Dewpoints in the upper 50's are far from ideal.

[Jay McCoy]

I am worried about moisture. You have great dynamics with this system, but t-td are around 20 degrees. That is definitely doable for tornadoes, but I like to see a little less of a spread in the Kansas-Oklahoma area where the mid-level jet streak crosses the LLJ (and you get the best 850-500mb crossover). It definitely looks like a good severe weather day, but I wouldn't go as far as to call it a tornado outbreak yet. Dewpoints in the upper 50's are far from ideal.

Temps will be in upper 70's and DP's in the low/mid 60's is 15 degrees or less. Not bad for the panhandle or west oklahoma by any means. If the triplepoint is trully in this area then I dont see many issues to a good event.

[Michael P. Morris]

Looks to me like there are two potential regions for this event. The northern target in Nebraska will be closer to the 850 mb/surface low, so the forcing will certainly be strong. Also looks like there may be some ongoing warm advection type precipitation at the start of the period, which could limit destabilization. Shear will be more than adequate if a storm is able to develop, but the moisture and hence the instability appear to me more limited. It's also too far for me to drive, so maybe that explains a little bit of my pessimism.

I think another area of concern lies along the Oklahoma/Kansas border, roughly in a Pratt-Alva-Woodward triangle, perhaps extending as far west as Dodge City. Here the instability looks higher and the presence of the stronger upper level jet will allow storms to sufficiently vent, perhaps favoring more classic-LP structures. Strong low level shear and mid level flow appear more than adequate for strongly rotating storms, and a subtle surface low will only act to further enhance the low level shear. Again, my main concern here is that the instability still may not be great enough to realize the intense shear that is being forecast for the area, but the GFS apparently thinks that storms will develop in the region, as evidenced by the vertical velocity fields.

Hopefully once this event is in the WRF's time window we can get some more details, but for those who worry about the moisture (including myself) just remember that the March 28th event was able to make do with only upper 50's!

[Ray Walker]

Well I dont see northern Oklahoma as having the best shot. I dont know if you all are talking about Monday of next week or not but shurly not saturday. This setup screams Western portions of Oklahoma and east Texas panhandle. Maybe I'm just missing something.

[Skip Talbot]

Well I dont see northern Oklahoma as having the best shot. I dont know if you all are talking about Monday of next week or not but shurly not saturday. This setup screams Western portions of Oklahoma and east Texas panhandle. Maybe I'm just missing something.

Well, the GFS has favorable shear across most of the dryline. As the jet noses in, storms may fire first up by the KS/NE border as the shear arrives here first, and then areas to the south should quickly follow. Much of the dryline may light up... like March 28. The GFS is still holding consistent at putting 1500 J/Kg from the NE/KS border down to western OK. With all areas in between seeing ample speed shear, and backed surface winds, you probably have a plethora of options to choose from. Earlier initiation up north might be favorable (if the dew make it that far) it terms of observing during daylight hours. Although instability may be hindered in NE from the morning junk. Down south in the panhandles, you'll have the bulk of the 500mb jet at 0z, but you might have a later show and the T/Td spread might be higher.

[Mikey Gribble]

I don't know what model you're going off of Jay, but the 12Z run of the GFS has dewpoints in the mid to upper 50's on Saturday.
http://www.rap.ucar.edu/weather/model/gfs108hr_sfc_dewp.gif
20 degree t-td are still doable, but that's pushing the limits for tornadoes.

[Shane Adams]

Well no one thought there would be tornadoes today either and low and behold.....nowadays nothing's taboo. My only concern for the next system approaching is will I see the tornadoes or not.

FWIW, my avatar occurred in 77/57 ;)

[Billy Griffin]

I didn't mean to create a controversy or argument in this or other closely-related threads regarding the forecast for this weekend, all I wanted to do was start the discussion on the "potential" upcoming weekend event.

To be quite honest, I don't put much faith in the models until 24-36 hours out. What I was really impressed with (and still am) is the familiar pattern off the west coast and into the Gulf of Alaska. I've studied these patterns over the past couple of years since moving here (Seattle area), and this pattern has me very impressed with what could happen here (Oklahoma and Plains) for the period of Friday through early next week.

To stay on topic with the forecast for 4/21/07, I still see a very impressive system and the potential for a great chase day! Seeing as it's the weekend, and I'll be back in OKC, I'm looking VERY forward to my first Plains chase in almost 15 months! Pattern looks good, moisture won't be an issue, great dynamics, models supportive... I can't wait 'til this weekend!

[Joey Ketcham]

Well no one thought there would be tornadoes today either and low and behold.....nowadays nothing's taboo. My only concern for the next system approaching is will I see the tornadoes or not.

February 23rd comes to mind too, many people said "ohhh nothing will happen", and the few of us that did choose to chase ended up seeing a tornado outside McLean Texas.

[Jay McCoy]

I don't know what model you're going off of Jay, but the 12Z run of the GFS has dewpoints in the mid to upper 50's on Saturday.
http://www.rap.ucar.edu/weather/model/gfs108hr_sfc_dewp.gif
20 degree t-td are still doable, but that's pushing the limits for tornadoes.

For 1 the morning run of the GFS is historically bad. Also the GFS has underdone Td's the whole year. Look at the Euopean. It has handled the systems this year much better and seems to be right on with this one including pulling the dryline furher back west. The GFS has been trying to push the dryline east all year but every event from Clovis to 3-28 to last week the dryline was on the Tx/NM border. The ground in this area is almost saturated 8 inches deep. We are well ahead of our average rainfall so mixing wont be much of an issue at all.

And like theses other guys have pointed out. even 50's have produced big days here. This is the High Plains!! elevation and the caprock itself changes the way you have to think. If we were forcasting for I35 or central Kansas then I would worry but not around here. Like Shane said.. We had Td's in the low 40s and we had tornados near LBB. Only landspouts but still had them. Thats why I have loved chasing this area for over 2 decades now.

[Rich Thompson]

I'm not trying to hijack this thread, but the dewpoint depression/LCL concern is worth a few comments. It's true that relatively low cloud bases (small T-Td, low LCL) have been associated with significant tornadoes, and there aren't too many serious supercellular tornadoes documented with LCL heights above 1750-2000 m. Still, that doesn't mean that no tornadoes can occur with higher T-Td spreads.

We think the LCL is related to tornadogenesis through it's impact on residual buoyancy in the RFD. How this downdraft with substantial buoyancy comes about has not been completely explained, so I'd be careful getting too literal with the application of this stuff in tornado forecasting. I'm comfortable saying that a major tornado cluster/outbreak is unlikely with large dewpoint depressions, but I don't start trying to rule out mesocyclonic tornadoes until we exceed 25-30 F. There's also some question about climatological bias toward the moister eastern areas where a larger % of tornadoes are rated F2+.

Rich T.

[SteveCarter]

How about waiting until Thursday night to worry about DP's and moisture? This is STILL way to early to plan, or NOT plan to chase this weekend. This is what happens when these threads are started this early. MODS...why not limit a FCST to a MAXIMUM of 4 days out? You can always post in the weather and chasing forum further out when there is not a difinitive time OR area, such as this was when the thread was started. And for the record.....because of the distance, I'll probably wait until the second event occurs next week. I can't chase for 7 straight days, even tho I'd like to! I guess there will be an immense fine-tuning process taking place Thursday and Friday. having said all this...I think the moisture WILL be ok, but...to far away to really know.

[Kenny Drake]

There's also some question about climatological bias toward the moister eastern areas where a larger % of tornadoes are rated F2+.

Rich T.

Good find, however could the higher population density in the eastern areas versus the western areas have something to do with the ratings?

Okay back to topic. I think this far out the best thing to look at is model consistency. Doing some quick comparisons vs the previous run (this website isn't tell me what the initialization time is, so I'm going to guess 00Z vs 18Z) the GFS has stayed almost exact in location of the 500mb wave at 00Z Sunday. Amplitude is also very similar however the new run is hinting at a slightly farther north (maybe about 50-80 mile difference in isobaric locations). Now to compare against the ECMWF I picked the 12Z Sat timeframe. This is where the major difference comes in, however as said these models are probably at different initialization times. The ECMWF still has the 500mb trough axis over the CA/Nevada/Arizona region while the GFS has the 500mb trough already ejected into the plains with the axis over the TX/OK panhandle. It appears the ECMWF kicks the trough out fairly rapidly and catches up with the GFS by 12Z Sunday. This of course has major implications, therefore its quite obvious why the SPC has not been able to pinpoint a closer local. This seems quite similar to the March 28th event, in which case we could possibily see a mirror event in terms of location. However, with the timing differences I'm merily just pondering the possibilities.

[Jim Tang]

This is really not a March 28 event. March 28 was associated with a deep closed 500mb low with extremely steep lapse rates and was neutral-positively tilted. This open trough is negatively tilted and will zip on out of here. Overall I'm not very enthusiastic; boundary layer moisture doesn't look too good. Now if the dryline does not mix east as quickly, which could happen if one goes by the pattern of this year, and the fact that the ground is fairly wet, the dewpoint depressions probably won't be as big as a problem. But if we look at Mar 28, we notice that that system was in the process of cutting off. This is not; many times models will prog deep systems too fast, but I've also noticed they can also prog mobile shortwaves too slowly. Another concern is how strongly the winds back. On Mar 28 they were extremely backed, but not in this case, especially if the sfc low is too far north/east. Given southerly 500mb winds, we'll need backing winds to keep those hodographs curved like they were on Mar 28. Finally, with the trough negatively tilted, the strong forcing will be right over the warm sector, and there is a chance that everything could line up very quickly. On Mar 28, the best forcing was lagging behind the warm sector which helped things remain discrete for a while.

[Gerard Jebaily]

When the wave first started showing up on the 180hr frame of the GFS, its looked much more impressive than it does now. I think that we may have a decent setup given the current model outputs. But the overall strength of this system is forecast to be much weaker than what was first anticipated. I do agree that moisture wont be a problem. But attm, I'm not betting this will be something significant. However, alot can change in the next 5 days.

[Jason Boggs]

I think some of us (including myself) try to get too technical this far out, and fail to look at the big picture. Let's look at right now. Moisture in deep south TX is in the mid 60's right now so the gulf is wide open for Saturday. Right now, no cold fronts are forecasted to scour out the moisture, so higher Td should keep making their way northward, especially as the trough gets closer. Td in the upper 50's will be adequate in the warm sector on Saturday. Speed and directional shear appear to be good enough to produce rotating updrafts. Obviously this far out, many things can change, but it does look like a rather interesting day on Saturday.

NOW JUST FOR FUN:
TARGET AREA IS PAMPA,TX

[Wesley Luginbyhl]

After some shifting in the models again, I am beginning to lean towards Saturday to having the best potential out of the up coming system. It will take a major change in the models to get me not to chase this day. There are just too many ingredients looking to show up for this storm not to atleast go for a little Saturday drive to the panhandles. The forecast soundings have been looking pretty good over the past few days. Storm motions seem to be slowing down a little from where they were a few days ago. Maybe in a few more days the models will start to give a clearer idea about what will happen. One thing I like however is that the GFS has been pretty consistant with almost SE winds ahead of the dryline for a few days now. I am prolly more curious than anything else just to see how my Alltel service is through the eastern panhandles and western OK region.

[Ray Walker]

I really dont know why anyone would not like this setup. It is going to produce tornadoes, an outbreak? well probably not but who knows. I think dewpoints will average 62 to possibly 65 degrees. I think instability will be in the 2000 +jkg range and wind profiles will be pretty darn good. I think we are looking at a good chase day, not a massive tornado outbreak day.

[Jim Bishop]

Every year the moisture argument comes up on this forum. For this particular setup the moisture should be fine. We are looking at mid to possibly upper 50s dewpoints in the high plains of Nebraska, Kansas and Texas. With 500mb temperatures forecast to be between -16C to -18C (possibly as cold as -20C), a 55F dewpoint should be adequate. Although I agree the quality of moisture could be better since we'll be mixing from the Southeast U.S.

To be honest I'm a little concerned the 500mb flow will be too backed and the storm motion will end up being nearly parallel to the dryline (north-northeast). This would promote linear storms of course. This concern is based on the 00z 4/18 GFS. But, this event is still 4 days away, so I'm sure the tilt of the trough will change a bit. In any case the shear supports supercells.

[Shane Adams]

Sorry to post slightly off-topic, but the subject has ben brought up. Regarding the moisture thing, it's important to realize it doesn't have to be an outbreak to be chaseable....in fact most of the best days aren't "huge" days at all. I think a lot of the anxiety comes from the fact it's a mental letdown when the scenario goes from Armageddon to just "average".

But average is gooood :-) I think Sat will be just fine.

[Mikey Gribble]

It's not that I don't like this setup. It is a synoptically evident severe weather event, but unless dewpoints are >60 then I really don't think this going to be a major tornado event. On 3/28, dewpoints were well into the 60's. You had a 64 degree dewpoint in DDC at 00Z. I would be very suprised if this event ended up being comparable (tornado wise) to 3/28, unless you get dewpoints in the 60's.
I didn't know the elevation increased as you moved West. Thanks for filling me in Jay. I'm really not trying to screw with you Jay lol. I just had to throw that in. I have chased the caprock, western Kansas, and Colorado plenty of times and I am aware of the effect elevation has on convection. I'm just not banking on geography to make up for the lack of better moisture. I'm not sold on the idea of the dryline not mixing East due to saturated soil conditions either. Evapotranspiration can only do so much. I do agree that the GFS has been a little off on the dryline location on some earlier setups this season, so that is obviously something that will need to be considered.
Don't get me wrong about this setup. I do think it has a lot going for it and I do believe it is a solid bet that this will be a major severe weather event, it is just the tornado potential that I am skeptical about. I haven't looked at anything closely yet, but I am planning on starting up my forecasting for this event today once the NAM picks it up.

[Billy Griffin]

Yesterday's rain event certainly won't hurt, albiet this is very shallow moisture near the surface. But with trajectories good the next couple of days, including today, evapotranspiration will aid in good return flow. Frankly, I don't see what the problem is regarding moisture. Most of Oklahoma is seeing ~ 50TDs and with 100% ground saturation. I can honestly see ~65 TDs near the OK/TX panhandle border by Saturday afternoon/evening.

To reinforce some positive thought, just take a look at WV imagery over the western US and especially the Gulf of Alaska. When I started this thread, I was excited in what I was seeing evolve up here in the Pacific NW. Last night, we had a good convective event for even here in Seattle. The trough continues to dig and looks like a solid pattern is setting in for the next series of systems, starting with this one progged for Saturday!

I guess, for once, I'm just being overly-optimistic. I see this being a very good day. No, not likely to have tornadic cells lined up from Texas to South Dakota, but I find this one more of classic dryline day.

Heck, at least one thing, we're not debating over how far east into the hills / trees we'll be chasing this one! (KOW)

[Brian Stertz]

Absolutely...Shane's right on the money...it's a dryline event and in a great viz chase area on a weekend. How much better can you get?? The morning DDC forecast discussion was some great reading...and Amarillo forecast discussion is also pretty charged up. :)

[Billy Griffin]

;) "Virtual" forecast as of now... MDT risk for much of SW Kansas, TX and OK panhandles and far western OK. By mid-afternoon Saturday, surface low intiates convection over the high plains of E Colorado, NW Kansas and parts of SW Nebraska. Potential will exist for low-topped cells and cold-core tornadoes along the WF in this area. Cold rain / snow over much of Wyoming and the Dakotas.

Farther south, along the dryline, initiation will hold off until late afternoon (3-5pm). As the jet interacts with the dryline, several isolated sups will erupt along the dryline corridor from near Liberal, KS south to near Claude, TX. 3-4 isolated supercells form up for a classic dryline chase! These storms will all have the potential to become long-lived supercells, capable of very large hail and a few tornadoes. The tornado threat will continue after dark, before the mess all congeals into a MCS over central KS & OK.

My target as of now = get an EARLY start on the day and arrive by 12 noon at good ol' McLean, TX. Wait for initiation along dryline, then adjust from there.

[Mark Farnik]

Depending on how busy I am and how I feel, I may chase the potential cold core event over eastern CO/southwestern NE/northwest KS. Unfortunately I'm going to have to sit out the southern target this time. I wouldn't be able to get down there in time, anyway, even if I really wanted to chase, as I'll be in Co. Springs tonight, Thursday and most of Friday. Our Fort Morgan HS Knowledge Bowl A Team (which I'm a member of) qualified for the CHSAA State Knowledge Bowl Tournament which they're holding at Colorado College. I won't be back from the tournament until late Friday evening.
So I'll take a look at the models then and again Saturday morning, as well as see what the SPC forecast and BOU/LBX/GLD's take on the setup are before I make up my mind. My preliminary target would be Holyoke, CO, which is only an hour and a half drive east of me, so it would be a relatively backyard chase. I'm in definite 'wait and see' mode on this one. But if it looks like anything less than surefire cold core 'naders, I won't chase it and save my gas money to chase the better setups once we get into May, which I just realized is only a week and a half away!:cool:
My college semester ends May 8th, and after that I won't have ANYTHING going till the very last week of May, so I'll pretty much be able to chase whenever I want from May 8th to May 26th... :D I'm sooo excited!

[matt patterson]

Just took a peek at the 12 NAM (84hrs out) and lets hope the NAM does not verify as it only has low 50s at best along the dryline... That is really pushing it for me. THe sfc low is all the way up into SW SD and NW NE... With SFC temps progged to be in the upper 70s pushing 80, that creates a nearly 30 F TD depression....Yikes!!!!!!

Will be interesting to see what the 12z GFS has to say...

[David Drummond]

Personally I have had enough "outbreak scenarios". The best chases I have ever had were not outbreak days at all except for one, but rather "normal" chase days such as what appears to be setting up for Saturday, or even plain ol slight risk days.

There are definitely some signs setting up for some classic panhandle severe weather and the devil will be in the details when we start coming in the T -3 day range, but I recognize this pattern as one that is VERY condusive to some excellent chasing in good chasable terrain.

And that's really what you should be concentrating on more than 3 or so days out...pattern recognition. As it looks now, I have seen setups like this produce what we used to call the "chain of pearls".....supercells scattered up and down the dryline....take you pick of one.

[Jim Bishop]

The 12z WRF sure shows some interesting changes. I like the area from Southwest Kansas down into the Texas Panhandle. Nice vort max and strong 250mb jet max moving right into that area. Surface and 850mb flow is more backed and the mid level flow is more veered.

The one really bad thing is how it mixes out the low level moisture across the high plains. I'm not buying into that yet, but it sure doesn't make me feel great about surface dewpoints. The one good thing is the -16C to -18C 500mb temps, which still support half decent CAPE values. We shall see. I never like the WRF solution beyond 60 hours anyways...it always seems to have some strange issue. I'd take the GFS over the WRF beyond 60 hours any day.

[Brett Roberts]

Just took a peek at the 12 NAM (84hrs out) and lets hope the NAM does not verify as it only has low 50s at best along the dryline... That is really pushing it for me. THe sfc low is all the way up into SW SD and NW NE... With SFC temps progged to be in the upper 70s pushing 80, that creates a nearly 30 F TD depression....Yikes!!!!!!

Will be interesting to see what the 12z GFS has to say...
Not sure if we're looking at the same thing, but overall, the 12z NAM looks good for the TX Panhandle target, IMO. Moisture could certainly be better, but this run does show mid 50s DP's over that area (granted, more like low 50s farther north into KS) - hopefully that will improve as we approach the event. Aside from moisture, things look great with a 997 mb sfc low in W NE at 00z, temps in the low-mid 70s (not sure where you're seeing 80 anywhere along the dryline setup), and backed sfc winds with great directional and speed shear. Still looks like a nearly-ideal chase day is in store, with some good potential for tornadic supercells if moisture return is a bit better than progged.

[Jay McCoy]

I didn't know the elevation increased as you moved West. Thanks for filling me in Jay. I'm really not trying to screw with you Jay lol. I just had to throw that in. I have chased the caprock, western Kansas, and Colorado plenty of times and I am aware of the effect elevation has on convection. I'm just not banking on geography to make up for the lack of better moisture. I'm not sold on the idea of the dryline not mixing East due to saturated soil conditions either. Evapotranspiration can only do so much. I do agree that the GFS has been a little off on the dryline location on some earlier setups this season, so that is obviously something that will need to be considered.

The GFS is starting to pull the dryline further back west as I expected. The dryline hasnt mixed east yet this season and I dont expect it to this time. Even SPC has pulled the day4 outlook back west. I expect the dryline to be close to Amarillo on Saturday. I would love for it to mix east a bit and set up close to hwy 70 (pampa to clarendon). Some of our best events come when the dryline is close to the edge of the caprock.

I am not worried about you messing with me Michael. Its all good. I take it in stride that people look at models and think differently. Since you have chased this area you know it doesnt take monster DP's to get incredible storms. I am not expecting a 60+ tornado event. I never said it would be but I do expect a number of tornados in the east panhandle and west oklahoma. No moster HP's just some beautiful classic motherships dropping a few cones :)

The more pessimists we have the less chasers we will have which is great. Shane and I both hate traffic jams. But I bet even the naysayers will be out in full force this weekend with their tripods in the road:rolleyes: . And Like Shane and Jason I like the smaller events just as much. 1 good rotating supercell is enough to keep me happy

[Mike Hollingshead]

The GFS is starting to pull the dryline further back west as I expected.

Is the new GFS run out somewhere? I don't see it yet(just coming in on ncep). It seems a lot of folks post mere minutes before new data is out and I never understand that. But looking at the 0z gfs it doesn't seem like it's moved all that far from past runs.

12z nam looks like A$$ for dews. I wonder if that is mostly "dew"(sorry lol) to those soundings right along the coast, just having the front pass over them. I wonder if it then doesn't account for how deep the moisture is just south of that(as it was pretty deep in the western gulf yesterday). That's all that gives me some(very little) hope for Saturday...hoping the model puts too much weight into the crap air now on those soundings in far south TX...and that it's not as bad as the NAM makes it appear. But I really have no idea how it handles all that. It seems there'd still be some good moisture down there in the western gulf, off shore.

[Donald Giuliano]

A quick glance at the 12Z NAM shows that the moisture concerns voiced by a number of other posters appear to be well-founded. The main problem with the moisture is, I think, related to a vort max rotating through the SE US coast Thursday night/Friday. Although the surface dewpoints in the Gulf don't show a huge effect from this, at 925mb dry air is advected into the northern Gulf, rounds the base of the ridge, and then advects (above the surface) up from the south into the Plains on Saturday and mixes out the modest moisture already present. In other words, we're not even getting neutral advection from the Gulf, we're getting dry air advection. Not what I want to see, and not what I was expecting, but perhaps should have. Further, I see no reason to doubt this solution, as it looks perfectly realistic, no matter how undesirable.

That said, even with the crappy moisture forecast by the NAM, there is still a large area of decent CAPE with no CIN, especially over NE/SD. The main problem is that because of the low dewpoints the LCLs over much of this area are also very high, and the best shear is forecast from southwest NE down through the TX panhandle, where the corridor of instability is narrower, as one would expect given the situation. Obviously this would limit the threat in both space and time.

In summary, things still look interesting as everything else remains in place, and I love the terrain out there; but it does indeed appear that moisture will be a limiting factor. Even though a wedgefest is looking doubtful, the lean moisture could lead to awesome LP/classic storm structure with high-contrast tubes, I think I can deal with that. :)

[Mikey Gribble]

I hate to make a frivolous post in here, but the "dewpoints look like A$$" comment was pretty funny and it does an excellent job of summing up the 12Z NAM. If the NAM holds true and moisture return is that bad, I would definitely be sitting this one out. As much as I love chasing, I'm just not going to drive 5 plus hours without a reasonable chance of tornadoes.
I haven't seen an update of the GFS on Rap, so I don't think it's updated.

[Donald Giuliano]

Is the new GFS run out somewhere? I don't see it yet(just coming in on ncep). It seems a lot of folks post mere minutes before new data is out and I never understand that. But looking at the 0z gfs it doesn't seem like it's moved all that far from past runs.


The status of all real-time NCEP model runs can be found here:

http://www.nco.ncep.noaa.gov/pmb/nwprod/prodstat/

[Karen Politte]

I hate to make a frivolous post in here, but the "dewpoints look like A$$" comment was pretty funny and it does an excellent job of summing up the 12Z NAM. If the NAM holds true and moisture return is that bad, I would definitely be sitting this one out. As much as I love chasing, I'm just not going to drive 5 plus hours without a reasonable chance of tornadoes.
I haven't seen an update of the GFS on Rap, so I don't think it's updated.

I've been biding my time watching most of the discussion evolve along with the models for this event, largely because I'm burned out on a lot of the discussion. At least I'm not burned out on forecasting LOL. But I'll keep plodding along I suppose. I like Michael's take on things, and I too was tickled enough by MikeH's funny on the A$$-points that I had to join in again.

The 12Z ETA is out today and I guess I'll play with it like it was actually a useful tool, although most of us know that the NAM went to lunch sometime a couple of years back and hasn't been seen since.

In pinpointing what's ailing with this upcoming system for Saturday 4/21, I think one has to look no further than the current surface map and put 2&2 together. A cold froPA took place across the south-cntrl/southeast US overnight and has dagged a front off the Gulf shore again - as has been progged by the models since the outset. Surface winds are northerly and have swept some modified wintry (alright - early springtime) air out into our moisture supply.

The associated ridge with the backside of this departing system causes Saturday's hopes some real grevious issues, the large HI having it's foot planted firmly in the Gulf all the way through the weekend assuring us not one ounce of true moisture to play with. I appreciate that many are hoping this event will unfold on higher terrain and compensate for the lack of instability and I also appreciate the respectable shear setup. As for evapotranspiration......yes a lot of the ground may be wet out there but I don't think that the atmosphere will be able to actually harness that to it's fullest potential. In my experience, I think that the time to really sit up and take note of evapotranspirational enhancement of the available mositure is in the humid, green months of late May, June and July when the cornfields are in overdrive. In mid-April in the high Plains I don't know how much we can rely on that.

It is feasible to expect T-Td depressions in the range of 20 degrees or so. This may provide some classic supercells and possibly some aesthetically pleasing LPs - which is all one should expect when chasing with limited moisture. As the sun sets, the atmosphere cools and LCLs lower I would imagine that any chaser's tornado potential would rise, assuming that you have a supercell ongoing and the convective mode has not been destroyed for the event by a cluster of multicells or an MCS/squall.

I can totally understand why those who can chase this event will chase - it is, after all, all about hope. This naysayer, however, will not be chasing due to many much more important engagements and the fact that a 10+ hr drive from central AR is simply not worth it. So my tripod is one you won't encounter out there this weekend. I will wait until May and am completely happy to do so. In the meantime I will be interested to watch this system develop from a comfortable armchair somewhere on Beale St. in downtown Memphis. :cool:

KL

[Skip Talbot]

I find it interesting that both GFS and WRF are plotting 60 degree dewpoints in South Dakota. Boths models are comparable in the strength of the surface low pressure at 997-998 mb over central Nebraska (but vary in placement). I guess this is contributing to good moisture convergence in these northern areas. SPC's day 4 has areas all the way to the North Dakota border highlighted, so they certainly think this event isn't limited to the southern plains. Winds will be better backed closer to the low's center and there will be better veering through the 850-500 levels in these areas as well. Areas south through the panhandles still look good for supercells despite the blow WRF dealt us with its moisture forecast, but I think those of us who aren't already in the southern plains should be considering Nebraska and even South Dakota for chase options on Saturday.

[Stan Rose]

. I will wait until May and am completely happy to do so. In the meantime I will be interested to watch this system develop from a comfortable armchair somewhere on Beale St. in downtown Memphis. :cool:
KL

yeah, that's what i said last year! :mad: Not this time. Don't know what will happen in May. Do know that Sat has a lot of potential, even if it's not a major tor outbreak. I'll be happy with the 60 dews or 58 dews or even 55 dews. Just another detail. I won't be surprised if the lousy moisture doesn't show, but a ridge over the gulf doesn't cause me any undue concern. 24 hours of limited return on friday looks fine to me. As Karen said, gotta be hopeful.

[Brian Emfinger]

Dewpoints may only be around 55 degrees but that will work for me. And we are so far out we could still end up having 60+ degree dewpoints. I think dewpoints around 55 degs will be sufficient however to at least have some nice structure (which is extremely sufficient for me at this point) and if the T-Td spreads come down a bit then we could be set for another NW Texas Outbreak. I imagine there will still be a ton of people out there Sat.. anyone going further north JUST because of the March 28th crowds?

[Jeff Snyder]

Just as a note, let's try to keep all posts about the 4/21/07 forecast. If you have commens, thoughts, or ideas about forecasts or chase ideologies in general, please use an appropriate Weather and Chasing thread.

[Matt Hines]

Looks to be a pretty good set-up here in the Texas Panhandle region on Saturday. Models continue to pull the upper level dynamics back further west with each run. Euro. has been the most consistant the past few days and now GFS seems to be trending that way as well. We will not have deep gulf flow, but dewpoints in the 50s seem like a good bet. That is definatly all we need in April up here at 3,600 ft. If this were late-May or June...I would not be too excited. Case in point, all the tornado events here in my area this year have been with dewpoints less than 60F. Looks like sheer will be very nice...(500mb) 210 at 50-55 kts....(700mb) 190 at 45-50 kts...(850mb) 170 at 35 kts. Again, still a few days out...but looking better with each run!

[matt patterson]

If you look at the OK mesonet there are widespread 50+ tds all over Ok with some upper 50s in southern OK... WIth a couple good days with southerly winds we should start to see the moisture increase. I think that if there was a problem it would be concerning the depth of the moisture as the 850mb is very dry, relatively speaking, and that seems to be what is causing the models to mix down that dry air... If it were anywhere else, i would be worried but its the caprock, its saturday in Mid April, and i want to see some storms, so thats my reasoning....
The next 48 hours will be interesting.

[Michael O'Keeffe]

I'm not sure why the WRF has such sucky tds across the region when we have alomst 65F tds on Friday. I'm wondering how the Tds can drop that much over the course of 24 hours. That's why I think the dewpoints are a bit underforecast and there are mid-upper 50s tds across the TX Panhandle and that along with the Caprock will be plenty for at least a tornado or two along with the amount of favorable parameters associated with the system. Like I stated earlier on the Fenruary 28 tornadoes the TDs at Iola were 54F. So anything can happen, I'm still planning on chasing this event.

[Jason McKittrick]

yeah, that's what i said last year! :mad: Not this time. Don't know what will happen in May. Do know that Sat has a lot of potential, even if it's not a major tor outbreak. I'll be happy with the 60 dews or 58 dews or even 55 dews. Just another detail. I won't be surprised if the lousy moisture doesn't show, but a ridge over the gulf doesn't cause me any undue concern. 24 hours of limited return on Friday looks fine to me. As Karen said, gotta be hopeful.



For everyone on the board that is waiting for the season to magically start when the calender changes to May I would like to point out a couple of things to you. Not to say there have not been any May tornadoes the last couple of year but that despite my good efforts I have not seen a one since 2004. Last year I chased 3 times during the first half of April and got some great video (3 tornadoes). This year I chased twice before April 1 and again more great video (6 tornadoes, yes on 3/28 but not in TX). My point is you need to judge each event on its own merit and not just expect great storm chasing when the calender switches to May. The last several runs of the GFS are hinting that a PNA pattern will develop during the first week of May I sure we all remember what great chase opportunities that pattern produced last May.

As for Saturday I like what I am seeing in this mornings models. Last night I was about to give up but with each new day comes new hope. The models today are trending more toward a good chaseable event.

Both the GFS and NAM have abandon the idea of a closed low jetting out of the Rockies and are now indicating a nice negative tilt open wave trough. However the devil is in the details. There is at least one major difference between the two which has a huge impact on the potential storm types and locations for this event.

The NAM is the best model in terms of a chaseble severe weather event. It ejects the trough over southeast Colorado and brings out an associated 60knt westerly 500mb jet which is juxtaposed with 30-35knt south southeasterly 850mb flow. This yields great directional and speed shear for storms across SW KS south to the South Plaines of TX to work with. Model forecast 3km and 1km SRH's of 300 m^2/s^2 and 150 m^2/s^2 respectively are present across this area along with CAPE's from 1000-2000 J/Kg. Another feature which is present in the NAM is a mid level dry intrusion which the NAM RH fields show overtaking the dryline across the South Plaines late in the day Saturday. As a result if the NAM solution were to verify we should see several supercells across west TX into SW KS late Saturday.

As far as things that could go wrong one has to do with moisture. It is true that it does not take a 70F DP to get great storms at 3500ft. However with DP's around 55 this afternoon along the TX coast will the quality of moisture return be good enough to get low 60 DP's on the cap rock. The event is still three days away that's more than enough time to transport moisture northward into KS and west TX and I am sure that evapotranspiration will also help but if the moisture does not make it onto the cap rock I fear the dynamics and moisture may not meet until sunset and that might not be such a great thing as far as chasing goes. One thing to note however is that both models are indicating low 60 DP's in Amarillo by 7PM Saturday.

The other concern stems from the GFS solution which has the trough over southeast New Mexico late Saturday with the 500mb jet moving through Guadalupe Pass. With this more southerly solution the more favorable super cell kinematics are confined to west TX south of I 40, with more and more unidirectional shear profiles as you go north through KS favoring a more linear storm mode. I suppose this solution would not be so bad for many but its a little to far away for my liking.

Since I have an opportunity to chase in the panhandle this weekend I for one will be rooting for the NAM solution.

[Kenny Drake]

Okay it now seems the NAM, ECMWF and GFS are in fairly good agreement with location of a 500mb open wave across the eastern sides of the Rockies centered over New Mexico by Saturday evening. I have noticed a fairly large difference in the positioning of the surface low between the NAM and GFS, the NAM having it centered over SW NE while the GFS over the western sides of the ND/SD border. The ECMWF has the surface low in relative agreement with the GFS which has just recently come into agreement with the ECMWF. Given this agreement between the GFS and ECMWF along with ECMWFs consistency in the previous couple of runs I will base my current forecast along these lines. GFS has recently decreased the surface Td's pretty much from the northern plains to the coast showing isolated pockets of 60s but mostly upper 50s. It appears the reasoning behind this relatively lax moisture return given current readings across the southern plains (upper 40s, low 50s) is the origin air mass the LLJ will be advecting being the east coast, with a quick route over the Gulf immediately along the southern coast.
Here is a map of the SST in the Gulf recorded at 1200Z today:
http://marine.rutgers.edu/mrs/show/?file=../regions/gulfmexico/sst/noaa/2007/img/070418.108.1205.n15.jpg

Forecast model valid 4-13 through 4-18:
http://www.opc.ncep.noaa.gov/sst/images/gmex/GMex_GoesENH.gif

The surface temperatures near and where the 850 and surface winds will be advecting are less than desired (54-65F). Normally when we have seen the gulf wide open the 850 winds were coming from the western Gulf region where SST's are shown to be in the mid 70s currently.

Maybe I'm just a natural skeptic, but I'd rather be surprised than dissapointed. Given the current surface Td forecasts of upper 50s I'm still not blowing this off. The dewpoints were in the upper 50s during the May 28th event. The main difference between the events of March 28th and the current forecasts for Saturday is the 500mb trough, based off of NWS AMA's analysis of the March 28th event that had the low much deeper and farther west. The AMA sounding at 18Z on the 28th showed a temperature of 70F, with a TD of 57F.

Bringing the forecast for Saturday into this, temperatures near Amarillo seem to be very similar (upper 60s to low 70s) so, upper 50 Tds (about a 12F spread) are definitely doable. Upper level winds will likely be slightly less than the 3-28 event. The surface low if I recall was across NE/SD area which is also relatively similar to this forecasted event. Given all of these assesments based on model output that is 3 days in advance of the forecast day in question I will not pick a target zone but a target area that would extend from central NE southwards into the TX panhandle where surface winds ahead of the dryline are out of the SE. My hope is that models trend towards more of a southerly LLJ across the Gulf. The implications of a LLJ coming out of the SE obviously lead to a much stronger convergence zone up and down the dryline, which may spell disaster for cell isolation. However, the negative side would also be a disaster if the LLJ veered too much and we ended up with SSW surface winds ahead of the dryline northwards all the way to the surface low. It appears the NAM is taking the weaker east coast ridge solution while the GFS and ECMWF are taking the stronger. If models end up overplaying the magnitude or western extent of the east coast ridge then the LLJ would likely become more favorable for deeper moisture advection and less convergence ahead of the dryline/pacific front. For deeper moisture some of the larger scale surface convergence would have to be given up.

[Bob Schafer]

I have to laugh a little at the posts made by folks who obviously haven't read any of the previous posts. I'm also laughing at the raging debate over Td's. I don't think I've ever seen eight pages of banter about Td's for one event before.

That said, something on-topic (gasp!):

18Z WRF run has this to offer for 00Z Sunday:

http://www.wxcaster.com/CONUS_MESO-ETA_LCL_GPH_78HR.gif

Broad area of 1000-1200m LCL's from PH's-northward. Love it or hate it. Believe it or don't.

http://grib2.com/nam/CONUS_NAM218_ATMOS_BRF_78HR.gif

You have to like that Derived Radar Reflectivity forecast, even it it does look a tad linear.

[Michael O'Keeffe]

I am starting to look at the South Dakota area for Saturday near the TP. Tds in the 60s and SFC temps in the 70s along with CAPE to 3000 j/kg north of I90 also the area of instability is far greater area than the panhandles. I'm quite confident that SD may be the play for Saturday.

[Jason A.C. Brock]

March 28 also had some interesting ineractions in the Panhandle that were not see up north. If the differential heating area hadnt occured etc. would the storms in the panhandle have been as impressive? I think so but it would be nice to find some type of an intersecting or close by boundary along with the dryline. You can never find these things till maybe even hours ahead of time. If dewpoints do recover SD may very well be the place to be near the TP. Ill chase the panhandel tho....moisture is more likey to make it at least to this point as well as storms being a little more islolated and a bit of a cap kepping them that way. Atleast initially.
Still way too far off to make any decision. All I know is unless things change dramatically ill be in the Panhadle.
Get out and chase people! I know its not May but I belive the first week of May or the first 5-10 days at least are gonna be very very quite....this week doesnt look like any massive outbreaks but it looks like one thing to me

STORMCHASING:
Storm chasing is broadly defined as the pursuit of any severe weather condition, regardless of motive. A person who chases storms is known as a stormchaser, or simply a chaser. While witnessing a tornado (http://en.wikipedia.org/wiki/Tornado) is the biggest objective for many chasers, many chase thunderstorms (http://en.wikipedia.org/wiki/Thunderstorms) and delight in seeing cumulonimbus (http://en.wikipedia.org/wiki/Cumulonimbus_cloud) structure, watching a barrage of hail (http://en.wikipedia.org/wiki/Hail) and lightning (http://en.wikipedia.org/wiki/Lightning), and seeing what skyscapes (http://en.wikipedia.org/wiki/Skyscape) unfold. There are also a smaller number of storm chasers who chase hurricanes (http://en.wikipedia.org/wiki/Hurricanes).

;-)

On a side note Friday will have a isoalted sup or two.....its just amatter of finding where...NE Colorado....TO Abiliene.....:-p It will be somewhere in there lol.

[Bob Schafer]

http://weather.cod.edu/forecast/AVN/OFFAVN/avn_usUS_500_rhum_84.gif

There ya go, everyone! There is clearly a tornado at Lakeview TX based on that anvil you see there.

[Alex Lamers]

STORMCHASING:
Storm chasing is broadly defined as the pursuit of any severe weather condition, regardless of motive. A person who chases storms is known as a stormchaser, or simply a chaser. While witnessing a tornado (http://en.wikipedia.org/wiki/Tornado) is the biggest objective for many chasers, many chase thunderstorms (http://en.wikipedia.org/wiki/Thunderstorms) and delight in seeing cumulonimbus (http://en.wikipedia.org/wiki/Cumulonimbus_cloud) structure, watching a barrage of hail (http://en.wikipedia.org/wiki/Hail) and lightning (http://en.wikipedia.org/wiki/Lightning), and seeing what skyscapes (http://en.wikipedia.org/wiki/Skyscape) unfold. There are also a smaller number of storm chasers who chase hurricanes (http://en.wikipedia.org/wiki/Hurricanes).

;-)

Thank you! :D This is what I always say. There is so much said about seeing tornadoes...personally I'm happy to see nature's power unleashed in a variety of ways, whether it be hail or wind etc. I'm really happy to be on the Plains now in the hopes of seeing a few LP supercells sometime...those tend to be nice to photograph...even if it's just a wall cloud.

Basically we're looking at a narrow corridor of severe weather that should encompass most of the CONUS between those magic longitudes. We could wind up seeing a severe weather risk region from near the Canadian border all the way down to the Big Bend/Trans-Pecos area of Texas. It will be interesting for sure! I think the final reports graphic may look similar to 3/28 in terms of where/how the reports are concentrated not necessarily specific numbers of certain types of severe weather. Only exception would be I think it may extend a bit further east and should extend a bit further north as well.

I went down the SPC ('98) / Miller ('72) Severe Weather Checklist that the WFO SGF has out on their webspace and 94.7% of the parameters came out favorable for a Moderate-Strong severe weather event (52.6% for a strong one). We can debate dewpoints and LCLs until we're blue in the face but the thing I take away from current model progs is that almost all models develop widespread convection along the composite dryline & cold front boundary between 18z and 00z Saturday. The shear and instability is sufficient to support severe weather and based on what I've seen on the checklist and various parameters, I'd say it should be pretty widespread.

The area I'm looking at for the best concentration of severe weather would be from W KS and far E CO southward along the dryline into the OK/N TX Panhandles (anywhere between Goodland and Amarillo). People may argue in favor of a position closer to the low off to the north and the nose of the LLJ but I like the southern end of the event more. PVA and H5 height fall maximums concentrated here as a potent shortwave ejects at a negative tilt off the Frontrange and overspreads the low-levle boundary. Bit of a "butterfly" to the jet as it bends around this feature and exit region of the upper level jet streak positions over W KS. Methinks this could help induce a mesolow in the low levels in W KS and perhaps a secondary LLJ.

Basically here I see the best phasing of strong dynamic lifting and thus vertical motion, wind shear, and instability. Low level moisture layer extends up to about 5000' so plenty of depth. Shear is sufficient for supercells but with high LCLs/LFCs I see them being higher based. Therefore I see large hail being far-and-away the greatest threat. However storm-scale interactions and variations in the environment could prove sufficient for some tornadoes. Further north may have the best shot for tornadoes overall, near the warm front, but I'm unsure how good chasing conditions would be in that area (NE/SD)...and that's far too far to drive.

AJL

[Jeff Snyder]

I haven't paid much attention to the models given that, until the 0z runs come in tonight, we're still looking >72 hours out. When it comes to specifics such as whether the dewpoint at AMA and DDC will be 54F, 58F, or 62F, I think we all could agree that model forecasts at >72 hours out shouldn't be taken too strictly.

My biggest concern regarding a potential significant tornado event on Saturday is moisture quality and depth. As others have noted, at the elevation of the TX and OK panhandles and western Kansas and Nebraska, lower dewpoints are expected compared to areas farther east (the dewpoint of a surface parcel will decrease naturally as the elevation increases). However, there have also been other events that have looked similar but that have not produced significant severe weather. We can debate Tds to death, but the "Td required" or "Td sufficient" to produce a particular level of severe weather will always vary from situation to situation. The depth of the moisture is important, the degree of turbulent mixing in the boundary layer is important (both by mechanical mixing by strong shear and convective mixing caused by buoyancy as insolation occurs at the surface), and the vertical virtual potential temperature profile is important, amongst many others. I've never liked the use of "magic numbers", so I won't continue any further.

Let's remember that forecasts are the opinions of others. Nobody is a perfect forecaster, nor should anyone think they are a perfect forecaster. As such, let's try to be less critical of each other. Everyone has been burned before. In addition, everyone chases for different reasons -- some chase for the sole purpose of seeing a tornado, while others chase to see any strong convection. It's a personal decision, and I don't think any motive is any more "hardcore" or is any better than another. Let's not try to make ourselves feel more "hardcore" by putting down those who may only want to chase on "big days". Live and let live, as they say.

The strong negative-tilt nature of the trough, which appears to the first such trough of the spring, argues for a very strong ageostrophic response as strong pressure falls occur downstream of the trough and vorticity maximum. The surface reflection and response to the strong disturbance aloft will seen in the backed flow east of the dryline. Depending upon the depth of the southeasterly and south-southeasterly flow, we may be able to mitigate the amount of mixing that occurs in the boundary layer, which may help keep dewpoints from dropping too much through the day (or it may at least help off-set the positive affects from advection). In actuality, I'm increasingly becoming a fan of days with SOME cirrus in the warm sector, since ci may help prevent an "overmixing" of the BL (preventing the BL from becoming too deep, which tends to reduce low-level shear as winds in a well-mixed boundary layer tend to be relatively vertically homogeneous).

With deep-layer shear on the order of 50-60kts out of the west in the OK and TX panhandles, we should see a discrete convective mode preferred (deep-layer shear vector normal to the dryline), largely aided by mid-upper level flow out of the west at the base of the trough. Model performance in the past few days has been quite good, but let's remember that we are still 72 hours before the event. Despite how things look now, we should all know that they can change quickly and unpredictably.

So again, my primary area of interest is in the OK and TX panhandle region. I'm not confiden

[Ray Walker]

Well I may be missing something because im no expert on reading models but from what im looking at on the lates GFS is not very promising for saturday. It shows the low to be even further north, across south dakota and not as strong. Also it shows most of the upper jet energy remaining out to the west at 18z. Also the moisture content is only proged to be in the low 50s. All this is not promissing from what im looking at. Any thoughts, is it me? Do you think this is just one of the famous GFS model bounces or is the system just starting to fizzel?

[Mike Hollingshead]

May as well wait on the new data that will be updating now. NAM should be out by 10pm or so and GFS should be starting to update by 10:30.

[Matt Hines]

No matter what...1) There will be storms...2) Some will be severe...3) There will be a few tornadoes...Bottom line...There will be a chase on Sat. ;)

[Mikey Gribble]

I would agree that there will be some severe storms Matt, but I wouldn't make the statement yet that there will be tornadoes.
I like chasing a non-tornadic supercell just as much as the next guy, but I'm not going to drive 6 hours to do it. If it's within 2 hours of home that is when I consider going out without a reasonable chance of tornadoes. I have to be selective though because obviously I can't chase everything with precipitation, so I typically make my decision based on whether or not there is a decent chance of tornadoes. I still log 20,000 plus miles a year chasing, so at this time in my life that criteria is not too strict for me. I am getting all the chasing in that I want.
If I lived in the panhandle, this would be a definite chase day for me (even if dewpoints were in the low 50's). If the moisture return isn't good you still have the chance of an isolated tornado or two. I haven't paid much attention yet because of the lack of tornado potential, but if I were going to chase, as of now I would target the far northern part of the TX panhandle (not even looking at the northern half of the target).

[Matt Hines]

Just trying to lighten the mood a little. I do think that there will be a few tornadoes though...Heck, we had 7 yesterday in West Texas...and yes, they were landspouts, but still are "tornadoes." IF, the models continue to show the sheer they have now, I do think there will be some spin-ups.

[Brandon Clement]

Thank you! :D This is what I always say. There is so much said about seeing tornadoes...personally I'm happy to see nature's power unleashed in a variety of ways, whether it be hail or wind etc. I'm really happy to be on the Plains now in the hopes of seeing a few LP supercells sometime...those tend to be nice to photograph...even if it's just a wall cloud.

AJL

Sounds good when you live in Norman. I have the same concerns as many others on this board....dews. Yes, their have been many events that produced tornadoes with td's in the 50's. However, the tds in the 50's is not ideal, certainly not with temps forecasted to be in the mid 70's yielding a big spread. I know this is still a ways out and a lot could change but over the past few years models have certainly overestimated td's far more often than they have underestimated them. So before I drive 1300 miles while gas prices are creeping towards $3 a gallon I want a lot of confidence in the setups.

I do love the Tx panhandle and playing the dryline but the last thing I want to do is drive all night, spend $300 in gas, be in position by noon and then hope td's creep up all day. I have done this several times and more times than not I am looking at high blue skies by late afternoon with all the moisture mixing out or chasing after high based thunderstorms all day. Don't get me wrong, I love the structures but I don't stay up all night, drop a few hundred bucks, drive over a 1000 miles and eat junk for two days for structure.

Lets just hope the L drops back farther south and the moisture is not even a question by Friday afternoon. If that is the case then I will be there, if not, lets hope this is just the start of the season and not the end like last year.

[BrandonWhittington]

Lots of people are worried about the amount of moisture there will be and deep it will be able to go. As of right now at 11:00 pm, dewpoints are already reaching 50 degrees in the southern TX Panhandle....I dont know if I'm missing something that others are seeing, but moisture is the last of my worries. This year so far, tornadoes have formed when dewpoints have been in the mid 50s, and the tornadoes have been large.....Throw the dryline in the whole mess and it looks to be a very interesting day on SAT.

[Jeff Snyder]

I dont know if I'm missing something that others are seeing, but moisture is the last of my worries. This year so far, tornadoes have formed when dewpoints have been in the mid 50s, and the tornadoes have been large.....Throw the dryline in the whole mess and it looks to be a very interesting day on SAT.

Brandon,

The problem is that, although we are seeing southeasterly flow in western TX now, the surface ridge on the eastern US is north enough that there is easterly flow across most of the central and norther Gulf. In essence, the position of the high is leading to the recirculation of drier continental air from the southeastern US, across the Gulf, then up into the Plains. So, we may be seeing low-mid 50s across areas of western TX now, but models continue to indicate that, despite southeasterly flow across much of the risk area, better Gulf moisture is unlikely to make it into the Plains by Saturday evening.

The 00z BRO and CRP soundings look pretty bad in terms of low-level moisture. Buoy 42002 east of CRP is observing a northly wind at ~20mph, with a Td of 66.6F (likely quite shallow). 00z soundings from Mexican locales in the southwestern Gulf (e.g. MMVR --> http://www.rap.ucar.edu/weather/upper/mmvr.gif ) show relatively deep northerly and northeasterly flow in the low-levels, another strike.

Pattern recognition indicates that confluent, mid-level flow associated with mid-level ridging over the Gulf and a deep trough off the southeastern US coast is often not favorable for the advection of rich Gulf moisture into the Plains. I'd much rather see the mid-level trough off the coast slide east faster, which would allow more ridging on southeast coast, and may allow for more favorable trajectories.

Per the latest 00z NAM, it looks like the trough will come through quite a way farther south and later than previous models outlined. Jeez, looking at the surface progs, some chasers may even target Tucumcari NM for Saturday. Of course, a slower solution shouldn't really surprise anyone, given the tendency for the NCEP models to be too fast / too far east with many of the dryline days this year so far (prime example being 2-28). With the slowdown, timing has now become an issue. Ah well, we certainly can't expect 72+ hour forecasts to verify perfectly within 50-75 miles and 1-3 hours.

EDIT: All this is NOT to say that Tds won't reach 60F in spots. There is the possibility that tds will verify higher than models are currently forecasting, and there's no way we can know for sure at this time. All that said, I think there is a better than 50% probability that we'll see widespread mid-50s Tds across western KS and areas northward, with widespread upper50s farther south. I've be pleasantly surprised, but surprised nonetheless, to see low-60F Tds widespread in the warm sector, with the way it currently looks. I hope I'm wrong, and I'll be the first to admit it if I am. Ah, the unpredictability of the atmosphere.

[Jay McCoy]

Well as the models come into agreement they are now bringing the dryline well back close to the Tx/NM border and the latest NAM and GFS breaks out the storms in that area. Like I said in previous posts the dryline hasnt mixed east yet this year and looks to stay well west again. I expect the best storms to be the central panhandle as they mature. Not sold on tornados in NM. And I agree with Jeff about the slower resolution. This year the GFS has been constantly too fast until the last 48-72 hrs or so where it has been slowing it down.

Now that the 72hr models are out I am anxious to see what the SPC is thinking for day3. Will they pull it back that far west also?? Do they expect moisture to be sufficient? etc..

I expect a good chase day. I wouldnt be surprised to see a moderate by day2 on for saturday. Maybe as late as the 1730z. Doubt they will go mod until then. Just a huge slight risk area.

[rdale]

May as well wait on the new data that will be updating now. NAM should be out by 10pm or so and GFS should be starting to update by 10:30.

No need to guess...

http://www.nco.ncep.noaa.gov/pmb/nwprod/prodstat/

[Adam Atkins]

Well I'm not gonna jump into the battle but I'd like to point out one fact...and one observation...

Observation: The models have consistently slowed down and deepened the short-wave trough from run to run...meaning this event will likely be much later in the day, further west, and over higher terrain...possibly upslope flow in eastern NM, CO.

Fact: Surface based air parcels are composed of moisture that is a mixture of the entire boundary layer. Good surface moisture will mean nothing beneath an airmass with continental origin. The moisture fetch just off the surface is off the Mexican plateau leading up to the event...yes it's southerly flow, but it's not Gulf of Mexico southerly flow until late fri/early sat. So our surface based parcels will have low moisture content once the boundary layer does mix out.

Another thing...shallow moisture along the dryline and the lack of eastward progression shows how strong the southeasterly flow and convergence will be, coupled with significant upper level forcing, shouldn't' have a problem breaking the cap (all models break out convection).

[Kenny Drake]

I'm glad to see the NAM coming into agreement with the GFS and ECMWF which have been consistently agreeing with each other. Td's still a question mark with progged trajectories leading to the source region of the advected air mass being the east coast. The air mass progged to be over the east coast at the time of the LLJ initiation on Friday is still being advected southwards across the east coast at this time. If the source region turns out to be the east coast air then the real question would become the exact trajectory the air mass will take as its forced westwards. I'd say the more southerly route it takes the better. Latest GFS run (00Z) is encouraging in that it pushed the trajectories much farther south, into the heart of the Gulf. If this verifies, the next question is how much moisture can be evaporated into the air mass before it reaches the coast of Texas. Air mass modification during advection is quite complicated but the warmer and faster the air moves the higher evaporation rate over warmer water (very basic.) The moving air will help mix out the moisture and the lowered temps near the water surface due to evaporational cooling. Higher temps will increase the water load capacity.
As a final note, its important to answer these questions when forecasting moisture: where is the air coming from? what is its characteristics at its source location? What are the characteristics of the land/water its being advected over? What is the speed of advection (will determine the time for modification)? At this point in my forecasting career I have to rely on models to make this forecast for me, as I haven't seen enough weather for lack of a better word in my days to have that knowledgeable instinct on just how this air mass may modify with regards to moisture content (if it comes from the east coast).

I know I'm a skeptic, but I'd rather be surprised than disappointed. I also hope that I'm wrong as Jeff stated, because Saturday is about the best possible chase day of the week for me and probably the majority of people here.

Now, I'm fairly sure that there will be convection on Saturday, and there will be some supercells... and chasing to me isn't about catching a tornado every day. This will definitely be a fairly long drive for us from Norman though. Norman to Amarillo: 277 miles. Given the dryline location progged by ECMWF/GFS/NAM: about 300 miles. That is a straight shot, add in the return trip and the jogging around this could be an 800 mile day easy. Making my trip from Norman to Minneapolis, MN is about 870 miles which is about 13 hours of driving including very limited stops traveling I-35 the entire way.

[Tony Laubach]

For those worrying about moisture only being in the 50s... High Plains + 50s TDs = pretty good results....

Two targets for Saturday; Burlington, Colorado or Dalhart, TX

[Ray Walker]

The GFS is showing very low dewpoints across the texas panhandle for saturday now, dewpoints in the upper 40's!!!!!!!! Most of the deep moisture will be in the eastern panhandle and Oklahoma points north. Evidently the model is showing some massive amounts of mixing taking place, which I think if this accurs then the dryline should mix further east, closer the the moist axis. Also the model is showing a very nice setup for SW Oklahoma down into Texas for next tuesday. Look at the low level wind feild setting up in that area, nearly DUE EASTERLY FLOW!!!!!!!!! This should make for some large curved hodographs.

[Mike Hollingshead]

No need to guess...

http://www.nco.ncep.noaa.gov/pmb/nwprod/prodstat/

That's already been posted...might read previous posts. Guessing works just fine 99.9% of the time(since it's generally always the same time that it updates). Not like I need some second by second update.

And so this post has meteorological value. Looks like TDs should be above 20 along the dryline, so anythings possible! I mean the dryline is getting very close to the mountains now, so that should do.

[Rich Thompson]

I'm glad to see the NAM coming into agreement with the GFS and ECMWF which have been consistently agreeing with each other. Td's still a question mark with progged trajectories leading to the source region of the advected air mass being the east coast. The air mass progged to be over the east coast at the time of the LLJ initiation on Friday is still being advected southwards across the east coast at this time. If the source region turns out to be the east coast air then the real question would become the exact trajectory the air mass will take as its forced westwards. I'd say the more southerly route it takes the better. Latest GFS run (00Z) is encouraging in that it pushed the trajectories much farther south, into the heart of the Gulf. If this verifies, the next question is how much moisture can be evaporated into the air mass before it reaches the coast of Texas. Air mass modification during advection is quite complicated but the warmer and faster the air moves the higher evaporation rate over warmer water (very basic.) The moving air will help mix out the moisture and the lowered temps near the water surface due to evaporational cooling. Higher temps will increase the water load capacity.
As a final note, its important to answer these questions when forecasting moisture: where is the air coming from? what is its characteristics at its source location? What are the characteristics of the land/water its being advected over? What is the speed of advection (will determine the time for modification)? At this point in my forecasting career I have to rely on models to make this forecast for me, as I haven't seen enough weather for lack of a better word in my days to have that knowledgeable instinct on just how this air mass may modify with regards to moisture content (if it comes from the east coast).

I know I'm a skeptic, but I'd rather be surprised than disappointed. I also hope that I'm wrong as Jeff stated, because Saturday is about the best possible chase day of the week for me and probably the majority of people here.

Now, I'm fairly sure that there will be convection on Saturday, and there will be some supercells... and chasing to me isn't about catching a tornado every day. This will definitely be a fairly long drive for us from Norman though. Norman to Amarillo: 277 miles. Given the dryline location progged by ECMWF/GFS/NAM: about 300 miles. That is a straight shot, add in the return trip and the jogging around this could be an 800 mile day easy. Making my trip from Norman to Minneapolis, MN is about 870 miles which is about 13 hours of driving including very limited stops traveling I-35 the entire way.

Kenny,

You've touched on one important aspect of moisture return forecasts - the "source" region and the underlying ocean conditions. Another critical component is the vertical structure of the air mass being modified. Most chasers fear cold frontal intrusions into the Gulf, but they're not all bad. The depth (as in vertical extent) of the cold intrusion and the sea-air temp difference drives sensible and latent heat fluxes (heating and moistening) in the boundary layer. If the cold air is really deep (say up to 700 mb), your moisture return can end up looking like a deep moist layer with relatively mediocre dewpoints. A very shallow cold intrusion results in a shallow moisture return.

The worst case scenario is sort of what's happened over the past few days. Not only are trajectories questionable (emanating from sern US ridge with short over-water paths, much of which isn't the warmer ocean), the sea-air temp differences are also quite small. The frontal intrusion this time was more of the mild and dry variety, which limits the potential for sensible heat flux over the water (which in turn drives boundary layer depth). The net result is a very shallow marine boundary layer that is very susceptible to vertical mixing over land. If you loop through the NAM dewpoint forecasts, or even forecast soundings, you see the moisture "mix out" each day across TX, for a few days in a row. These moisture "holes" then spread nwd/nwwd on the prevailing flow. The whole situation won't improve much until the background trajectories change and we bring in either an air mass from farther NE that has had more time to modify and consists of a deeper boundary layer, or we simply draw the previous maritime tropical air mass back northward. The low-level wind forecasts and persistence of the big trough/low off the SE Atlantic coast makes the latter scenario less probable.

With all of this in mind, mid 50s Tds aren't too bad with surface temps in the low-mid 70s at 3500 ft. Perhaps the situation will end up looking more like the NAM with the Kain-Fritsch scheme (upper 50s), instead of the operational NAM with the Betts-Miller-Janic convective scheme (low-mid 50s), or the GFS and its 48-50 F Tds. I'm pretty sure we'll have it figured out by about 21z Saturday :)

Rich T.

[Stan Rose]

Kenny,

I'm pretty sure we'll have it figured out by about 21z Saturday :)

Rich T.

Rich, hope the NAMs convective scenario doesn't play out, or we'll know by 18Z Sat :eek: Namely, a roaring linear mess. Not to say there wont be embedded sups, but if that verifies might have some thinking they should have braved the high-risk of bust on friday!

[Rich Thompson]

Rich, hope the NAMs convective scenario doesn't play out, or we'll know by 18Z Sat :eek: Namely, a roaring linear mess. Not to say there wont be embedded sups, but if that verifies might have some thinking they should have braved the high-risk of bust on friday!

Stan,

Strong linear forcing and the potential for cold pool formation could lead to squall line development. However, wind profiles still appear quite favorable for supercells with plenty of cross-boundary flow and shear. Again, we'll know by 21-00Z ;)

Rich T.

p.s. This Saturday system looks vaguely similar to 6 April 2001, with more cross-boundary flow and (perhaps) and little weaker instability. The outcome of that event was a rapid transition to a squall line, with lots of wind damage and ridiculous storm motions. See
http://www.spc.nssl.noaa.gov/exper/archive/events/010406/index.html

[mcrowther]

I don't know- based on the new NAM the really strong forcing from the primary vort max does not really get cranking until 00Z or after- one thing I do notice is that the main threat really seems to have shifted well back into extreme eastern NM and near the TX border by 00Z-If this scenario unfolds as forecast by the NAM then there looks to be a decent chance of a few supercells from CVS to HOB before the squall line sets in?

[Donald Giuliano]

One interesting thing to note about April 6, 2001, is that the 300mb jet core was on the east side of the trough, extending from the TX panhandle through western KS/NE. Now I may be comletely off-base on this, but it seems as though having the strongest winds on the east side of the trough may tend to lead to more squall lines and fewer supercells, at least in the Plains. FWIW (probably nothing), Saturday doesn't have this configuration of 300mb flow.

That said, I would like to see more moisture available Saturday, and though 53-55 in eastern NM is not terrible, I'd rather see 56-58. Also, if this thing slows down any more, we'll be chasing above the treeline. ;)

[Alex Lamers]

Rich T.

p.s. This Saturday system looks vaguely similar to 6 April 2001, with more cross-boundary flow and (perhaps) and little weaker instability. The outcome of that event was a rapid transition to a squall line, with lots of wind damage and ridiculous storm motions. See
http://www.spc.nssl.noaa.gov/exper/archive/events/010406/index.html

Rich -

That's a pretty good match! How exactly do you go about finding analagous events - do you guys have a good database or is it just a keen memory? I'd just say the only obvious difference at first glance is that the surface low in the 4/6/01 case was much deeper, by ~10 mb.

Otherwise -

The dryline keeps getting pushed further west. If you're going to chase this on Saturday and had originally planned on going to the TX Panhandle you may need to head close to the NM border. I'm still debating on whether or not to head out and a lot depends on whether or not I can ride along with someone.

Still, plenty of shear spilling past the dryline. Despite moderate instability the strong shear favors supercells. Still difficult to tell how quickly it will go linear. The 12z NAM does have a little pocket of 150-200 m2/s2 0-1km SRH in the C TX Panhandle in the mid-late afternoon. Winds are out of the southeast so good veering on wind profiles. I wouldn't rule out a few tornadoes. I think that the volume of hail and to a lesser extent wind reports will probably dictate a mod risk somewhere unless a lot changes between now and Saturday. Don't give up the hope on isolated tornadoes yet though! Shouldn't be a tornado outbreak but a decent severe weather outbreak.

AJL

[Bob Schafer]

AMA recorded 700mb wind speed of 140kts at 00Z 010407?? Is that to be believed? I assume that's RAOB data? I will say I was trying to keep ahead of the line on US60 at about Panhandle TX, exceeding the 70mph limit, and the line was STILL overtaking me. Of course, that's not news to anyone who chased that day.

[Rich Thompson]

Rich -

That's a pretty good match! How exactly do you go about finding analagous events - do you guys have a good database or is it just a keen memory? I'd just say the only obvious difference at first glance is that the surface low in the 4/6/01 case was much deeper, by ~10 mb.

Otherwise -

The dryline keeps getting pushed further west. If you're going to chase this on Saturday and had originally planned on going to the TX Panhandle you may need to head close to the NM border. I'm still debating on whether or not to head out and a lot depends on whether or not I can ride along with someone.

Still, plenty of shear spilling past the dryline. Despite moderate instability the strong shear favors supercells. Still difficult to tell how quickly it will go linear. The 12z NAM does have a little pocket of 150-200 m2/s2 0-1km SRH in the C TX Panhandle in the mid-late afternoon. Winds are out of the southeast so good veering on wind profiles. I wouldn't rule out a few tornadoes. I think that the volume of hail and to a lesser extent wind reports will probably dictate a mod risk somewhere unless a lot changes between now and Saturday. Don't give up the hope on isolated tornadoes yet though! Shouldn't be a tornado outbreak but a decent severe weather outbreak.

AJL

Alex,

That loose analog was from memory (I worked the event back in 2001). I'm not saying the outcome will be the same, but it does offer some insight into the possibilities with the upcoming system.

Interestingly, that was one of the first "dry hole" forecasts I can recall *before* anything was apparent in observations across the conus.

The notion of a squall line Saturday comes from the solid band of precip on the dryline. Of course, the model resolution is so course (compared to the storms) that it could mean any number of things from lots of supercells to a line, to anything in between. We (SPC) have some post-processed output from the NAM that shows the potential for mixed convective modes (both line segments and discrete cells) along the boundary Saturday evening. The 4 km WRF runs will be amusing to see by late Friday night.

Rich T.

[cdcollura]

Good day,

We also have to bear in mind something that tends to happen if low-level backed winds, developing low-pressure, moisture convergence AND a negatively tilted trough being not "quite" in the right places above and below each other.

I have this explained in GREAT DETAIL in the interesting post in WEATHER AND CHASING below ... Please feel free to look at it.

http://www.stormtrack.org/forum/showthread.php?p=132377

The point about this is that if there is too much of a negative tilting of the trough as it is moving east of the Rockies, the developing surface low will have to be right beneath a small area where a difluent WSW wind is aloft, assuming surface winds back to SE. If they are both SE aloft AND at the surface, that's about the same as SW at all heights (unidirectional, speed-only shear).

If this pattern happens, then a squall line will be likely.

[Craig Maire II]

I'll be taking off tomorrow morning or early tomorrow afternoon and heading towards western Kansas (somewhere between Dodge City and Goodland). I believe that sitting along the Kansas/Colorado border will serve me best and am definitely confident that supercells and a few tornados will develop Saturday afternoon. By camping out in western Kansas tomorrow night I won't have to rush to the target area as I had to do on 03/28/07 (won't be tired and insane LOL:p ). Both the Goodland and Dodge City NWS offices are talking about tornados in they're hazardous weather outlooks and SPC has a 30% outlook for that area. I do believe that a moderate risk will be issued either tomorrow or on Saturday and that Kansas will bring me some beautiful storms and maybe even a tornado or 2.:D There are far to many atmospheric variables and this system is still a few days out for anyone to say if this will or won't produce a decent number of tornados, supercells etc., one thing I have learned from my 9 years as a storm chaser is that nature is FULL of surprises!

[Joey Ketcham]

To chase or not to chase..

My main worry is the storms forming into a squall line, it's not too bad with embedded supercells.. but depends on how fast they're expecting the storms to move. Don't want to spend all day playing catch up.

[Stan Rose]

Sig tor parameter (which incidentally was an excellent predictor on Mar 28) based off the 12z nam puts a bullseye in wrn TX panhandle, coincident with the highest helicities, and where the model seems to like discrete cells coming off the NM terrain. Id rather not drive that far, and there are plenty of other factors to consider, so w'ell see (by 21z sat)

[Brian Stertz]

Yeah I was seeing that too Matt...the higher tornado/sig severe parameters from 12z ETA are focused between Lubbock and Clovis. Looks like the higher lapse rates are favored across EC/SE New Mexico into the SW Texas Panhandle and NW TX South Plains area. Very strong 0-6km shear and probably just enough instability to make things fun before dark and the squall line scenario plays out.

[Jeff Snyder]

AMA recorded 700mb wind speed of 140kts at 00Z 010407?? Is that to be believed? I assume that's RAOB data? I will say I was trying to keep ahead of the line on US60 at about Panhandle TX, exceeding the 70mph limit, and the line was STILL overtaking me. Of course, that's not news to anyone who chased that day.

No, that isn't to be believed. The 700mb level is the last mandatory level at which winds were reported, and, the unrealistic speed, I'd disregard that wind observation as well.

Another distinction from 4-6-01 is the strength of mid- and upper-level flow -- At 500mb: 60-85kts on 4-6-01 across the warm sector ahead of the dryline, as opposed to 40-55kts forecast Saturday evening. Well, perhaps it's not so much that the 500mb flow is that much weaker than the analog, but it's that the jet streak ejects later in the day than was seen earlier. The 12z NAM storm motions are <40kts from storms east of the NM/TX border, so I wouldn't be too cocnerned about storm motions with this system. Of course, as the jet streak lifts out of the base of the upper-level trough/low, storm speeds should increase with time.

Not that Rich brought up the 4-6-01 case to be an identical setup... It is an analog, but I wanted to point out one difference that should result in more chaser-friendly storm motion.

IMO, the upper-level trough passage is about 12 hours too late. If we could take the 12z forecast, and force it to verify at 00z (12hr later or 12hrs earlier), I think we'd be much better in terms of significant supercells in the Plains. As it is now, I expect a potential MDT risk, mainly for the risk of hail and wind damage. One mitigating factor for wind damage is that the strongest mid-level flow doesn't really work into the warm sector until 6z and thereafter, when the low-levels should be considerably more stable given nocturnal cooling. With more stable air near the surface, it's more difficult for damage winds to make it to the surface.

With the fact that it looks like initiation will occur in eastern NM through far southeasern CO, I'm more likely than not going to sit this one out. The next system coming through looks as though it'll have much better moisture to work with, so I'm not down-and-out yet. Of course, this all depends upon the NAM verifying, which is certainly not something in which I'm putting a whole lot of confidence.

EDIT: Stupid me -- I forgot to make a very important "NOTE": The 12z GFS solution is faster than the 12z NAM solution, with the dryline located in the central OK panhandle and not too far west of AMA by 0z. In addition (and not unrelated), the upper-level trough is a little farther east on the GFS compared to the NAM by 0z (though there's some convective feedback/modification in the fields in northern TX panhandle by 0z).

[Gabe Garfield]

Another distinction from 4-7-01 is the strength of mid- and upper-level flow --