WINTER STORM FCST 11/30/07-12/02/07: IA, NE, SD, MN, MI, WI, IL [Archive]

Jeff Miller

11-27-2007, 01:37 PM

I'm not certain of the procedure in this new forum for winter storm forecasting. The rules seem to be much more lax since this is not a part of Target Area, but many are used to the format so I will follow that in here for now unless more direction is provided.

The GFS 12Z run and latest Ensembles are both projecting at the least a significant winter weather event for a good part of the Northern Plains and upper midwest this weekend. The model path is subject to great change, and the next 48 hours will be critical to exact path and strength.

Ensembles are painting a greater than 90% Categorical POP for snow - and GFS is now painting on the 12Z run a 993 low over central Kansas.

Ensembles:
http://www.emc.ncep.noaa.gov/gmb/ens/fcsts/ensframe.html

This low is then forecast to rapidly deepen over Central lower Michigan to an impressive 986 low - with a 125kt Jet Stream fueling the storm. Ensembles are also in great agreement with a large storm developing this weekend as well. This storm could have all varieties of severe winter weather with snow, wind, and freezing rain looking quite likely.

We have 3 to 4 days until cyclogenesis - but it's looking better on this run so the official winter storm forecast begins.

http://rap.ucar.edu/weather/model/gfs132hr_sfc_mslp.gif
http://rap.ucar.edu/weather/model/gfs108hr_sfc_prcp.gif
http://rap.ucar.edu/weather/model/gfs108hr_250_wnd.gif

Here is the 108 Precip total snowfall accumulation potential as of latest run:

http://i29.photobucket.com/albums/c278/cloudtoground2004/WINTER_GFS0P5_SFC_ACCUM-SNOW_108HR.gif

Mike Hollingshead

11-27-2007, 02:42 PM

http://rap.ucar.edu/weather/model/gfs102hr_sfc_prcp.gif

http://www.rap.ucar.edu/weather/model/gfs102hr_sfc_ptyp.gif

Could be a fun event if it can slow down some and move slower. I had only looked at RAP's 12 hr interval prog's of 6 hr precip and wasn't thinking much of it till I saw your snow total map. I was like, what the hell. Guess the system likes the 6 hr off intervals...as seen above most comes in that time frame. The rest in the other one.

http://rap.ucar.edu/weather/model/gfs114hr_sfc_prcp.gif

http://www.rap.ucar.edu/weather/model/gfs114hr_sfc_ptyp.gif

Looks like it wants some of that freezing rain to change to rain Saturday evening. It seems often the cold air is thicker than prog'd and it's less likely to change to rain in the central US. But maybe I'm thinking of more inverted trough situations and this more e-w setup may be more likely to warm some as shown.

Too bad it's so many days out, as I like it just how it is for us right now. Looks like later in the period there may be quite the LES event, while at the same time the west coast gets hammered.

Will be fun to see if a severe icing event unfolds around here this weekend(looks like gfs is showing around an inch of that stuff near here).

Edit: Nevermind. I kept thinking red was freezing rain, but it is sleet. An inch of sleet would be the worst case scenario! BORING.

rdale

11-27-2007, 02:43 PM

I'm much more comfortable with the GFS today than yesterday when it did that funky low-pressure split over eastern TN. That would have been a big ice event, but just looked unrealistic.

ECMWF certainly MUCH colder and stormy for the Great Lakes. GEM has similar low track but warmer.

Dan Robinson

11-27-2007, 02:47 PM

The GFS is painting quite the arctic air intrusion - subzero temps in Wisconsin by Tuesday morning. Ptypes should easily be all snow with this system by the end of the weekend. Wind fields look great for LES and upslope in PA-WV-OH-NY through Wednesday.

rdale

11-27-2007, 03:02 PM

The last thing I'd ever believe on GFS is a cold snap at long-ranges... ECMWF offers support, but this is the first run it's done that so I'm still not too excited.

Dan Robinson

11-27-2007, 03:09 PM

Very true. I don't expect the full brunt of what it's showing, but with such a strong cold signal I have more confidence in at least getting everything frozen by Monday night.

Jeff Miller

11-27-2007, 05:22 PM

I want to mention some reasoning from DDC. It appears that precipitation type could be a serious concern with this clear down toward the OK border in Kansas with the prospects of freezing precipitation or a downright chaotic wintery mix.

THE COMBINATION OF INCREASED
MOISTURE (http://www.crh.noaa.gov/glossary.php?word=MOISTURE), COLD SURFACE TEMPERATURES AND WARMER TEMPERATURES ALOFT
COULD CREATE AN ICY SCENARIO ACROSS PORTIONS OF SOUTHWEST KANSAS
THIS WEEKEND. ISENTROPIC LIFT (http://www.crh.noaa.gov/glossary.php?word=ISENTROPIC%20LIFT) IS IMPRESSIVE WITH 5-7 G/KG (http://www.crh.noaa.gov/glossary.php?word=G/KG) SPECIFIC
HUMIDITY (http://www.crh.noaa.gov/glossary.php?word=HUMIDITY) VALUES ACROSS THE BULK OF THE FA (http://www.crh.noaa.gov/glossary.php?word=FA) SATURDAY. 06Z GFS (http://www.crh.noaa.gov/glossary.php?word=GFS)
CONTINUES TO POINT TO THE WARM FRONT (http://www.crh.noaa.gov/glossary.php?word=FRONT) LYING ACROSS THE KS/OK BORDER
WHILE THE 12Z GFS (http://www.crh.noaa.gov/glossary.php?word=GFS) IS NOT TAKING THE WARM FRONT (http://www.crh.noaa.gov/glossary.php?word=FRONT) WELL INTO NE. THE 00Z
ECMWF (http://www.crh.noaa.gov/glossary.php?word=ECMWF) LIFTS THE WARM FRONT (http://www.crh.noaa.gov/glossary.php?word=FRONT) FURTHER NORTH INTO NE. THIS WILL BE AN
INTERESTING STORM TO KEEP AN EYE ON OVER THE NEXT COUPLE OF DAY.
PRECIP ACROSS SOUTHWEST KANSAS APPEARS LIKE A GOOD BET BUT PRECIP
TYPE IS A HUGE QUESTION AT THIS POINT. ACROSS THE NORTHERN FA (http://www.crh.noaa.gov/glossary.php?word=FA),
TEMPERATURES SHOULD BE COLD ENOUGH TO PROVIDE FREEZING RAIN OR SNOW
FOR MUCH OF THE EVENT WHILE ACROSS THE SOUTHERN FA (http://www.crh.noaa.gov/glossary.php?word=FA), NEAR THE OK
BORDER, RAIN OR FREEZING RAIN LOOKS LIKE THE DOMINATE PRECIP TYPE.
AREAS IN THE MIDDLE FA (http://www.crh.noaa.gov/glossary.php?word=FA) COULD BE A MIXED BAG OF RAIN, FREEZING RAIN,
SLEET AND/OR SNOW. FOR NOW, WILL BUMP UP POPS (http://www.crh.noaa.gov/glossary.php?word=POPS) SATURDAY AND CHANGE
PRECIP TYPE TO ACCOUNT FOR COLDER AIR NORTH AND WARMER AIR SOUTH.Des Moines reasoning in this early point is clarifying the confusion as central Iowa as well could see at least a period of freezing rain and sleet - but is not discounting that it could change to snow earlier should the cold air swing around earlier than anticipated.

Kansas City also brings across a very important point:

ALSO AM NOT CONVINCED OF THE NICE AND CLEAN PHASING THE MODELS
SUGGEST LATE THIS WEEK. FIRST CONCERN IS IF AND WHEN THE UPPER LOW
OFF THE BAJA COAST EJECTS NORTHEAST. THIS FEATURE WILL LIKELY (http://www.crh.noaa.gov/glossary.php?word=LIKELY) HAVE A
SIGNIFICANT IMPACT ON THE TIMING AND TRACK OF THE SFC (http://www.crh.noaa.gov/glossary.php?word=SFC) LOW THIS
WEEKEND. WITH THE LATEST MODEL GUIDANCE SUGGESTING A MUCH WARMER
SCENARIO FOR THIS WEEKEND...WILL BEGIN TO HEDGE TOWARD THAT DIRECTION AND AWAIT FUTURE GUIDANCE.

And as a note of humor, don't you love it when NWS uses the term "Interesting storm"? That always seems to get my attention!

Brandon Sullivan

11-27-2007, 05:34 PM

http://img407.imageshack.us/img407/9876/conusgfs0p5sfc12hraccumpl5.gif

Looks like at this time, big accums may stay north of me. im trying to watch the thermal profiles to see where the frz line is gna be... Dec 1st last year we had 12 inches overnight preceeded by .5 inches of ice.. I think it would be weird but cool to have 2 "December 1sts"

And as a note of humor, don't you love it when NWS uses the term "Interesting storm"? That always seems to get my attention!

I also love this wording too.. From DVN CONCERN IS COLD AIR WILL BE IN PLACE OF ARCTIC
ORIGIN FOR ICE AS STRONG WAA KICKS IN. INSPECTION OF ANOMALIES SHOWS 2 UPWARDS
TO NEAR 4 SIGMA.../90 TO 99 PERCENT STRENGTH/ OVER FORECAST REGION
WHICH SUPPORTS STRONG TO NEAR RECORD STRENGTH SYSTEM.

Jeff Miller

11-27-2007, 05:52 PM

Question for those knowledgable with relation to this system:

Is this SIGMA referring to SIGMA Theta or SIGMA the System of Icing Geographic identification in Meteorology? And with that said, how does this point to a record storm.

Davenport states:

VOLATILE AND VERY DYNAMIC...A CHANGEABLE PATTERN CAUSING SIGNIFICANT
MODEL ISSUES THROUGH PERIOD. THIS CULMINATES WITH A STRONG TO VERY
STRONG...POTENTIALLY NEAR RECORD WINTER STORM THIS WEEKEND INTO EARLY
NEXT WEEK...

ECMWF (http://www.crh.noaa.gov/glossary.php?word=ECMWF) IS STRONGEST WITH INTENSE
SYSTEM...GFS (http://www.crh.noaa.gov/glossary.php?word=GFS)...VERY STRONG AND UKMET (http://www.crh.noaa.gov/glossary.php?word=UKMET) JUST STRONG. UPDATED HWO FOR
CERTAINTY OF SIGNIFICANT WINTER WEATHER WITH GREATEST RISK NW OF A
SQI TO OTM LINE. CONCERN IS COLD AIR WILL BE IN PLACE OF ARCTIC
ORIGIN FOR ICE AS STRONG WAA (http://www.crh.noaa.gov/glossary.php?word=WAA) KICKS IN. THIS RISK SHOULD BE CLARIFIED
NEXT 24-36 HOURS. HAVE UPPED POPS (http://www.crh.noaa.gov/glossary.php?word=POPS) TO LIKELY (http://www.crh.noaa.gov/glossary.php?word=LIKELY) TO HIGH CHANCE WITH SNOW
NORTH OF HIGHWAY 30 AND A MIX SOUTH.

For being this early in the system I find it eyebrow-raising for Davenport to use the wording
"Potentially Near Record Winter Storm".

For a city in the Midwest, that's certainly saying a mouthful and a big gamble.

Joel Wright

11-27-2007, 06:18 PM

Question for those knowledgable with relation to this system:

Is this SIGMA referring to SIGMA Theta or SIGMA the [size=-1]System of Icing Geographic identification in Meteorology? And with that said, how does this point to a record storm.

Davenport states:

For being this early in the system I find it eyebrow-raising for Davenport to use the wording
"Potentially Near Record Winter Storm".

For a city in the Midwest, that's certainly saying a mouthful and a big gamble.

There's a certain forecaster (I won't mention any names) at DVN that notoriously seems to say that a few times each winter. I think he seems to let his excitement for a potential storm system cloud his better judgement sometimes. But, on the other hand, I always like reading his forecasts. :)

Definitely looks like a very large storm system, with plenty of cold air to work with. Somebody may end up getting a nice snow and or ice this weekend. Maybe one of you guys reading this right now lol. ;-)

Danny Neal

11-27-2007, 07:17 PM

Skilling here in Chicago seems to be hyping up the storm. He seems to think well get a 1-2-3 punch of snow storm, lake effect snow, then bitter cold. All the models that are provided seem to at least hint at that. GFS if it stays on its current path the next run would in all likelihood inundate Northern IL and Southern WI with 6-10 inches of snow, we are talking over 120 hours here so the certainty is definitely in question. From a personal standpoint I hope this storm is record breaking, nothing I can't stand more than 3 inches of snow then 5 degree temps to follow :(

Jeff Miller

11-27-2007, 11:24 PM

00z coming in...

Storm developing stronger and with higher QPF in Nebraska and South Dakota...

http://rap.ucar.edu/weather/model/gfs084hr_sfc_prcp.gif

Also, minimum pressure 3mb lower than the previous run so we have a wetter and stronger solution with the 00z run with what we have right now up to the 108. The overall path looks to be on basic target, but a little faster. How this will play with the overall snowfall/frozen potential has yet to be seen.

A change however is occuring with this run at 108 with the low opening up and weakening once it gets to the Chicago area - same basic path but an obviously weaker solution.

As the model gets back to Michigan - again, no change in path this run, SE Lower Michigan, the low deepens again, and a moderate rain changes to snow and wind over Chicagoland by the 120.

by 132 the low bombs near Toronto with a minimum pressure of 983 and a great deal of light to moderate snow and wind over practically all the Great Lakes!

at 144 we have a tremendous powerhouse of a low with a pressure of 979 and snowshowers and light snow with strong winds continuing over the Great lakes as arctic air rapidly intrudes over the Northern Plains.

So far then with this run, same general path, but some fluctuations in intensity and in timing with higher snowfall accumulations in Nebraska at the onset then first predicted last run at the 12Z.

Here is the latest GFS predicted total snowfall accumulations. This is up to hour 99. Note the increase in total predicted snowfall over the FSD area - some predictions bringing total accumulations to near 16"!

http://i29.photobucket.com/albums/c278/cloudtoground2004/SNOWLATEST.gif

V. Gensini

11-27-2007, 11:36 PM

Question for those knowledgable with relation to this system:

Is this SIGMA referring to SIGMA Theta or SIGMA the [SIZE=-1]System of Icing Geographic identification in Meteorology? And with that said, how does this point to a record storm.

Jeff:

Sigma is referring to the deviation from the norm. For purposes you probably could care less about ;) Statisticians use standard deviation (sigma) to refer to the deviation of a data set about a mean (average). The 4 sigma category would put the system in the 99th percentile (only 1 percent of storms would rank above it).

Anyway, back to the storm :D The model differences we are seeing right now are IMHO purely due to the fact that we are not getting good sampling of either systems at the moment! How are the models supposed to resolve these features 100+ hrs out, if they are interpolating data to begin with? Everyone should sit back and let this storm get on shore before making any proclamations! After all, its still 5 days away!

On a side note, I had the pleasure to sit in at DVN today and I must say that there was some disagreement as to what was put in the HWO & AFD. You must remember that these are written solely by one forecaster with little collaboration. (One forecaster working short term, one working long term). Therefore, things such as this should be taken with a grain of salt. Isn't it much more fun to form your own opinion about the storm anyway?

Jeff Miller

11-27-2007, 11:39 PM

Vince,

completely understand the model disagreements and poor sampling. I am however impressed with the overall consistency of GFS.

As far as what you say - sit back and let the storm get on shore - what fun is that? I personally enjoy keeping up with the models and forecasting even 3-4 days away. That's the fun of it, Vince :) Remember, no one's proclaiming anything at this point, just analyzing the data we have and explaining what we see.

Joel Wright

11-27-2007, 11:40 PM

Boy it's gonna be a challenging forecast for areas just north of where that surface low tracks. It's showing a period of snow, changing to sleet and freezing rain, and even all rain in areas of northern MO, the southern half of IA, and about the northern half of IL. Some pretty strong warm air advection just off the surface most of the day on Saturday over those areas.

Much of NE, southern SD, the northern half of IA, up into parts of MN and WI may get a lot of snow.

It's interesting the way the storm weakens, and then regains strength again out east.

V. Gensini

11-27-2007, 11:49 PM

Mike Hollingshead

11-28-2007, 12:28 AM

http://www.rap.ucar.edu/weather/model/eta84hr_850_wnd.gif

http://www.rap.ucar.edu/weather/model/gfs084hr_850_wnd.gif

Now there's some model agreement! lol Nam is apparently 12+ hours slower.

http://www.rap.ucar.edu/weather/model/eta84hr_500_wnd.gif

http://www.rap.ucar.edu/weather/model/displayMod.php?var=gfs_500_wnd&hours=hr072hr084

Meanwhile, what the hell is that off the west coast? Wish Nebraska got things like that. http://www.rap.ucar.edu/weather/model/gfs120hr_sfc_mslp.gif

Jason McKittrick

11-28-2007, 01:12 AM

I am excited about the potential for the first major winter storm of the season here in Omaha. As I have been watching the models over the last few days I have identified, what I believe, are important characteristics of this weekends setup.

1. There is a very impressive cold airmass in place accross the northern plains. Highs in parts of ND failed to get out of the single digits today and highs here in Omaha were about 10 degrees cooler than the 24 model forecasts indicated.

2. The Gulf of Mexico is very, very closed right now. There are northerly winds as far south as the Bay of Campici with dewpoints in the 50's over the GoM and near 40 in south Texas. A lot of modification has to occur before good moisture return can be realized.

3. Cut offs definitely tend to take there sweet time ejecting and the GFS has a northerly bias.

4. Most importantly as has already been pointed out these systems are over the Pacific and are poorly sampled. The current model forecasts, while they have been quite consistant they could change dramatically once the energy is sampled which probably won't be until Friday afternoon.

I think that we will probably see some decent accumulating snows across parts of NE and IA however at this point it does not look like a 12+ inch event for anyone, to me, I do however see some serouis ice storm potential.

Michael O'Keeffe

11-28-2007, 07:50 AM

Brandon Sullivan

11-28-2007, 08:17 AM

Wow.. I think this system will have a very sharp rain/snow/frz line, and I think that line is going to be very hard to pin down. This does look like a good icing event. I will be interested to see HPC's thoughts tommorow night. Personally, I dont think local gov't/highway dept is ready at all, so this ought to be interesting..

rdale

11-28-2007, 08:18 AM

I think Jason may be right the CPC hazards assessment was issued yesterday and they outlined E Nebraska, most of Iowa, and S Wisconsin/N Illinois under Heavy Snow on Dec 1. While most of Kansas, MO, C IL/IN/OH all the way to W New York could see freezing rain on Dec 1-2.

Remember that the CPC synopsis is just a general outlook, there's very little "real forecasting" going into that product.

Jeff Miller

11-28-2007, 12:22 PM

Latest information...

GFS...

GFS continuing heavy QPF over the Central Plains and essentially the same path but a slightly more northern component. This model takes the rain/snow/mix line farther north, and almost has even KSUX in an all rain pattern as warmer air works in according to the model. It also takes the heavier snows farther north into Minnesota and Wisconsin, leaving Minneapolis with close to a foot of snow - thus also leaving cities such as Davenport and Des Moines in the warm sector with just rainfall. ECMWF also in agreement with general path of storm so we have some continuity here with GFS/ECMWF. UKMET just coming onboard with the storm at the 72. Latest total snowfall potential:

http://i29.photobucket.com/albums/c278/cloudtoground2004/SnowfallLatest2.gif
NAM...

NAM Model is producing really different results. It is coming in slower and with a farther south solution...much farther south on the 84. NAM is hedging towards a freezing rain/sleet event for areas such as Kansas City and Columbia, MO and Topeka, KS. NAM throws a monkeywrench in the consistency of model runs reducing the certainty of the northern scenarios for sure - but I am discounting for now.

For the time being, due to model discrepencies as expected at this timing, I'm not expecting a lot of change in the forecast with this run with GFS a little farther north with a warmer solution and ETA way south with a colder solution north and a significant ice storm KS/MO... perhaps by tonight things will be a little clearer on exact path/strength/precip type.

The big concencus is the storm itself is looking extremely likely, and the questions are as always on path and precip type/strength.

.

Jeff Miller

11-28-2007, 04:13 PM

NEW INFORMATION including WFO Early Evening Forecast Reasonings:

According to Hastings NE WFO AFD, the Hastings office is considering issuing a Winter Storm Watch as early as tonight for the FRI-SAT period for snow, freezing rain and sleet accumulations. I'm awaiting to see what the other WFOs will think about that - in my mind, it's a little premature due to the possibility of stronger warm air intrusion than currently indicated.

Omaha is very specific. Specific for NE/IA area according to Omaha reasoning: (This reasoning is for NEBRASKA)

1. Blend of GFS ensemble and SREF mean for forecasting.
2. Confidence is very high, According to Omaha that surface air will remain subfreezing with lack of sunshine and the depth of the airmass Friday Night through Sunday.
3. Strong isentropic lift/Warm air advection should be strong enough to advect over surface based arctic airmass by, at the latest, Saturday afternoon.
4. Convective Slantwise Instability or just Upright Instability will exist along and south of I-80 Sat afternoon and evening with heavy precipitation likely, 1" of liquid precipitation is possible.
5. Two possible scenarios for the winter Precip:
I. Significant icing event if an elevated warm layer exists. This appears to be a threat along and south of I-80 with signigicant snowfall possible north of I-80.
II. Snow in the north and also to the southern counties if NE if warm elevated layer stops short of Beatrice, NE, however heavy snow concerns are if the highest instability doesn't match the highest area of dendretic growth - and also sleet and non-dendretic snowflakes hampering accumulation potential.
6. Winds will be a problem as well with surface gusts of 20-25 MPH.
7. Final analysis is
MUCH OF THE FORECAST
AREA SHOULD SEE A VERY MESSY MIX OF SNOW/SLEET/FREEZING RAIN
COMBINED WITH GUSTY WINDS... OR VERY HEAVY SNOWS COMBINED WITH GUSTY
WINDS.

Jayson Prentice

11-28-2007, 04:44 PM

Mike Hollingshead

11-28-2007, 04:55 PM

http://weather.cod.edu/forecast/GFS/gfsUS_850_thetae_78.gif

http://weather.cod.edu/forecast/GFS/gfsUS_2_temp_78.gif

Someone's going to get icy, especially with a sort of morning timing going for things too.

http://weather.cod.edu/forecast/GFS/gfsUS_2_temp_60.gif

That air ending the day on Friday, then being rapidly overrun above the sfc front into Saturday morning...fun times. Guessing the sfc warming surging north Saturday is a little overdone. The cold loves to win these battles. Give it a morning go with heavy precip and clouds all day, sure ain't going to do the sfc front any favors in surging north like it wants.

That's really one of the most important factors to a big ice event. Getting the front to stay in one position for a long duration of time. Otherwise each area only gets some of each form. It's when one area gets locked into the freezing rain zone forever things get crazy. That's what happened with last years major ice event in NE and KS. The sfc freezing line did not budge for 2 days, so the same areas kept getting the same precip type. That probably won't happen like that this time, unless the system wants to evolve a bit slower like the nam seems to want. I bet with the cold that will be in place and the system wanting to at least fight it back north, I would guess there'd be a strong tendancy for that wannabe warmfront to stay stationary.

Sort of sucks hoping for a major ice storm when you know it could cause you to be without power for weeks.

Jeff Miller

11-28-2007, 04:57 PM

Jayson,

I agree with you on the snow/frz rain to rain transition in Central Iowa. I also looked at Davenport's reasoning and they are also thinking of a wintery mix to rain scenario with mainly snow in the northern counties of Iowa. Regardless, I think its fairly certain the rain snow line will set up somewhere from central Nebraska through Central Iowa - with a plethora of winter weather along the transition zones. The question will be how long the shallow cold air will hang on to keep rain freezing or just plain rain in those zones.

Mike Hollingshead

11-28-2007, 04:58 PM

The Des Moines NWS office mentioned how they may begin to put out watches on Thursday night if the run to run consistency can stay good for the next 36 hours or so... Des Moines's currently thinking that their CWA will encounter all modes of the storm, mainly snow north, sleet/frz rain and rain across the central/southern zones.

I took a look via BUFKIT today at the GFS runs from last night and this morning, some fluctuation in how much of a warm air intrusion we will get. But, for here in central Iowa it continues to look like this may be a nice event of snow/frz rain and possibly rain to finish it out?

Yeah if one looks at the gfs for exactly what it says, it sure wants to change things to rain. My problem with that is, it seems like a rare thing in these parts. Along the ocean, yeah, but here in the plains it seems things rarely change back to rain. I guess they do from time to time though.

Edit: Looks like the 18z gfs is liking you Jeff. http://www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/18/images/gfs_p60_084m.gif

This 18z snowfall total out 96 hours should be interesting when it updates(whenever the link ends in 18z not 6z like right now) http://wxcaster4.com/gfs/CONUS2_GFS0P5_SFC_ACCUM-SNOW_96HR.gif Guessing it'll show 12+ over Mr Miller's house, lol. (yeah silly prog's I know, but I'm bored)

rdale

11-28-2007, 05:16 PM

Rhyuan Janssen

11-28-2007, 05:32 PM

Jaysen, I seriously hope we don't wind up with rain at the end, that would bum me out incredibly. IMO I would say we'd wind up with snow at the end due to cold air behind it, but one is never certain.

Rdale, are you serious about the 48 hours thing? Because someone needs to let the NWS office in DSM know because they have issued as far out as 72 hours before.

Sioux Falls seems to think warm front might reach Highway 20 or beyond to the Iowa great lakes, which would not make me happy since I live on 20 just about. However they are trending towards NAM which keeps more cold air in place than GFS and according to La Crosse NAM handles shallow air masses better than GFS does.

At any rate I think someone is headed for a big snow and/or ice event in Iowa

Jeff Miller

11-28-2007, 05:40 PM

Rhyuan, Welcome to Stormtrack!

In answer to your question, directed at Rdale, I took the liberty of looking up the NWS Directives. This was issued in 2001, so Im not sure if there are any changes...

6.2 Winter Storm Watches. Winter storm watches shall be issued when
conditions are favorable for hazardous winter weather conditions, as defined
in section 4, to develop over part or all of the forecast area, but the
occurrence is still uncertain. Winter storm watches should be issued for the
second, third, or occasionally fourth forecast periods, when the forecaster’s
confidence is 30 percent or greater of a hazardous winter weather event
meeting or exceeding local warning criteria. Watches shall be updated
whenever there is a change in the timing, areal extent, or expected
conditions.
Watches shall be issued via the WSW product category and subsequently
headlined in all appropriate state, zone, and local forecast products. The (Z)
form of the UGC shall be used as described in WSOM Chapter C-63. The MND shall
be "URGENT - WINTER WEATHER MESSAGE."

Therefore, a Winter Storm watch can only be issued up to the fourth forecast period, or 48 hours - but with only a 30% criteria chance according to this document.

Rdale, feel free to correct if I am in error.

Michael O'Keeffe

11-28-2007, 05:52 PM

I think this event will be further south than the GFS is showing. Closer to the NAM. I mean how can a pseudo warm front move all the way up into Nebraska when we have snow and freezing rain falling in the morning and in addition having cloud cover socking us in. So with that said I think it will be a snow maker for much of Nebraska, Iowa, and SE South Dakota east into Wisconsin while much of Kansas, Missouri, and extreme S Nebraska and S Iowa will see a significant ice storm maybe some sleet as well.

But I'm biased. ;)

Rhyuan Janssen

11-28-2007, 05:59 PM

Thanks for the welcome Jeff.

This was back in 1999 when it snowed every day in December just about so it is possible they changed guidelines after that. :-)

I must agree with Michael, I think that GFS is incorrect in bringing a warm front possibly as far north is just south of the MN/IA border; and as I am opting for the first big snow of the season where I live...I might be biased too! :-)

rdale

11-28-2007, 06:34 PM

Therefore, a Winter Storm watch can only be issued up to the fourth forecast period, or 48 hours - but with only a 30% criteria chance according to this document.

All the directives are http://weather.gov/directives -- updated with 50% chance in 2004. http://www.weather.gov/directives/sym/pd01005013curr.pdf

Joel Wright

11-28-2007, 06:45 PM

The NAM performed horribly with the big system last week until about the 48-60hr time frame. So at this point I'm really taking the NAM with a big grain of salt.

The trend has been to take the storm further north, which may be the correct notion. Look at how strong the WAA is at 850mb. The cold air that will be in place won't be that especially deep, so I would think that the warmer air should make a good push as the storm ejects.

That double barrel low may mess things up though. If the southern low ends up being the dominant one, then that may keep the colder air considerably further south. I wouldn't be surprised if that ends up being the case. I think a lot of that depends on the timing of the shortwaves that have yet to even be sampled all that well...

Bring on the 00z!:-)

Bill Doms

11-28-2007, 06:57 PM

FWIW the WFO up here is leaning towards the GFS solution on the latest AFD. I'm still miffed with the moisture looking at the H85 RH saturating out Friday night all the way to the Canadian border. A little strong on the WAA?

Jeff Miller

11-28-2007, 07:10 PM

Yeah, a little bit. I noticed the discussion concerning overdoing it on the WAA and the snowfall amounts with the GFS - so I'm not sure if that can all be believed at that point and I believe that Minneapolis is using their heads cutting down on some of the accums up that way, especially as the wave seems to open up a bit just to the south of Minnesota before deepening out there in Michigan.

Mike Umscheid

11-28-2007, 07:47 PM

This could be a big freezing rain and sleet event for a good chunk of the Plains and Midwest...with the northern periphery of the pcpn axis being snow. Be careful looking at the 850mb 0C line, the warm layer aloft is typically centered around 750-800mb, so a mix precipitation type might be farther north than what you might expect looking at 850mb Temp alone. I'm interested in this storm for Kansas given the freezing rain vs. rain forecast problem.. I'm working mids right now, so I get to have fun forecasting this one for SW KS :)

Michael O'Keeffe

11-28-2007, 09:44 PM

The NWS in Norman has issued a special weather statement concerning the winter storm. They are siding with the NAM I think mentioning how they think the front may not even move north into Kansas! So parts of N Oklahoma may get in on the action too.

Mike Hollingshead

11-28-2007, 10:10 PM

http://www.rap.ucar.edu/weather/model/eta72hr_sfc_prcp.gif

http://www.rap.ucar.edu/weather/model/eta72hr_sfc_ptyp.gif

NAM looks like it likes the general area the GFS did to me. That looks nasty.

Joel Wright

11-28-2007, 10:20 PM

Wow. The NAM sure has changed it's ways lol.

It's all going to come down to how stubborn the cold air is at the surface. That warm air advection wing sure is loaded with substantial precip.

Joel Wright

11-28-2007, 11:26 PM

Well, it's looking more and more if there's going to be any decent snow accumulations, you're going to have to be in South Dakota or parts of Minnesota and Wisconsin. The GFS really fills the low as it leaves the high plains Saturday. Really disappointing. Always hate to see a storm in a weakening trend as it leaves the rockies.

Storm looks fairly uninteresting now. Mainly just from my local perspective. Someone up in South Dakota and Minnesota/Wisconsin may be shoveling some respectable snow by Saturday night however.

As far as the icestorm aspect, maybe somewhere out in Nebraska or parts of Kansas, where the zone of icing sort of pivots as the low goes by would be a better spot for that. Other areas further east will likely get just a quick shot of ice before it changes over to just regular rain...

Dustin Wilcox

11-28-2007, 11:29 PM

Looks real nasty for areas along I-80 in NE, not sure if SC NE really deserves another ice storm.

This would just be straight up nasty across E CNTRL and SE NE SW IA !!
http://grib2.com/animate/nam218nojava.php3?fcsthour=69&type=1000-500_SLPTHKPRP&region=C-PLAINS

SRFC TEMP
http://grib2.com/animate/nam218nojava.php3?fcsthour=69&type=SFC_SLPTMPDPT&region=C-PLAINS

850
http://grib2.com/animate/nam218nojava.php3?fcsthour=69&type=850_GPHTMPRH&region=C-PLAINS

700
http://grib2.com/animate/nam218nojava.php3?fcsthour=69&type=700_GPHTMPRH&region=C-PLAINS

Brandon Sullivan

11-29-2007, 12:07 AM

While computer forecast models continue to differ with the timing and strength of the system, it appears that significant amounts of snow could fall in Northern IA, MN and back into SD and then move into Northern IL and WI. 0z GFS takes the center of the low a little further west/north than previous runs, and brings in strong Warm Air Advection, transporting warm surface temps into the low, across IL and MO. As it moves northeast, the system weakens as it moves over IL/WI. At this time I don't see a major wrap around Deformation Zone type snow accumulation setup..

It appears that upwards of 10 inches could fall in IA/MN/SD, where thermal profiles would support snow to be the dominant precip type.

Of the greatest uncertainty at this time is the location of the freezing line and the associated ice accumulations. The GFS solution at would indicate all snow over MN/SD/Northern IA with freezing rain/snow in IL/Southern IA. The WRF is not as impressive with the amount of warm air being brought up into the system however. The GFS would indicate a complicated forecast in the fact that numerous precip type/intensities could occur in areas such as western/central IL and southern/eastern IA, where thermal profiles hover close to the transition line. Personally I feel as though the GFS temps are too warm, and are overdone.

HPC has introduced an area in the .25 inch minimum ice accumulations, spanning from Northeast Kansas, through Central Illinois.

As the system moves onto shore, and better data sampling can be obtained, hopefully the models will have a better handle on it by this time tommorow.

Jeff Miller

11-29-2007, 08:25 AM

The following forecast is not an official product. Always refer to your local National Weather Service for official Weather information for your area. Use at your own discretion.

This Morning's Highlights for early onset situations in NE/IA/SD/MN:

1. Consensus is very high that there will be a major storm system this weekend over the plains. The details are the tricky aspect of this storm due to many, many factors which include exact storm track, moisture, surface and mid-upper air temperature profiles, convective instability, and exact precipitation type which makes forecasting this winter storm at the least extremely challenging.

2. Due to the extreme variability and uncertainties of exact precipitation path and model discrepencies of exact path, the National Weather Service forecast offices have for this fourth period before the storm held off on the issuance of storm watch products for this event.

Discussion:

Model discrepencies are as follows:

1.NAM Keeps Surface low down in the southern Plains

FSD Forecast area discards this solution.

2. GEM Runs Surface Low up to near Sioux City.

3. GFS Brings Surface Low Farther South then GEM but not as southerly as NAM.

OAX has thrown out the extremely warm solution with GFS and believes the strength of the warm air will not pan out at all.

RESULT: FSD siding with GFS/EMCWF solution bringing undercutting colder air under warm air aloft resulting in tremendous precip issues.

Precipitation issues:

Mixed precipitation will be a high concern over Central/Southern NE and much of Iowa with this winter storm. Model profiles also indicate sleet could be a pretty big concern also especially in Central NE and Central IA should the path take the GFS solution. Thunder will be an issue with precipitation in Central Iowa with CSI/UI. Snow is likely to be the predominate precipitation type per DMX reasoning north of Highway 30 in Iowa, with mixed precip central and south. Warning criteria ice and snow likely over IA, NE, with mainly warning criteria snow likely over SD/MN.

Potential totals:

up to 1" of liquid precipitation equivelent in the mixed zones to liquid precip zones of central/southern IA expected, as well as SE/SC Nebraska. Thunder is possible with PL/FZ precipitation as well.

Heaviest snowfall should occur just north and northwest of the main axis of mixes precipitation and 4-8" of total snowfall is likely in these areas with isolated 10" amounts possible in banding/bursts and exact location for this remains highly uncertain and will be based on a very volatile thermal scenario. A slight chance of thundersnow.

Winds:

Winds should be no greater than 25-30 MPH and gusty throughout the duration.

Overall Summary:

Storm certainty high, precipitation amounts may approach 1" liquid equivalent, however exact precipitation types and amounts uncertain, and thundersnow is possible as well.

Watches/Warnings:

Watches and Warnings will likely be issued if needed in the third period between 1:30 PM and 4:30 PM this afternoon CST.

Andrew Revering

11-29-2007, 09:48 AM

I haven't read every post here, but I thought I'd throw in my 0.02 regarding the model output and the graphics people are using.

The graphics from Earl Barker's site concern me actually. I think much of the heaviest snow areas on his maps are actually in greatly liquid form, or at best mixed precipitation. I have a set of winter weather maps that I generate for my F5 software product and I'm comparing it to the Earl Barker output using Kuchera's liquid to snow ratio method.

First off I'll say that my liquid to snow ratio is slightly different using a combination of methods but generally the ratios are ballpark. The big difference I think is in where you place your rain/snow or snow/mix lines. For me, I interpret the 06z NAM to start having accumulations at least 150 miles north of the Earl Barker image valid the same time using the same model run.

In comparison, Barker's image has snow in northern Iowa, where I think you will have to be at least past the 1st tier counties in MN before accumulation begins (using this data--not that this is actually going to happen, but more about the interpretation of the data presented to us and reading it religiously).

The end results by Barker's image is 7-8" in the Twin Cities, when I look at the same data and end up with 3-6" in the Twin Cities.

So what's my point? My point is I'm not sure that a rain/snow line is even defined in Barker's image. It may just be a ratio applied to all QPF, or all QPF where the surface is above freezing. I don't know.

In any case, pay close attention to where you think that snow cutoff line is going to lay... because that's the key.

Dustin Wilcox

11-29-2007, 11:29 AM

In any case, pay close attention to where you think that snow cutoff line is going to lay... because that's the key.

Key and almost next to impossible to pin down this far out, and might be hard to pin down all the way right up to crunch time. The 12Z NAM is much more agreeing with previous GFS runs and brings the Warm Air Advection much further North then previous runs. The temperature Gradient is sharp. http://grib2.com/animate/nam218nojava.php3?fcsthour=60&type=SFC_SLPTMPDPT&region=C-PLAINS This mornings NAM favors the Sioux city NE East Into IA for a sig Ice Event

Mike Hollingshead

11-29-2007, 02:38 PM

Anyone looked at just how cold the northern plains are right now?

http://www.rap.ucar.edu/weather/surface/sfc_pir.gif

Low single digits early afternoon in northern ND! That is heading this way(yeah modifying, but jeesh). I'm having a pretty hard time imagining that kind of air sinking southeast, them being quickly slammed back northeast on Saturday with 60s prog'd in se NE! Someone's getting nailed on this one I'm affraid. Very strong warm air advection off the ground returning north above what looks like a very very cold air mass. Sure it will modify some and want to warm from the top down a bit.....but. Should prove very interesting to see what that boundary is like Saturday. RUC is already indicating a fair amount colder than the GFS by 6z Friday.

Chad Cowan

11-29-2007, 04:10 PM

LOT has just issued a Winter Storm Watch:
.DISCUSSION...
256 PM CST

MAIN CONCERN THIS AFTERNOON IS WINTER STORM EXPECTED ACROSS THE
REGION SATURDAY AFTERNOON AND SATURDAY EVENING. HAVE DECIDED TO
ISSUE A WINTER STORM WATCH WITH THIS FORECAST...MAINLY DUE TO THE
THREAT OF SEVERAL HOURS OF FREEZING RAIN.

...AS THE WARM AIR LIFTS NORTH OVER THE COLDER AIR AT THE SURFACE...
SOUNDINGS SHOW THE POTENTIAL FOR A 3-5 HR PERIOD...PERHAPS LONGER
OF FREEZING RAIN. BEST TIMING ON THIS PERIOD WOULD BE FROM ROUGHLY
20Z SATURDAY TO 03Z SUNDAY. QPF AMOUNTS RANGE WIDELY FROM A TENTH
OR SO ACROSS THE SOUTH TO A HALF INCH OR MORE TOWARD THE IL/WI
BORDER. WHETHER ALL THIS FALLS AS FREEZING RAIN OR AS A MIXED BAG
IS TOO DIFFICULT TO PIN DOWN THIS FAR OUT. BUT THE LESS MIXED
PRECIP...THEN THE MORE SNOW OR AT LEAST POTENTIAL FOR SNOW...AGAIN
WITH THE MOST QPF EXPECTED TOWARD THE WI BORDER. AS WITH ANY
WINTER STORM SITUATION...VARIATIONS IN THE TRACK OF THE STORM AND
EXTENT OF COLD AIR AT THE SURFACE CAN HAVE LARGE IMPACTS ON THE
FORECAST. BUT GIVEN THE THREAT OF A PROLONGED PERIOD OF FREEZING
RAIN...A WINTER STORM WATCH IS REASONABLE.

Henry Margusity at Accuweather is dead set on the system taking a more southerly track than has been shown by recent model runs. His reasoning is that model initialization is poor with not being able to sample the UA where the main storm is (currently off Baja). This, he says, is resulting in initialization of the system being stronger than it actually is which, when it comes ashore and is absorbed by the trough, a weaker storm won't buckle the jet as much as progged resulting in a more southerly track.

Jason Bolt

11-29-2007, 04:17 PM

DDC also seems confident on a more southerly track as a Watch has been issued for SW KS... Funny to me DDC has been predicting storm to hit their CWA from 5 days out, LOT has been predicting it to hit their area, and if you read the posts from forecasters on here, everyone seems to be wishcasting too, predicting it to be in their backyard. I say SD/NE can have it!

Dick McGowan

11-29-2007, 04:56 PM

EAX just issued a winter storm watch for tomorrow night through Saturday afternoon for just a couple of counties north of here. They think Highway 36 (in Northern Missouri) and further north are going to get the ice/sleet accumulation the worst. I'm hoping it's further south, I have a class on Saturday and don't feel like going!

SIGNIFICANT ICE AND SLEET ACCUMULATIONS APPROACHING ONE QUARTER OF AN
INCH WILL BE POSSIBLE ACROSS THE WATCH AREA BY LATE SATURDAY MORNING. AS
WARM AIR SURGES NORTHWARD...PRECIPITATION WILL GRADUALLY TRANSITION TO
RAIN BY THE EARLY AFTERNOON HOURS ENDING THE THREAT OF WINTRY WEATHER.
HOWEVER...WITH A RAPID PRECIPITATION RATE EXPECTED ACROSS NORTHERN
MISSOURI DURING THE HEIGHT OF THE EVENT...SIGNIFICANT ICING COULD OCCUR
IN A SHORT DURATION OF TIME.

And from their AFD from this morning:

WHILE THE NAM (http://www.crh.noaa.gov/glossary.php?word=NAM)-WRF IS THE OUTLIER
VERSUS THE GLOBAL WAVE MODELS...FEEL HOLDING THE COLD AIR FURTHER
WEST LONGER IS IN PART DUE TO BETTER BOUNDARY LAYER (http://www.crh.noaa.gov/glossary.php?word=BOUNDARY%20LAYER) RESOLUTION...AND
PAST EXPERIENCE SUGGESTS NAM (http://www.crh.noaa.gov/glossary.php?word=NAM) HANDLES THESE TYPE OF SHALLOW COLD
AIRMASSES BETTER THAN THE GFS (http://www.crh.noaa.gov/glossary.php?word=GFS)

Maggie Kahman

11-29-2007, 05:29 PM

This was just issued at 3:22PM CST
From WeatherBug

The National Weather Service In Hastings Has Issued A Winter Storm Watch... Which Is In Effect From Friday Evening Through Saturday Evening.

A Widespread Wintery Mixture Of Precipitation In The Form Of Freezing Rain... Sleet Or Snow Is Expected To Develop Friday Night. The Mixture Of Precipitation Will Continue Into Saturday Morning... Then Warm Air Is Expected To Be Drawn North Into South Central Nebraska. The Precipitation May Transition To Rain From South To North... Or Potentiallly Remain As Freezing Rain If The Warm Air Does Not Arrive. Ice Accumulation Is Possible From The Freezing Precipitation Friday Night Into Saturday.

A Winter Storm Watch Means There Is A Potential For Significant Snow... Sleet... Or Ice Accumulations That May Impact Travel. Continue To Monitor The Latest Forecasts.

Mike Hollingshead

11-29-2007, 05:30 PM

Patrick Marsh

11-29-2007, 06:07 PM

First of all, don't use the actual air temperature use the wet-bulb temperature. The air will cool toward the wet-bulb temperature as precip falls into any dry layer aloft and evaporates.

If the Warm Layer Wet-bulb temperature is greater than 3C then you will most likely get complete melting of the snowflake and thus freezing rain is likely. If the maximum warm-layer wet-bulb temperature is between 1-3C then you will only get partial snowflake melting and thus sleet is likely.

This only holds when you are in a non-convective environment.

You can also use the tau method to further refine sleet-freezing rain environments by comparing the mean temperature of the warm layer to the depth of the warm layer.

Edit: The following is taken from a PDF from the HPC on winter weather forecasting. http://www.forwarn.org/temp/tau.gif
(http://www.forwarn.org/temp/tau.gif)

Craig Maire II

11-29-2007, 07:29 PM

My forecast calls for blustery conditions in my area on Saturday and Saturday night with winds of 20 to 35 miles per hour along with rain, freezing rain, sleet and snow plus "chilly" temps. Precip chances stand at 100% for Saturday and 100% for Saturday night, precip accumulations may vary but we will get atleast a trace of something...:D lol With this forecast can I possibly be wrong? :p The Craig Prediction Center has now issued a Messed Up Weekend Weather Watch for the entire state of Iowa along with portions of Nebraska, Minnesota, and Wisconsin. A Messed Up Weekend Weather Watch is issued when the threat of nasty, potentially dangerous winter weather may screw up my plans for this or any other upcomming weekend, please stay tuned to this thread for updates and possible warnings regarding this potentially cra$py weather situation!

Mike Umscheid

11-29-2007, 08:57 PM

Is there any sort of maximum near ground freezing height to get freezing rain rather than sleet? Like is there a set 0c height where you become more likely to get sleet than freezing rain?

I guess that would depend on how thick any above freezing level may be too, as far as rain drop velocity...or maybe not. I should head on over to haby hints.

Well there's two ways to get sleet. The more "traditional" way is when maximum warm layer aloft T (or Tw) is greater than about +0.5°C and less than about +3°C. When snow falls into this kind of a thermal profile, it will only partially melt. it takes less thermodynamic energy to re-freeze a partially melted snowflake as sleet than to re-freeze a completely liquid rain drop. The other more "non-traditional" way is to re-freeze a completely liquid rain drop (after it has fallen through a ~ +3°C or greater maximum warm layer aloft) in the shallow cold layer, if it is cold enough. There are some studies that have tried to correlate these kind of soundings to freezing rain vs. sleet, and from a purely simplified "minimum cold layer temperature" approach, if the minimum near-surface cold layer temperature approaches -6 to -6.5°C, then the probability of liquid freezing back to solid before hitting the ground becomes much greater. The latter is what happened during the central Oklahoma/southeast Kansas sleet storm last Mid-January.

To expand on what Patrick said, he is right in that the more appropriate way to diagnose precipitation type is to use wet bulb temperature vs. temperature. That said, an event that is underway, typically the sounding is pretty much saturated such that Tw and T are almost identical. Since water vapor quantity is a highly varied and hard-to-predict variable above the surface, resultant Tw is going to suffer from uncertainties due to water vapor uncertainties. This is when I develop tools at work, I just use T, b/c it is less sensitive than Tw.

Jeff Miller

11-29-2007, 09:15 PM

http://rap.ucar.edu/weather/model/eta42hr_sfc_prcp.gif
http://rap.ucar.edu/weather/model/eta42hr_sfc_ptyp.gif

New NAM really kicking out the QPF on the wintery mix! Quite a bit more QPF then the last run.

Jeff Miller

11-29-2007, 09:17 PM

http://rap.ucar.edu/weather/model/eta48hr_sfc_prcp.gif

Ice Storm anyone??? That right there is some pretty incredible ice storm QPF.

Mike Hollingshead

11-29-2007, 09:39 PM

Thanks for the advice Patrick and Mike. I'm rather clueless on it yet though, lol. I like the 1/3-5 thing on the haby page for wet bulbing. That may prove interesting/useful come Saturday morning around here.

I don't recall seeing such similar precip forecasts run after run. They almost always look the same each run. Compare that one to the gfs run from this morning Jeff...

http://www.extremeinstability.com/stormpics/temp/gfs_p60_072m.jpg

The speed and the surface low track have been extremely consistent too. The biggest difference is the GFS sfc temps compared to the NAM. It will get nasty around here fast given the current NAM may still be warming things too much. Hell it's already 12 degrees as far south as Mitchell SD now. Back towards the center of the High headed for this region it's single digits below zero. NAM looks like it probably initialized 5 degrees too warm up there(given it shows 0-5 above zero at midnight where it's already -3 to -6......areas in southern SD already to the temps prog'd by midnight for there).

Mikey Gribble

11-29-2007, 09:39 PM

Here are some notes from Tim's Weather Forecasting Handbook that I had on my website Mike. They aren't as detailed as the other two explanations you got, but helpful none the less.

Rules of Thumb (Vasquez)
- solids that pass through a warm layer depth of at least 600ft. will melt partially
- solids that pass through a warm layer depth of 1,200ft. will melt completely
- liquid that passes through a cold layer depth of 800ft. will freeze
- liquid that refreezes will always form a frozen droplet. Snow is not formed in this manner

Snow Through a Warm Layer (Vasquez)
(this section is referring to layers of warm air near the surface that snow falls through)
- warm air layer > 1,200ft. = most likely rain
warm air layer of 900ft. = 50% chance snow 50% rain
warm air layer of 700ft. = 70% chance snow
warm air layer of 300ft. = 90% chance snow
- the freezing level often lowers 500-1,000ft. during the first 1-2 hours after the precipitation begins
falling and may rise again to its original level 3 hours after the layer becomes saturated
- be wary of evaporational cooling in the lower dry levels
- wet bulb temperature is usually about halfway between the temperature and the dewpoint
temerature
- it has been shown that 0.38 inches of liquid equivalent precipitation can contribute 8 degrees F of
cooling towards an air parcel's wet bulb temperature

Mike Hollingshead

11-29-2007, 09:47 PM

Thanks Mike. Now those terms I can handle a little.

- liquid that passes through a cold layer depth of 800ft. will freeze

That is pretty much what I was wondering on. I'm actually sort of surprised liquid will refreeze that quickly.

Joel Wright

11-29-2007, 10:22 PM

Wow, what an interesting storm. The warm air advection wing precip is going to be quite significant. Many areas are going to see all four forms of precip within a 12 hour period of time. Just look at that tremendous WAA at 850mb! Incredible.

The NAM shows quite a wide band of sleet from Nebraska through Iowa, and into Illinois for much of the day on Saturday. Judging by the QPF forecasts, some of these areas may receive quite a bit of sleet. For example, Des Moines and Davenport are forecasted to have sleet for over 6hrs. If that were indeed the case, it would accumulate quite significantly over that duration.

Jeff Miller

11-29-2007, 11:00 PM

New GFS run in and pretty much everything totally on - with no significant changes then what we have here

Expect upgrades to Warnings in the morning package and suspect some of them will be of the Ice Storm variety.

Brandon Sullivan

11-29-2007, 11:13 PM

Taking a look at the 0z NAM and GFS...

The NAM/WRF takes the low on a further south path, keeping freezing surface temps over the IL area the whole duration of the event. However, it still shows nice WAA advection northward, with 850mb hovering between 6-9 degrees Celsius. This would spell s nice ice storm setup, with surface temps forecast to be sub freezing by at least a couple degrees..

Taking a look at the GFS.. It continues to drag the low further north, this time taking the low into central and northern iowa up into central wisconsin!! The WAA did not cut back too much either, and this would mean a lot of rain for areas in Western IL into north central Illinois and eastern/southeastern IA.

Hopefully things will converge one way or the other here soon..

rdale

11-29-2007, 11:25 PM

Remarkable how the GEM & EC had this track all week long - what do those foreigners know about long-range models that NCEP doesn't?!? Remember the GFS track from Sunday / Monday's runs that had everyone excited, as it brought the low across Tennessee :>

Rhyuan Janssen

11-29-2007, 11:30 PM

I do not buy into the GFS taking the low from SC Iowa and then up thru wisconsin. I think it will be further to the south; unless the low bombs out and is much stronger than anticipated

Some of our local stations where I live are saying an inch of ice on top of 3-6 inches of snow; then 35mph gusty winds. They are also mentioning thunder will occur with our freezing rain. I'd like to see that!! That would cause major issues for us, in fact, maybe worse than last years Feb/March Ice storm.

I would also agree with morning update that some of these WSW will become Ice Storm warnings (cent. IA and eastward); especially if thinking edges towards NAM QPF output.

In any solution someone is going to get a major ice storm IMO, and I hope it is me!

Dustin Wilcox

11-30-2007, 12:23 AM

Still looks as though the Sioux City NE to Storm Lake to Spencer IA area looks most at risk for a sig icing event. Models have been pretty consistent warming areas south of a Columbus NE to Harlan IA line to above freezing, though some early morning frozen precip looks likely. I think being in Lincoln I will probably be missing out on this one, I would head North towards my hometown, and maybe even farther North to enjoy the storm, but have to stick around Lincoln just in case we get any snow, as its my weekend to move snow on campus.

Corey Sloan

11-30-2007, 01:15 AM

I am new to the winter weather discussion, usually only involved in the spring season....does anyone put any weight into the models not verifying the temps in the Upper Midwest during the latest 00Z run today....they are anywhere from 8 to 14 degrees warmer than the actual station plots....if I am correct, could this affect the precip type across southern IA and northern MO?

Jeff Miller

11-30-2007, 06:46 AM

What a mess.

Am Ice Storm Warning is hoisted for extreme NE KS and extreme NW MO.

Winter storm watches are now in effect for SE SD, Central and Eastern NE, Central and southern MN, all of Iowa, NW/NC KS, Northern IL, All of Wisconsin, Extreme N IN, All of MI, Extreme NW OH, West NY and Central PA.

Freezing Rain advisories are in effect for Central and portions of NE KS

Major winter storm developing and this one is giving headaches to forecasters all over this country this morning. There are many, many highly complicating issues at play and this one is going to be a storm that is going to be evolving even at and beyond crunch time, and I would expect frequent changes as the "now" becomes more real then the "forecast".

The biggest issue will be how strong the mid level warm air advection can overcome the shallow cold air at the surface for many locations. I've noticed some weather service offices this morning look at the strength of midlevel WAA and agree with models on a warmer scenario. However historically colder air is not as easy to shallow out as models like to indicate. It seems that in this case just a few degrees difference is going to spell out the difference between a paralyzing ice storm and a cold rainy day with little consequence.

At any note, this is going to be a nailbiter and a challenge to even the most experienced forecasters.

Again, what a mess.

Joel Wright

11-30-2007, 07:37 AM

The DVN NWS calls for up to 3/4" ice accumulation by Saturday evening. They note that internal gravity waves combined with VERY strong winds just off the surface may induce some wind gusts up to 50mph. That exact thing happened in the icestorm this past February and it was a real mess.

The real question for many of us though is how much of that ice will just be sleet. That would make a huge impact on glazing accumulations.

Rhyuan Janssen

11-30-2007, 10:20 AM

For me I am hearing very conflicting forecasts. Several NWS stations say it will NEVER get above freezing in NE Iowa and Far North NE and NW Iowa in which case, we are all in for it.
Then on the flip side of things some say rain by afternoon? IMO I still think the warmer air is being overdone and the shallow cold air will stick around longer than anticiapted. In fact this morning my local station said 1-2 inches of ice??!! I think they are overdoing that a bit.
Now DVN says possibly 50+mph winds and crippling ice storm and Des Moines is talkin about lightning and thunder with the freezing rain! If the wind and thunder truly occur I think someone in Iowa is going to have an Ice storm beyond what we had last march. Again, hoping is it as my house :-):D

Brandon Sullivan

11-30-2007, 11:37 AM

Glancing at the 12z WRF. It has come in somewhat slower and holds precip into sunday night. It has also shown warmer trends, likely due to the more northern track.

The 9z SREF shows probabilities of Freezing Rain being higher thatn Ice Pellets/Sleet.

The GFS in all honesty hasnt changed much... It is however slightly slower this run.. Not too much to add

Edit: Emergency Manager Conference Call has been scheduled by DVN for their CWA.

Darren Addy

11-30-2007, 11:55 AM

The NE forecasters discussion continues to be interesting:
BELIEVE THE 00Z MODEL RUNS DID NOT HAVE A GOOD HANDLE ON
THE STRENGTH OF THIS COLD AIR...WITH 1032MB SURFACE PRESSURES INTO
NORTHERN NEBRASKA. CLOUDS ALSO NOW SLIDING OVER THE CWFA. WINDS
PROGGED TO TURN NORTHEAST/EAST TODAY AS WELL. THOSE TWO FACTORS
DON/T SUPPORT MUCH WARMING...AND HAVE LOWERED TODAY/S HIGH TEMPS.
THAT ALSO MEANS THE COLD AIR WILL BE SLOWER TO DISLODGE TONIGHT.

Naturally, they are discussing only their CWA, but they seem to think that the most significant ice will be north and east of Grand Island (at this time). It does sound like somebody is going to get a repeat of what we got in south-central Nebraska last year (and that ain't pretty):

BUT THE MOISTURE TRANSPORT
ASSOCIATED WITH MASSIVE WARM ADVECTION IS ANOTHER BIG POINT.
PRECIPITABLE WATER VALUES ARE UP TO 4+ STANDARD DEVIATION ABOVE
NORMAL FOR EARLY DECEMBER...AND ABOVE 1 INCH. SUCH ANOMALOUS
INGREDIENTS CAN RESULT IN ANOMALOUS RESULTS (SEE ICE STORM
2006).

The good news is that this will be much more short-lived event with temps warming significantly rather quickly. (This is NOT altogether good news if you happen to be under a warming traffic light or business sign that decides to drop a significant block/sheet of ice off when you - or your car - is under it.) The most damage will occur where there is the most ice (weight) combined with wind which is often the straw that breaks the camel's (tree's) backs.

As someone who lived through multiple days of no power last year, I hope anyone who lives in the affected area is telling friends/relatives to stock up on food that does not require cooking, candles, etc. (Count your blessings if you have a gas stove/oven and gas water heater). If you live on a farm, you probably know the drill: The pump won't be pumping with no electricity, so fill the bathtub with cold water before going to beddie tonight. In particular, before the storm, check for branches over the lines that bring electrical power from the pole to your house. If that branch gives: you are unplugged from the grid, even if the grid is UP. That's the line that will be the lowest on the power company's priority list to fix.

Bill Doms

11-30-2007, 04:31 PM

Really curious to see how the warm air works in tomorrow. Last GFS run puts me in the 8"-10". I'm still thinking may see a fair amount of sleet first really cutting down on the totals up here based on forecasted soundings for MPX. FWIW west metro suburbs started pre-treating the road surfaces this morning up here.

http://i173.photobucket.com/albums/w80/mnskies/winter/071130001.jpg

Darren Addy

11-30-2007, 04:43 PM

Well, crap.
The ice storm warning area shifted from this:
http://144.216.6.33/images/icestorm1.png
to this:
http://144.216.6.33/images/icestorm4.png
I can't believe we are going to get it again. This will be the third significant ice storm for this area in 12 months.

Brad Case

11-30-2007, 04:52 PM

Eastern Nebraska and Western Iowa weather updating at

http://w0tvs.net

Brad - Bellevue NE

Rhyuan Janssen

11-30-2007, 05:07 PM

Jeff Miller

11-30-2007, 05:10 PM

I am definately liking the forecast for my area now. DSM says this could turn into an 'EXTREME ICE STORM EVENT' for nothern Iowa with pws pushing record values...YEE HAW!!

I hope the hastings forcaster is right and we get even HIGHER winds...will be a distructive ice storm for me I believe.

What I do not understand and maybe someone can explain to me is DSM did not put anyone north of hwy 30 in the Ice storm warning but has Northern Iowa getting almost twice as much ice as the south?? That does not make any sense to me?! Besides that the southern 1/3rd of the state is to raise above freezing!

The northern part of Iowa will experience heavy ice accumulations but will also experience significant snowfall accumulations. This justifies the Winter Storm Warning instead of an Ice Storm Warning for that part of the state. The Ice Storm Warning is issued for areas that will receive almost pure ice of sleet or freezing rain forms.

Mike Hollingshead

11-30-2007, 05:23 PM

I'm guessing the reasoning is more likely due to timing. Just like ne KS and nw KS got an ice storm warning issued earlier today...and now we have been upgraded to it here. It won't be in northern Iowa till later than it will the other areas outlined. Just a guess.

Darren Addy

11-30-2007, 05:31 PM

Nebraska forecasters have declared a Crap Your Pants alert:

http://144.216.6.33/images/ohcrap.png

Danny Neal

11-30-2007, 05:32 PM

I suppose LOT is waiting a little longer into the event to decide what to do for the CHI-Metro. NW counties in WSWarn while ILX placed nearly all of the counties north of I 72 in ISWarn. That lake throwing curve balls into the forecast again no doubt. Southeast winds still blowing over "warm" lake waters into the city and north-side might spell all rain or no significant icing.

Joel Wright

11-30-2007, 06:13 PM

The DVN area has gone with an icestorm warning as well. The real key is the sleet aspect. This thing is going to be almost impossible to forecast until we start seeing some observations late tonight and early tomorrow. Hopefully this thing is loaded with sleet to cutdown on the glazing. Personally, I don't want to lose power.

Looks like the southern half of MN could get a real big snow. Maybe into eastern South Dakota too.

Josh Richardson

11-30-2007, 06:40 PM

Well as a few others have touched on....it appears we are in for a major ice event up here in Northern IA. DMX is calling for up to .7 inches of ice and that is after 3-5 inches going with a more ETA solution. Hoping to not lose power, will try to post some updates on Sat.

Jim Zandonai

11-30-2007, 06:58 PM

Looks like extreme N.IL is in for a mixed bag. Typically up here we end up with a mix of all three or all snow. Rarely have seen a significant ICE STORM in RFD. due to being in the slightly colder air. Timing and a couple degrees will mean everything. Mixed frozen precip and a slight glaze sound like the thing here.Down in the Ice storm warning area though I would be worried about losing power..that looks like a wicked ice storm set up.
I'll post on Sat when some significant precip moves in here..
Not too worried here but Good luck to you all in the Ice Storm warned areas..
Interesting to see how this pans out as far as precip type goes.

Andrew Revering

11-30-2007, 09:28 PM

I'm going with the pure snow/mix line staying just south of Minneapolis, and therefore with varying ratios as suggested by the NWS/FAA liquid to snow tables and the Kuchera method, i'm going with 12-16" of snow for the Twin Cities from the 18z NAM and GFS.

Looks like snowfall rates around 2" an hour between 3pm and 6pm on Saturday here.

PS... I posted earlier about not agreeing with what appeared to be a rain/snow line in the Kuchera maps on Earl Barker's site. I still maintain there's something goofy there, but now it appears the line is too far north in my opinion... IE more snow on the southern sides of the snow band than what is depicted, maybe by 30 miles or so... but I'm probably getting picky here, and perhaps detail is lost in image rendering too. So I'll shut up.

Also regarding a post with the significant ice event in Iowa... I would agree that the reason for the winter storm warning is because of the involvement of snow along with the ice... I am guessing 2-4" of snow across northern Iowa in addition to the freezing rain.

Good luck everyone! Put sand bags in your pickups, use 4 wheel drive, ESP and anything else you got. Keep your chase vehicles in one piece for when this crap melts.

Joel Wright

11-30-2007, 10:16 PM

Damn. Anyone notice the 925mb winds tomorrow? NAM is forecasting 50+kts at 00z in the cornbelt. That's insane. That's only a few thousand feet up. At 850mb winds are over 60kts! There could be some very high wind gusts tomorrow in the midst of some serious glazing. This could make even a somewhat marginal glaze MUCH worse. Definitely going to have to watch that.

The RAP NAM precip model shows quite a wide band of sleet much of the day tomorrow. I'm kind of wondering how accurate that is. That sure seems like a wide band of sleet. Usually sleet is sort of a transitional precip type that doesn't last all that long. (Unless you're positioned in an area where the storm is sort of pivoting around you.) So I'm questioning how much of that band could be just plain freezing rain, or, freezing rain with just a bit of sleet mixed in, in spots. Hmmm.

It's sure going to be cool to watch this thing unfold. In my particular area, looks like snow will begin around 11am. That may whiten the ground a bit before the sleet moves in by early afternoon. Depending on which model you look at, it looks like freezing rain should fall in my immediate area for 3 to as much as 6 hours. What's really interesting is that temps may eventually spike above 40° as midnight approaches. All the while winds will be howling.

I'll try to post some updates in the future NOW thread tomorrow as things unfold. Maybe post a few pics if anything interesting occurs. Hopefully all of us will retain our electricity so we can track this baby as it unfolds tomorrow! :-)

Bill Schintler

11-30-2007, 10:24 PM

Specific forecasts for eastern and central IA.

All locations in central and east central IA can expect 1" of snow on Saturday, 0.25"-0.5" of ice accumulation on Saturday, and then 1" or less of additional snow on Sunday. The following are forecasts for specific locations:

Ames:
Snow will start at 8:15 AM Saturday, change to sleet at 10 AM, freezing rain at
12:30 PM, (liquid) rain at 4:15 PM, and then back to light snow around noon on Sunday.
Snow will end by Sunday afternoon.

Cedar Rapids:
Snow will start at 10:15 AM Saturday, change to sleet at 11:30 AM, freezing rain at 1 PM, (liquid) rain at 7 PM, and then back to light snow around noon on Sunday. Snow will end by early evening on Sunday.

Iowa City:
Snow will start at 10:00 AM Saturday, change to sleet at 11:15 AM, freezing rain with occasional thunder and lightning at noon, (liquid) rain at 6 PM, and then back to light snow around noon on Sunday. Snow will end by early evening on Sunday.

Marengo:
Snow will start at 9:45 AM Saturday, change to sleet at 11:00 AM, freezing rain with occasional thunder and lightning at 11:45, (liquid) rain at 6 PM, and then back to light snow around noon on Sunday. Snow will end by early evening on Sunday.

Synopsis:
Visible satellite imagery clearly showed a plume of moisture in the form of an ST deck surging N into WRN KS and SERN CO, along with an impressive subtropical plume in the SWRN CONUS. Upstream PW/S were 1 inch per 00Z KOUN and KAMA soundings, and 0.5 inches on KDDC and KTOP, which is well above normal. Looking at guidance; the NAM, which had trended slower and further S then the consensus (GFS, UKMET) on earlier runs, now is in better agreement. Earlier discrepancies were associated with sampling of ULVL energy in the PAC NW and BAJA areas, and with the SFC reflections beneath phasing ULVL energy.

Discussion:
WRN CONUS trough will phase with system in the BAJA area, and move slowly EWD and flatten slightly while a lead 90kt H5 jetlet races from NM towards SERN IA Saturday afternoon. Precipitation will rapidly develop in a region of strong isentropic up glide on 290K-300K SFCS, frontogenetic forcing along the NRN edge of precipitation, and weak elevated CAPE as a 60kt LLJ noses into IL/ ERN IA. Kinimatic forcing increases towards 00Z with the approach H5 streak left-exit region. Given the impressive moisture feed for this system, precipitation amounts may be underdone, especially in areas receiving convection. During the day Sunday, the primary H5 S/WV will race E from CO towards the WRN Great Lakes. Precipitation will be concentrated along a deformation zone axis extending from NWRN IA into WI. The ULVL system should maintain precipitation for a longer period then most MDLS suggest for QPF.

The all important question is the type and timing of winter precipitation that will fall. Prior to the onset of precipitation, a very cold and dry polar airmass will be entrenched over the NERN 2/3rds of IA. Top-down saturation of the atmosphere will take place, with SFC-3km temperatures falling over five degrees F owing to evaporative cooling. Very strong WAA will commence at the nose of a 55kt H925 LLJ, bringing the 0C isotherm to a line bisecting IA from NW to SE by 00Z Sunday. NAM is probably too cool with SFC-3km temperature fields given strength of WAA, and so sided with the GFS for LLVL fields. Another issue will be with gusty winds Saturday afternoon/evening as 60kt LLJ rides just above inversion, and perturbations may provide for momentum transfer to the SFC resulting in gusts in excess of 40 MPH.

- bill
9:23 PM CST, 11/30/07

Jeff Miller

11-30-2007, 10:36 PM

NOTED FROM FSD:

WELL...SOUNDINGS ARE IN AND ONE THING JUMPS OUT RIGHT AWAY. BOTH DDC
AND LBF ARE MUCH DRIER THAN THE 18Z NAM (http://www.crh.noaa.gov/glossary.php?word=NAM)/GFS (http://www.crh.noaa.gov/glossary.php?word=GFS) INIDICATED IN THE
700-550MB LAYER. SO THE QUESTION IS...WILL THIS BE A HELP OR A
HINDERENCE. GENERALLY...LESS MOISTURE (http://www.crh.noaa.gov/glossary.php?word=MOISTURE) IS BAD. IT EITHER MEANS THINGS
WILL BE SLOWER OR PRECIPITATION AMOUNTS WILL BE LESS. BUT...IN THIS
CASE...WITH MUCH OF THE FOCUS FOR PRECIPITATION DEVELOPMENT COMING
IN THE 850-750MB LAYER...THIS COOLER AIR MAY IN TURN AID IN
INCREASING INSTABILITY (http://www.crh.noaa.gov/glossary.php?word=INSTABILITY).

IF THE WARM SECTOR (http://www.crh.noaa.gov/glossary.php?word=WARM%20SECTOR) DOES IN FACT COME IN A
COUPLE OF DEGREES COOLER THAN PREVIOUSLY THOUGHT...WE COULD END UP
WITH MOSTLY SLEET...AND QUITE A BIT TO BOOT. MODELS CONSISTENTLY
SPITTING OUT ABOUT THREE QUARTERS OF AN INCH TO AN INCH OF QPF (http://www.crh.noaa.gov/glossary.php?word=QPF) AND
IF EVEN HALF OF THIS IS SLEET...WE ARE LOOKING AT ABOUT 1 TO 2
INCHES OF SLEET.

Pretty significant changes with much drier air than NAM/GFS indicated.

ALL bets are off.

Andrew Revering

11-30-2007, 10:51 PM

Here’s my snowfall forecast map based off of 00z NAM data.
This is storm total snowfall accumulations.

[This is not yet a feature in F5, but coming in future upgrades]

http://www.f5data.com/screenshots/eta00z12012007.png

Brandon Sullivan

11-30-2007, 11:11 PM

Oz WRF holds onto precip over IL all the way into Sunday Night, surface temps are to be below freezing, so we will likely switch back to snow. The GFS has not moved further south really, but it and the WRF appear to have pulled the highest precip values further north...

Andrew Revering

11-30-2007, 11:30 PM

I'm a bit baffled by the local Minneapolis news casts. They still want to work in ice in the Twin Cities. I must not be as smart as I think I am because I just don't see it. Maybe they are playing it safe because the line will be close.

For Minneapolis from TV stations at 10pm newscast:
WCCO (Paul Douglas) calling for 4-8"
KMSP calling for 8-12"
KSTP (Dave Dahl) calling for about 6-10"
KARE (Sven Sundgaard) calling for 4-8" by 6pm with more after that, so lets say 8"
NWS calling for 10"

I think 12-16" is a fair forecast.

I'm sure I'll be eating crow. Sounds like a lot of snow. I'm going to pull apart some point soundings from GFS and see if I overlooked something there.

Andrew Revering

12-01-2007, 12:16 AM

Well at 30 hours there is a brief period above freezing. Removing the time period above freezing from the equation I'm still calculating out 11-17" of snow from GFS over Minneapolis.

Here's a question for ya...

We've discussed the wet bulb or warmest wet bulb rules... I think Mike Umschied mentioned he prefers to use the environmental temperature... when looking at model data, the models produce precipitation and therefore shouldn't they already be lowering the environmental temperature appropriately, and therefore looking at the wet bulb temperature in model graphics is attempting to reduce the temperature twice?

Or do you still look at the wet bulb of the cooled model air because it's not capturing the actual cloud physics and just the air "around the cloud"

???

Darren Addy

12-01-2007, 12:25 AM

OAX mentions the DDC/LBF moisture progs (that Jeff noted above) and says:
THERE WERE SOME DISCREPANCIES IN MOISTURE PROGS OVER
THE HIGH PLAINS...ESPECIALLY ABOVE 700MB...WHEN COMPARING 00Z NAM
AND DDC/LBF SOUNDINGS. HOWEVER ABUNDANT MOISTURE WAS DEPICTED ON
UPPER AIR CHARTS AND SATELLITE IMAGERY JUST TO THE WEST OF THESE
AREAS...AND THINK REALITY WILL CATCH UP WITH MODEL FORECASTS IN
THIS REGARD. UPPER TROUGH WAS STILL ROLLING INTO THE SOUTHWESTERN
U.S. WITH 120M HEIGHT FALLS NOTED IN SOUTHERN CALIFORNIA.
IMPRESSIVE TROPICAL MOISTURE PLUME WAS SURGING NORTH AND NORTHEAST
AHEAD OF THIS SYSTEM AS PER WATER VAPOR IMAGERY...AND IS ON TRACK
TO OVERSPREAD EASTERN NEBRASKA LATE TONIGHT. BROAD SCALE ASCENT
WILL BEGIN IN OUR AREA AFTER MIDNIGHT IN DIFFLUENT UPPER
FLOW...INDUCING STRONG CYCLOGENESIS IN THE CENTRAL ROCKIES.
INCREASING SOUTHERLY FLOW WILL PUMP LOW LEVEL MOISTURE NORTH AHEAD
OF THIS SYSTEM...AND STRONG ISENTROPIC UPGLIDE WILL ENSUE LATE
TONIGHT. 00Z NAM IS JUST A TAD SLOWER IN BREAKING OUT
PRECIPITATION OVER THE CWA...

GID has been silent since 5:55 PM CST (probably because they are quite busy puzzling this one out). All I can add is, "that's one heck of a trough (http://weather.cod.edu/loops/Southwest-WV.loop.html) coming this way!

Mike Umscheid

12-01-2007, 12:36 AM

Here's a question for ya...

We've discussed the wet bulb or warmest wet bulb rules... I think Mike Umschied mentioned he prefers to use the environmental temperature... when looking at model data, the models produce precipitation and therefore shouldn't they already be lowering the environmental temperature appropriately, and therefore looking at the wet bulb temperature in model graphics is attempting to reduce the temperature twice?

Or do you still look at the wet bulb of the cooled model air because it's not capturing the actual cloud physics and just the air "around the cloud"

???

Yes, when the model generates precipitation, it is in effect "wet-bulbing itself", which is one reason why I use just the model temperature once precipitation starts in the model. Wet bulb temperature is most useful for me at the onset of precipitation, in any layer of the atmosphere, prior to saturation, of course -- but once the model produces precipitation and saturates the column, the model has "wet-bulbed" itself, which is why it's really no different, in my eyes as an operational forecaster, to use ordinary T versus Tw when forecasting precipitation type. Each model has its own precipitation parameterization schemes, but in my experience, the NAM does a better job of "wet-bulbing" model Temperature -- the RUC as well, or at least it is most noticeable in these models due to better resolution.

That said, I've noticed the NAM has a really dry bias on occasion, like near the center of an anticyclone in a very shallow near surface layer, such that when it generates precipitation through this layer, it wet-bulbs the temperature too much, b/c the model was too dry in some arbitrary shallow layer. This is the problem with models and trying to resolve the highly varied distribution of water vapor in the atmosphere, so sometimes when a high resolution model shows -20°C dewpoint, for instance, in reality, that particular level might actually be closer to -5°C, and thus the model would forecast too cool of a temperature at onset of precipitation and subsequent wet-bulb effects. I'm basing most of this on my own experience as a high plains winter wx forecaster :)

Maggie Kahman

12-01-2007, 12:57 AM

according to the Omaha Wx Page (http://w0tvs.net/) has just extended the Ice Storm Warning from 12 AM (or now..lol) to 6PM Sat. night; to from 3AM Sat to 12AM Sun. Morning

John Farley

12-01-2007, 01:11 AM

For what it is worth, this storm has been warmer than predicted in New Mexico. Snow levels thus far have stayed pretty much above 10,000 feet (except in the NE plains, where the air is much drier), versus earlier forecasts of more like 8,000. Hope it comes down tomorrow (as it is forecasted to do, but not sure how much this will occur before the precip lightens as the day goes on), as I am out here in hopes of getting in some decent skiing.

I'm not sure I can ever recall seeing such a widespread, persistent precipitation shield in Arizona - this storm has a ton of tropical moisture to work with. I would think that the warmth combined with the abundant tropical moisture would tend to drive the p-type gradients north, as it has raised the snow levels higher than predicted in NM, but time will tell.

Ryan Pfannkuch

12-01-2007, 01:57 AM

Thought I would throw out a few tidbits as of 6z regarding guidance vs. reality:

Russell KS: Actual T=30 NWS fcst=28 0z MAV fcst=31 0z MET fcst=31
Hastings NE:Actual T=25 NWS fcst=21 0z MAV fcst=27 0z MET fcst=21
Omaha NE: Actual T=25 NWS fcst=24 0z MAV fcst=24 0z MET fcst=21

Not to make too much out of this...but at least at 6z it would seem that in general...reality seems to be trending at least a few degrees warmer than the NWS forecasts...and 4 degrees warmer than the MET fcsts for HSI and OMA. Up until now, I had thought all along that the GFS was way too warm with the sfc T evolution vs. the colder NAM...but now I am starting to wonder if WAA may be kicking in a little better than I expected.

Taking it one step further...I noticed that the 5z GFS LAMP output had a very good handle on the 6z obs. So looking 9 hours ahead...here is a forecast comparison for 15z:

Russell KS: NWS fcst= 38 LAMP fcst= 41 0z MET fcst=38
Hastings: NWS fcst= 22 LAMP fcst= 36 0z MET fcst=29
Omaha: NWS fcst= 28 LAMP fcst= 33 0z MET fcst=31

Hmm...in this case the GFS LAMP fcsts are anywhere from 3 to 14 degrees warmer than NWS...and even the "cold" MET numbers are 3 to 7 degrees warmer at OMA and HSI. So, bottom line here is that at a quick glance...it appears maybe the WAA may surge north quicker this morning than currently reflected in the NWS grids...which could mean a shorter duration ice/sleet event...especially across northern KS and southern NE. Of course, this is a very simplistic analysis...and does not necessarily account for factors such as substantial cooling from wet-bulbing etc. That being said, its gonna be interesting to see just how close (or far off) these warmer guidance numbers play out and any impact this would have on ice accumulations.

Update at 8z: Sometime around 145am CST very light freezing rain kicked in here in Hastings...with a surprisingly decent little glaze already covering my vehicle. Sidewalks are just starting to show that "feel" of getting slick. At any rate...didn't expect this for at least a few more hours so things may be getting underway a little early. I figured this might start off with some sleet first...but so far just light ZR.

Craig Maire II

12-01-2007, 06:45 AM

From this mornings Desmoines NWS forecast discussion:

COBB OUTPUT SUGGEST ONE HALF TO ONE INCH
AMOUNTS (of ice) ON TOP OF ANY SNOW WHICH COULD BE QUITE CRIPPLING. ON TOP
OF ALL THE PCPN (http://www.crh.noaa.gov/glossary.php?word=PCPN)...EXPECT TIGHT GRADIENT (http://www.crh.noaa.gov/glossary.php?word=GRADIENT) TO PRODUCE AS STRONG
SOUTHEAST WIND WITH GUSTS OF UP TO 35 MPH OR SO WHICH WILL AGGRAVATE
THE SITUATION BY DOWNING POWER (http://www.crh.noaa.gov/glossary.php?word=POWER) LINES AND BREAKING TREE BRANCHES.
WILL CONTINUE TO HIT IT HARD WITH QUARTER TO HALF INCH ICING (http://www.crh.noaa.gov/glossary.php?word=ICING) AMOUNTS
IN THE SOUTH AND HALF TO ONE INCH ACRS (http://www.crh.noaa.gov/glossary.php?word=ACRS) THE NORTH. THIS MAY RIVAL
THE FEBRUARY ICE STORM (http://www.crh.noaa.gov/glossary.php?word=ICE STORM) AND CERTAINLY MAY BE MORE WIDESPREAD.

The February ice storm was the worst ice storm that I have ever seen so if this statement pans out :eek: we could truly be facing a VERY serious situation in my area. My next door neighbor has a generator so we will go there when our power gets knocked out to stay warm etc. I have warned all of my friends and family about the potential seriousness of this weather dituation so they too are as prepared as they can be.

Rhyuan Janssen

12-01-2007, 10:39 AM

Well it's only 22 degrees here and we have mostly freezing rain with a tad bit of sleet. DVN says this could rival the ice storm of last year and certainly could be more widespread with more power outages and the like...
YAAY for ice!!

Joel Wright

12-01-2007, 10:51 AM

Yeah, the 12z models are pretty much unanimously agreeing on a short lived, yet fairly impressive icestorm all across IA into IL. The band from just north of SUX, to Ames/Waterloo IA, to about Clinton IA, on towards Sterling IL look to get the most ice accums. Probably about 1/2", with some local higher amounts.

The latest GFS has a bullseye of 1"+ QPF in a six hour time period this afternoon over northeast IA, into southwest WI, and extreme NW IL. That's very intense for just stratiform precip.

The million dollar question is just how fast will the melting temperatures in MO and SW IA make it north to put an end to the icing. The NAM and RUC look way too cool, keeping the freezing line south of I-80 still at late this afternoon. The GFS is probably closer to reality, yet may be just a bit to bullish in bringing the warm air northward, taking the freeze line well into northern IA by late afternoon. Temps there now are still in the upper teens. I think a compromise of those models is the way to go. The GEM looks way too warm as well, yet it is usually very reliable.

In either case, still a substantial icing event to be sure, yet somewhat short lived...