Wind shear

[Jon Miller]

Can there be too much wind shear and , if so, what are the limits for tornadic storms ? Are there any good web sites that discuss wind shear and how it is deterrmined, etc?

 

[Jeff Snyder]

My answer would be.. yes. Given very weak instability (say, 100 j/kg CAPE), an updraft may not be able to sustain itself in a strong-shear environment. Like many other processes, we're talking about a balance between shear and instability. You can see this in action during many of the early-season storm days (Feb-March), which tend to be characterized by very strong winds aloft (presumably enhancing deep-layer shear, though this isnt always the case) with relatively weak instability (<1000 j/kg CAPE).

 

[Amos Magliocco]

There is some indication that increasing values of storm relative helicity, which is in the neighborhood of your question I think (unless you're specifically wondering about deep layer shear), become less supportive of tornadoes. This scatter plot was taken from a document used by the SGF office:

[Broken External Image]:http://www.crh.noaa.gov/techpapers/service/tsp-10/fig6.jpg

Figure 6. Scatter diagram showing combinations of CAPE in J/kg and 0-2 km AGL positive wind shear for 242 tornado cases during 1980-1990 (Johns et al. 1993). Figure reproduced from Johns and Doswell (1992).

This plot makes an interesting case that a sort of "sweet spot" exists between ~1000 - 3000 j/kg and between 100 and 400 m2/s2 SRH. Obviously plenty of exceptions on any of the extremes of the plot, but the cluster is intriguing, particularly how it suggests that, even within the cluster itself, higher CAPE values support strong tornadoes more frequently in lower SRH value environments and vice-versa.

Lots of disclaimers here. I don't know their methodoology; I don't know how you get near-storm SRH measurements with any precision since those values are so extremely localized and vary wildly, and even CAPE estimates for particular tornado studies are tough. (See Rasmussen's Baseline Supercell Climo papers for a discussion of this). However, all that understood, this piece of evidence supports what chasers have contended for a long time: that it's not so much about values reaching particular thresholds as it is about balance for supercells and tornadoes to thrive. Thus the pitfall of placing too much emphasis on indices or derived products.

Here's the paper in which this diagram appears:

http://www.crh.noaa.gov/techpapers/service...0/tbl-cnts.html

 

[Jeff Snyder]

Amos,
I think we need to be careful about drawing conclusions relating tornado frequency to a combination of helicity and CAPE. Sure, there may not be many strong/violent tornadoes with extreme helicity (say, >500), but how many times does that situation occur each year? I mean, how often do we see 500 0-2km helicity and 4000 j/kg CAPE? I think a better plot may be a 'relative frequency' plot... In other words, the probability of a tornado based on a given situation... or the number of tornado in a given combination relative to the number of times that particular combination occurs. Of course, I don't think there's really a way to do this, since it'd be difficult to come up with the "total number of times this combination occurred". In conclusion, I think that we don't see many tornadoes in the 5000 CAPE and 500 helicity range mainly because that situation only rarely occurs (and if it does, it's likely in a strong-cap situation).

 

[Craig Maire II]

Here is a link to a page concerning a late season tornado outbreak that occurred across portions of central Iowa on 09/05/04. There was very limited amounts of instability but very favorable wind shear on this day.

http://www.crh.noaa.gov/dmx/WX_events/Sep-05-2004.pdf

 

[Niki Darnaby]

Also, given a rotating updraft in an environment with high shear and marginal instability, the negative pressure perturbation associated with the initial mid-level mesocyclone often enhances the updraft strength such that large amounts of CAPE are no longer needed. That's why the updrafts often appear stronger on days with extreme shear and low CAPE versus days with marginal shear and high CAPE.

 

[Amos Magliocco]

Originally posted by Jeff Snyder
Amos,
I think we need to be careful about drawing conclusions relating tornado frequency to a combination of helicity and CAPE.

I wonder if your argument has some logic flaws. I think we're perfectly safe relating tornado frequency (and even strength) to combinations of CAPE and helicity. All tornadoes occur under some value of these two parameters, so there's no danger in measuring that occurence and trying to derive some ideas from it. That's why Doswell and Johns did the plot. As well, I agree that tornadoes are rare under rare conditions. Tornadoes are rare even under the most supposedly supportive conditions, so it follows they would be even less frequent in unusual circumstances.

However, the reason I posted the plot is to consider various environmental conditions under which the incindence of strong tornadoes is correlated with those particular ingredients, one of which is shear, the topic of the question. What this suggests about the overall frequency of tornadoes or their baseline climatology is outside the scope, though I think it does not run afield of some of the VORTEX conclusions. I think the results suggest that a particular environment of CAPE and SRH supports more strong tornadoes than other sets of those particular values. Nothing more and nothing less. But it seems germane to the idea of too much shear.

If only we could get Dr. Doswell to do a guest post!

 

[Jeff Snyder]

Originally posted by Amos Magliocco
I think the results suggest that a particular environment of CAPE and SRH supports more strong tornadoes than other sets of those particular values. Nothing more and nothing less. But it seems germane to the idea of too much shear.

But does it? Saying so means that one can conclude that there's a better chance of a strong/violent tornado occuring with a 300 0-2km helicity and 3000 CAPE combination than with a 500 0-2km helicity and 5000 CAPE combination. I don't think that graph shows that at all, since it only shows that there have been more tornadoes associated with the 300/3000 combo than the 500/5000 combo only in absolute number of tornadoes. This does NOT imply that, for whatever reason, the 500/5000 combo is less conducive to tornadogenesis, just that there are more tornadoes that occur in the 300/3000 combo. But this is likely because the 300/3000 combo occurs much more frequently than the 500/5000 combo. It's like saying that, because there are more severe hail reports with 3000 CAPE than with 8000 CAPE environment, that there can be too much instability for hail. One cannot conclude that accurately, however, since the 3000 CAPE situations occurs MUCH more often than the 8000 CAPE situation, and thus we'd expect to see more hail reports in terms of the absolutely number of reports.... From the chart, there were more reports with the 'moderate' degrees of helicity and CAPE than with the 'extreme' degrees of both, but again that's because the 'moderate' helicity/cape combo occurs much more frequently than the 'extreme' combo.

 

[Mike Hollingshead]

Their map is for 0-2km helicity. If we are worried about too much shear wouldn't we be talking higher up? I can't see anything wrong with extremely high 0-2km shear for any reason. I don't even think I'd be terribly concerned with extremely high winds up at 300mb or higher under any setup. Between those two heights I'd probably be more concerned with too much shear.

 

[Amos Magliocco]

Originally posted by Jeff Snyder+--><div class='quotetop'>QUOTE(Jeff Snyder)</div>

<!--QuoteBegin-Amos Magliocco

I think the results suggest that a particular environment of CAPE and SRH supports more strong tornadoes than other sets of those particular values. Nothing more and nothing less. But it seems germane to the idea of too much shear.

But does it? Saying so means that one can conclude that there's a better chance of a strong/violent tornado occuring with a 300 0-2km helicity and 3000 CAPE combination than with a 500 0-2km helicity and 5000 CAPE combination. [/b]

No, saying so doesn't mean that at all, at least not for me. I'm not being very clear, I suppose. Yes there are more tornadoes within the more common parametric conditions. No it does not prove that balanced but extreme conditions are somehow less conducive. While the lack of tornadoes in extreme circumstances may suggest nothing more than the rarity of those circumstances, it does not, in my opinion, invalidate the utility of the graph.

What I view as the utility of the graph--the reason I posted it---is the possiblity that within a fairly well-defined balance, the incidence of tornadoes is higher. I don't think we can dismiss this and say, 'well that's only true because there are more days with those particular values.' If that were the case, the plot would be meaningless. It would be a logical fallacy.

I think there's a little of the operational/theoretical dilemna here. I'm not proposing a theory. I'm not smart enough or qualified to do that. I'm only pointing to a small piece of evidence (which appears in an in-house operational handbook) that, within a balance of shear and CAPE of a particular range, a higher density of strong tornadoes appear. I can't imagine that all they're doing is showing us that more days happen with those ranges. As Chris said, "the cross-section of conditions conducive to tornadic supercells." It seems to me that must have been the intent of the graph.

Obviously somebody should download this paper and post Doswells and Johns conclusions or commentary. Now I really want to know.

 

[Glen Romine]

This paper is a good read, and is available online here:

http://webserv.chatsystems.com/~doswell/pu...&doswell_92.pdf

I would note that nowhere in the paper do they try and suggest an upper limit to CAPE and shear combinations - reality of nature is simply that you rarely have one large without the other being small. Both sreh and CAPE have units of energy - and as such it is when you have a synergistic pairing of the two that interesting things can happen. More recent work by Rasmussen and Blanchard add cautionary supplements, such as the need for the high shear layer to also be bouyant - so you's use a parameter such as 0-1 km EHI (they also show the 0-1 km helicty is most important) which combines the low-level buoyancy with the low-level shear. Of course, this alone is not enough, as you need favorable deep layer shear and CAPE to support the supercell storm. So, the tornadic environment is a supplement to the supercell environment.

Glen

 

[Gabe Garfield]

I think that, in general, storm mode is the most important factor. With stronger shear setups, storms tend to form convective lines and complexes rather than isolated cells. Most high shear/low instability tornado days feature fairly discrete storms (not always the case, but generally speaking). Storm mode is highly dependent on the orientation of the shear vector (relative to the pertinent storm initiating boundaries) and also on the cap strength.

So, while extreme instability and extreme shear would seem to be great, there is an extremely delicate balance between lots of storms (MCSs and MCCs) and no storms at all. I think that extreme shear is good only in cases where the overall forcing for convection is weak (which would be related to the instability and convergence along any surface boundaries).

A good case of an extreme shear/extreme instability (pseudo) bust would be May 24, 2004. Many thought NE KS and NW MO would see violent tornado after violent tornado given the extreme set of parameters. However, too much forcing along the warm front (correct me if I'm wrong) caused the convection mode to go to linear much sooner than most had expected. Subsequently, the significant tornado threat decreased considerably.

In my observation, extreme instability with moderate shear is much better than the volatile extreme instability/extreme shear combo.

Gabe

 

[Glen Romine]

Originally posted by Gabe Garfield

In my observation, extreme instability with moderate shear is much better than the volatile extreme instability/extreme shear combo.

Gabe

Can you give even one example? The paper reference couldn't find one, and I suspect you'd have a hard time as well. Extreme CAPE and extreme [edit]low-level shear[/edit] just don't coexist very often - so if you have an example when it happened, I'd be interested in hearing about it (seriously). Soundings are just too far apart, and launched too rarely to capture these rare meetings with any real frequency.

Also, we are talking about extreme low-level shear here - which does not directly affect storm mode (deep layer shear controls this - say 0-6 km). Cap strength does not regulate storm mode in any formulation that I've ever seen. Sounds like you are confusing environments favorable for discrete vs. linear convective modes, which is not the same as favorable environments for tornadoes.

BTW, there where quite a few tornadoes on the day you mentioned - so maybe not a good case of a bust.

http://www.spc.noaa.gov/climo/reports/040524_rpts.html

[edit]
Also, if you want to see the values of CAPE and helicity for the event you mentioned - you can see them here:

CAPE:
http://www.spc.noaa.gov/exper/archive/even...p_040525_00.gif

Low-level shear - note 0-3 km, which is more generous than 0-2 km
http://www.spc.noaa.gov/exper/archive/even...r_040525_00.gif

If you look carefully at the plots - you'll see that the highest low-level shear is offset to the north from the highest CAPE. Where the two overlay the best, you see roughly 450 helicity with 3000 CAPE, which is on the upper portion of the data envelope in the study, but we are using 0-3 helicity here vs. 0-2 in the study, which moves this case right into the average. Of course, these values weren't actually measured by a sounding. The best case sounding from this event was at Topeka, with a CAPE just over 3000 and 0-3 km SRH of only 318.
[/edit]

Glen

 

[Amos Magliocco]

I need to post one correction to my last post. "Logical fallacy" isn't the right term. What I mean is that if all the plot achieves is to document the frequency of DAYS under which these conditions occur, then it is a statistically insignificant scatter plot. I have not looked at their data, but the cluster at least 'looks' statisitically significant for those two variables, and I'm assuming the authors would not have published a plot that did not achieve this.

 

[Mike Hollingshead]

Extreme CAPE and extreme instabilty just don't coexist very often - so if you have an example when it happened, I'd be interested in hearing about it (seriously).

Well I tried to find the data on it, but the spc archive page doesn't have the shear/cape/lapse overlays for it. August 16, 2002 in ne NE had both, if I recall it right. I remember the ruc having an 80knot llj over SUX that night and a powerful surface low with the system, and a nose of like 5000+ SBCape. I think siggy tor was like 12 or something and the supercell index was over 30. But, the boundary was lined up all wrong.

http://www.spc.noaa.gov/exper/archive/even.../OAX_00_obs.gif

Shear and cape both were stronger more towards Yankton SD than this at Omaha.

 

[Glen Romine]

I was able to find some stuff at the NCAR archive. Let's look at where the precip was occurring at the time of the sounding:

http://locust.mmm.ucar.edu/case-selection/...00208170000.gif

and the surface map at that time:

http://locust.mmm.ucar.edu/case-selection/...00208170105.gif

You probably noticed the sounding at OAX still had a healthy cap of 50 J/Kg, and what we see is that storms were developing north of a boundary at the surface, where temps were only in the 70's, so convection was not occuring in the region with both large CAPE and large shear.

I think you get a better appreciation for the strength of the cap in this sounding plot:

http://locust.mmm.ucar.edu/case-selection/...020817_0000.gif

Thanks for the suggestion though - and please keep thinking of more.

Glen

 

[Mike Hollingshead]

You probably noticed the sounding at OAX still had a healthy cap of 50 J/Kg, and what we see is that storms were developing north of a boundary at the surface, where temps were only in the 70's, so convection was not occuring in the region with both large CAPE and large shear.

[Broken External Image]:http://www.extremeinstability.com/stormpics/02-08-16(11).jpg

LOL, I can assure you that was not north of the boundary in cool air. Look closer at your sfc temps and where those storms are. Hell that one return is almost on O'Neill at 7pm and O'Neill is 91/63. That would be this storm maybe 1hr later north of Norfolk.

 

[Jeff Snyder]

LOL I've got a good case for a combo of high low-level shear + strong instability...
[Broken External Image]:http://www.tornadocentral.com/now/July210z02MPXsounding.gif

0z July 21, 2002, MPX sounding... The tornado reports are empty because the cap held any deep moist convection at bay this day. I haven't seen too many soundings which show ~5800 J/kg MLCAPE (>6100 SBCAPE) and 262 m2/s2 0-1KM SRH (404 0-3km)... This may have been a good day to see this discussion at work considering that the deep layer shear (0-6km) was not terribly impressive at 40kts.

Originally posted by Glen Romine
Sounds like you are confusing environments favorable for discrete vs. linear convective modes, which is not the same as favorable environments for tornadoes.

I know this comment was directed at Gabe's post, but I just had to interject here... I think there is a pretty strong relationship between storm mode and tornado occurrence, and I don't htink many here would argue that there isn't a relationship between the two. I think storm mode is incredibly important to tornado potential... Yes, environments that are most likely to directly affect tornadoes (mainly low-level shear) are not directly associated with storm mode (e.g. squall lines, supercells, etc) and vice versa. But I do think that the parent mesocyclone plays a very important role in the development of tornadoes given strong low-level shear. Again, I know you're probably saying that we can't really use 0-6km shear for tornado forecasting and 0-1km/0-3km helicity for supercell forecasting, but the 0-6km shear does play a role in storm rotation most likely, which in turn can aid in tornadogenesis if you take the theory of tilting and stretching of streamwise vorticity. This is neglecting the fact that updraft rotation in general can significantly enhance updraft 'strength'/velocity relative to a nonrotating updraft...

 

[Gabe Garfield]

Soundings are just too far apart, and launched too rarely to capture these rare meetings with any real frequency.

Are soundings necessary to determine instability/wind shear parameters? There are a number of other methods for determining these quantities (satellite, wind profilers, etc.).

Can you give even one example?

Does model initialization from 00z 24 May 2004 count? As I recall, CAPE was well over 4000 j/kg.

From the 20 UTC Day 1 Convective Outlook:

THE AIR MASS IS EXTREMELY UNSTABLE AND THE DEEP LAYER SHEAR
IS FAVORABLE FOR TORNADIC SUPERCELLS.

Cap strength does not regulate storm mode in any formulation that I've ever seen. Sounds like you are confusing environments favorable for discrete vs. linear convective modes, which is not the same as favorable environments for tornadoes.

Cap strength does not regulate storm mode? Maybe not by itself, but a stronger cap means fewer storms and less storm mergers (e.g. change in storm mode).

From Edwards and Thompson's 3 May 1999 paper:

When forecasting a threat of tornadoes, the mode of convective initiation and the number and spacing of supercells that form are critical to the number of tornadoes expected. In the same mesoscale region, several supercells may develop in association with different forms of boundaries. These boundaries vary in detectability when using conventional data sources, and storms may form where there are no apparent boundaries. The initial storms in the 3 May 1999 outbreak evolved into tornadic supercells that each lasted several hours, with no early transition to a squall line or other convective mode. Storm spacing and motions were such that the supercells remained in an environment of favorable vertical shear and instability for several hours without numerous storm collisions, thus allowing the supercells to produce a large number of tornadoes.

The predominance of a supercell convective mode and lack of a squall line on 3 May 1999 may have been attributable to the lack of strong low-level convergence near the dryline(s). It is conceivable that the outbreak would not have materialized in such intense or prolific form had the convergence been stronger along a consolidated dryline, and had numerous storms formed simultaneously and merged into a larger-scale convective system in the weakly capped environment over northwestern Texas and western Oklahoma during the afternoon.

By the way, when I said May 24, 2004 was a (pseudo) bust, I was referencing the lack of a large number of significant tornadoes.

As far as 0-6 km shear determining storm mode, I can think of at least one exception from '04. Large 0-6 km shear was present in C. OK on May 29th (C. OK was under the mid-level jet), yet the storm mode was high precipitation for a good part of its lifespan.

Gabe

 

[Andrea Griffa]

Originally posted by Jeff Snyder+--><div class='quotetop'>QUOTE(Jeff Snyder)</div>

LOL I've got a good case for a combo of high low-level shear + strong instability...
[Broken External Image]:http://www.tornadocentral.com/now/July210z02MPXsounding.gif

0z July 21, 2002, MPX sounding... The tornado reports are empty because the cap held any deep moist convection at bay this day. I haven't seen too many soundings which show ~5800 J/kg MLCAPE (>6100 SBCAPE) and 262 m2/s2 0-1KM SRH (404 0-3km)... This may have been a good day to see this discussion at work considering that the deep layer shear (0-6km) was not terribly impressive at 40kts.

<!--QuoteBegin-Glen Romine

Sounds like you are confusing environments favorable for discrete vs. linear convective modes, which is not the same as favorable environments for tornadoes.

I know this comment was directed at Gabe's post, but I just had to interject here... I think there is a pretty strong relationship between storm mode and tornado occurrence, and I don't htink many here would argue that there isn't a relationship between the two. I think storm mode is incredibly important to tornado potential... Yes, environments that are most likely to directly affect tornadoes (mainly low-level shear) are not directly associated with storm mode (e.g. squall lines, supercells, etc) and vice versa. But I do think that the parent mesocyclone plays a very important role in the development of tornadoes given strong low-level shear. Again, I know you're probably saying that we can't really use 0-6km shear for tornado forecasting and 0-1km/0-3km helicity for supercell forecasting, but the 0-6km shear does play a role in storm rotation most likely, which in turn can aid in tornadogenesis if you take the theory of tilting and stretching of streamwise vorticity. This is neglecting the fact that updraft rotation in general can significantly enhance updraft 'strength'/velocity relative to a nonrotating updraft...[/b]

Seeing this case I've seen that in the morning sounding there was already strong cap that limited a spread convection during the day: in the 12Z sounding I see 404 J/Kg of CIN that it's a strong value. Probably the strong heating of the day eroded a part of this cap but this wasn't enough to erode it at all.This maybe can have suppressed convective initiation. Probably the only thunderstorms that developed that night were those that came from South Dakota and did some damagin winds: but there was no primary convection in MN.
I don't know why, but often when there are similar synoptic situation with a big amount of instability, cap is too much high and limited convection.
I think that the best range for having a deep tornadic convection is 1500-3500/4000 J/Kg of cape with a good amount of SRH: I think probably the same cape is the most important factor for having a good convection: when it's too high occur too factors that limit convection. This is obviously not a scientific observation but often things go in this way.
What do you say?

 

[Glen Romine]

Originally posted by Mike Hollingshead

LOL, I can assure you that was not north of the boundary in cool air. Look closer at your sfc temps and where those storms are. Hell that one return is almost on O'Neill at 7pm and O'Neill is 91/63. That would be this storm maybe 1hr later north of Norfolk.

Almost doesn't cut it - the cells were not at O'Neill, and the surface conditions (and the upper air) was not the same there as they were at Omaha, and an hour difference in time is notable as well. Let me see what I can do with what information I can easily get.

First, the warm front passed through O'Neill after 2000 UTC, as at that time it is clearly north of the boundary:

http://locust.mmm.ucar.edu/case-selection/...00208162105.gif

The cells developed within the next hour on the NE/SD border:

http://locust.mmm.ucar.edu/case-selection/...00208162300.gif

The southern edge of this development looks to be by Niobrara, which is a good 35 miles to the northeast of O'Neill. Let's now look at the visible sat image at 22:30, which splits the time difference between the radar and the surface.

http://locust.mmm.ucar.edu/case-selection/...00208162252.jpg

Note the inverted 'V' cloud pattern near O'Neill. That is the location of the warm front and dryline interesection. Now see the cells developing up along the state line, north of the warm front.

The laminar appearance of the cloud bases in your pictures from this event is further evidence that surface air was not buoyant until substantially lifted. Also, if we take the OMA sounding and make the (bold) assumption that the upper level environment was similar at O'Neill, then overlay the O'Neill surface conditions on the OAX sounding, you'll see that the CIN is even greater at O'Neill (91/63 vs. 88/70 at OAX) than at Omaha. Further, the wind direction at O'Neill is from the SSW and strong, so there goes much of the helicity as well. The best scenario of surface inflow north of the boundary is at Norfolk 2 hours before the cell crossed north of there, with 89/74 right on the edge of the warm front, but this fell to 86/67 by the time the cell approached this station.

So, I don't see any evidence that would convince me this cell was not north of the boundary. Did you have a thermometer with you to measure the air temperature of the inflow? The would be helpful in determining if the cell perhaps crossed south of the warm front later in it's life.

Glen

 

[Glen Romine]

Originally posted by Jeff Snyder+--><div class='quotetop'>QUOTE(Jeff Snyder)</div>

LOL I've got a good case for a combo of high low-level shear + strong instability...

[/b]

Thanks Jeff. I'll take a look at this later... you guys are overwhelming me a bit with all this at once.....

<!--QuoteBegin-Jeff Snyder

I know this comment was directed at Gabe's post, but I just had to interject here... I think there is a pretty strong relationship between storm mode and tornado occurrence......

Thanks Jeff, I guess I wasn't very clear on what I'm trying to say here. Let me try this again and see if it makes more sense. The biggest confusion I have with Gabe's arguments is the lack of distinction between deep layer shear and low-level shear. Perhaps it's best if we step through them, so here is Gabe's first set of statements:

With stronger shear setups, storms tend to form convective lines and complexes rather than isolated cells. Most high shear/low instability tornado days feature fairly discrete storms (not always the case, but generally speaking). Storm mode is highly dependent on the orientation of the shear vector (relative to the pertinent storm initiating boundaries) and also on the cap strength.

Here I must assume he is talking about deep layer shear. Shear strength alone does not dictate whether cells are discrete or organized, such as a squall line - instead the magnitude of forcing relative to cap strength will control this. If the cap is weak and dynamic forcing strong - mode is likely to be linear. Yes, the orientation of the shear vector relative to the boundary forcing ascent is very important. If the shear vector is oriented perpindicular to the boundary and is a straight-line hodograph - this favors splitting cells along a line which will tend to collide quickly - which often leads to a squall line, but can also have cell interactions that later cause a return to discrete cells. Also, yes you do need some cap to keep cells discrete - but there is a delicate balance in that too much cap means parcels won't become buoyant until high up in the storm - meaning the updraft is weak until you are well aloft. If you want tornadoes, you need a strong updraft as close to the ground as possible.

I think that extreme shear is good only in cases where the overall forcing for convection is weak

Again - are talking about deep layer shear or low-level shear? If you have too strong of deep layer shear for the amount of buoyancy present - storms are simply ripped apart - so you have no storms at all. If the statement here is that you can have too strong of low-level shear, that doesn't hold either. Deep layer shear combined with the amount of buoyancy regulates the type storm that will occur. If you have extreme CAPE, you don't need the as much deep layer shear (example - Jarrell Tx. 27 May 1997 F5), likewise, with extreme low-level shear you can get strong tornadoes with surprisingly little buoyancy (example Van Wert Oh. 10 November 2002 F4). Go back and look at this case - there was LOTS of forcing. That is what is shown in the Johns and Doswell plot, and really nothing more than there is a very broad range of favorable combinations of low-level shear and buoyancy.

In my observation, extreme instability with moderate shear is much better than the volatile extreme instability/extreme shear combo.

Again, I'd like to see this in an example - but note here I'm looking for extreme low-level shear, not extreme deep layer shear, combined with large CAPE. A good recent example of the two coming together, but unfortunately not directly sampled, is the Manchester SD tornadoes of 24 June 2003 - which appeared to have ~6000 CAPE and SRH ~500, but no soundings stations are in this area.

Ok.... on to the next one.

Glen

 

[George Tincher]

Good topic!

It does seem we rarely get to see a high shear/high CAPE day, as the early season action is usually characterized by high shear/low CAPE events and the late Spring season by high CAPE/relatively modest shear events.

So it would seem logical that most events are going to occur on days when these values are moderate and not extreme, simply because it's very rare to have an abundance of both on any ghiven day throughout the year.

I can remember one crazy day back on January 3, 2000. Shear was at an insane level (seems it was in the range of 800 m2/s2 at least locally!) while CAPE was barely present at all (500-800 j/kg). Regardless, there was a few tornadoes, including an F3 that touched down near the Owensboro, KY area. But the storms quickly became linear, rather thin and unimpressive looking on radar and lost their tornado potential soon after.

I have seen other examples in late May/June where CAPE was abundant, even extreme, yet shear was very weak. Yet these events can also spawn some tornadoes.

Now, if you could take those early season events where CAPE is low and bump that up into the 2000-3000 j/kg range to go along with the insane shear that's often present at that time of year, then you might be looking at a record outbreak. The same is true for the late Spring/Summer activity. If shear was increased in the presence of the extreme CAPE often available at that time of the season, things would likely go nuts.

It just seems the better setups are a comprimise made by Mother Nature. I suppose that is why the best days often have a good balance of each parameter, but not insane amounts of either. And also not surprisingly these days usually fall in April/May (and for the more NRN folks....June) where such a balance is most likely to take place. Again, it's just unlikely you are going to see a day in February with CAPE at 5000 or a June day with helicities of 800 m2/s2.

So are modest days really the best? I don't think so. I just think they are the best balance we are likely to see and thus, they end up being the best.

And yeah, I know, this is an overly simplistic view of things. But I am an overly simplistic kinda guy. Hehe.

-George

 

[Glen Romine]

Originally posted by Gabe Garfield+--><div class='quotetop'>QUOTE(Gabe Garfield)</div>

Are soundings necessary to determine instability/wind shear parameters? There are a number of other methods for determining these quantities (satellite, wind profilers, etc.).
[/b]

These are valuable tools for estimating the environment, but are no substitute for the real thing. Satellite soundings use model output for the first guess, use lot's of assumptions to come up with the profile, and have large error bars. While problems are being solved in this field quickly, they have a long way to go imo.

Originally posted by Gabe Garfield+--><div class='quotetop'>QUOTE(Gabe Garfield)</div>

Does model initialization from 00z 24 May 2004 count? As I recall, CAPE was well over 4000 j/kg.
[/b]

I'm a modeler - so I'd love to say yes, but I can't.

<!--QuoteBegin-Gabe Garfield@

Cap strength does not regulate storm mode? Maybe not by itself, but a stronger cap means fewer storms and less storm mergers (e.g. change in storm mode).

You could add hodograph curvature as being equally important. Large curvature favors one updraft over the other after the required split from tilting of environment horizontal vorticity, allowing for streamwise ingestion of the environment shear. If you have a straightline hodograph, then cell splitting is likely, and possibly deconstructive cell interaction and merging of cold pools that force more convective initiation and rapid transition to a squall line. The Edwards and Thompson paper you quoted notes the importance favorable shear and instability - and a weakly capped environment!!! So, a strong cap is not mentioned as a favorable characteristic of this case.

<!--QuoteBegin-Gabe Garfield

As far as 0-6 km shear determining storm mode, I can think of at least one exception from '04. Large 0-6 km shear was present in C. OK on May 29th (C. OK was under the mid-level jet), yet the storm mode was high precipitation for a good part of its lifespan.

Again, you are mixing up shear layers again.

0-6 km shear (really should use a mean BL wind instead of sfc) is generally used for discriminating whether you'll get air mass, multi-cell, or supercell convection.

If your BL-6 km shear is favorable for a supercell, then you can look at the shear above this to determine supercell type to be expected. If the 5-10 km shear is small (~< 5 m/s), expect HP storms, moderate values favors classic supercells and large upper level shear favors LP (~> 30 m/s) storms.

Finally, if we want to talk about one of these supercells having a tornado threat as well, then we need to also have large low-level shear (0-1 km shear vector length of at least 10 knots) and low-level parcel buoyancy (0-3 km CAPE > 50). See Rasmussen 2003 for more on this.

Glen

 

[Gabe Garfield]

I'm definitely getting my layers mixed up...LOL. I understand what you are saying. Writing these posts at 1 am is never good!

So, a strong cap is not mentioned as a favorable characteristic of this case.

I am sorry I implied this (if I did). I'm just saying there is typically a more delicate balance that has to be maintained with extreme deep layer shear and extreme instability.

I agree that extreme instability and extreme low level shear is just as good for tornadoes (if not better) than extreme instability with moderate low level shear. Now, regarding the case of extreme deep layer shear and extreme instability, I still maintain that it is better to have only moderate deep layer shear and extreme instability because the winds that create extreme deep layer shear are often associated with much stronger forcing (and thus greater potential for a linear storm mode).

I think that we agree more than we disagree...I failed to specify which type of shear I was referring to.

Gabe