Dynamics or Instability?

[Jeremy Lemanski]

What would you prefer if you had to choose? Example: Would you want a day with 750 or less of cape, but with huge amounts of shear/jet/etc. such as the case in early spring or late fall systems. OR would you take a day with Capes of 6,000 with only around 15-20 knots of 0-6KM Shear.

I have thought of this before and have seen both cases bust and be a huge event. I think i would perfer a low instability/high shear event over high instability/low shear event, but it is a hard choice.

What would you rather have?

 

[Scott Olson]

I'd take the shear setup because i've had setups like that produce for me. High instability with weak wind shear tends to usually be MCS/Pulse storms. Of course if there any sort of boundary to work with I'd take the high instability setup.

 

[Jeff Snyder]

What would you prefer if you had to choose? Example: Would you want a day with 750 or less of cape, but with huge amounts of shear/jet/etc. such as the case in early spring or late fall systems. OR would you take a day with Capes of 6,000 with only around 15-20 knots of 0-6KM Shear.

I have thought of this before and have seen both cases bust and be a huge event. I think i would perfer a low instability/high shear event over high instability/low shear event, but it is a hard choice.

What would you rather have?

LOL I cant probaby tell you which scenario would likely receive the higher SPC convective categorical risk -- high shear / low instability. We see that several times a year in the early spring (as you noted) in the southern/southeastern US. I can think of a few MDT risk days (maybe even a HIGH or two) on which there was very limited CAPE (~500) and strong deep-layer and near-surface shear.

That said, I've found these types of chase days to be very difficult to chase. Heck, we had several low-instability / high shear days in April in the southern plains, and I ended up empty-handed in April. I think one of the keys to these days are finding the areas of strongest low-level shear, namely by locating baroclinic boundaries (yay to baroclinically-generated vorticity).

On the other hand, I'm not sure I'd like any 15-20kt 0-6km deep-layer shear day. That's really on the too-low end (for the most part)... 25-30kts? Yeah, I suppose, since that's more likely to support a supercell than 15-20kts.

So, in the end, I guess I'd opt for high CAPE / lower shear days given that you can also end up with awesome structure. Sure, you probably won't get long-lived tornadoes, but a few intense, short-lived tornadoes are certainly possible given favorable near-surface shear and bouyancy profiles.

 

[Jeremy Lemanski]

On the other hand, I'm not sure I'd like any 15-20kt 0-6km deep-layer shear day. That's really on the too-low end (for the most part)... 25-30kts? Yeah, I suppose, since that's more likely to support a supercell than 15-20kts.

The reason i used the 15-20 knot shear idea is because i happen to be reading APX's discussion earlier and they talked about how cape would be near 2,500-4,000 today, but with meger shear of 20 knots or less, severe storms were unlikely with the lake breeze today. So, that gave me a pretty good idea at how low i should go in this question.

I still really don't know what setup i would rather have. I remember one sept. day a couple years ago, tornado watch out for a squall line ahead of a strong cold front. It was cloudy all day and i think temps in the low 70s if that. There wasn't any thunder with the line, and winds of only 40mph. No warnings anywhere that day. That was a classic low cape bust. But then the days when there is high cape there are busts also. I guess that is what makes chasing/forcasting fun for me, you never ever know what the day could bring.

 

[Glen Romine]

In short - I don't think I'd ever try to make a chase decision based solely on 2 pieces of forecast information (forecast CAPE and shear). That said, models are generally more reliable with forecasting shear than instability - so allowing a range of model error the expected 15-20 knots of shear is unlikely to be overcome, whereas a meager instability forecast could quite easily be way off, in either direction. If the forecasts are both accurate, neither day sounds very promising.

Glen

 

[Jarrod Cook]

In short - I don't think I'd ever try to make a chase decision based solely on 2 pieces of forecast information (forecast CAPE and shear). That said, models are generally more reliable with forecasting shear than instability - so allowing a range of model error the expected 15-20 knots of shear is unlikely to be overcome, whereas a meager instability forecast could quite easily be way off, in either direction. If the forecasts are both accurate, neither day sounds very promising.

Glen

I agree with you here. A good example of low CAPE/high shear was the April 20 2004 IL/IN tornado outbreak. SPC and NWS didn't expect much with CAPES around or lower than 500, but with the warm front that came through with some afternoon heating, mini supercells erupted everywhere. 8 people in Utica, IL died from an F3 that day. I remember that day well since I was caught offguard just like everyone else.

More on that day can be found here: http://www.crh.noaa.gov/lot/science.php

 

[Shane Adams]

I would definitely prefer the instability over dynamics anyday. I've chased enough storms in 1000 CAPE/70kt h5 flow to last the rest of my life. Dynamics mean squat if the rest of your environment can't handle them. A huge sweeping, Spring time trough in a modest instability axis spells storms being ripped apart before updrafts can even root. And they move so fast by the time you realize you're wasting the day, you're in another state.

With extreme instability and little dynamics, you at least have a trigger. The life of any storm might be limited, but there's early potential for incredible development. I'm not sure of this, but I believe the Jarrell, TX event was a high instability/low dynamics day. The storms propogated SWerd along a boundary and the tornadoes stood still or drfited slowly southwest at less than 5mph. But yet they were violent in some cases.

I've always had better luck with the instability.

 

[Mike Peregrine]

For the most part, I feel like I also prefer the days with higher instability (I absolutely love it when the "extreme instability" language starts showing up in outlooks!) ... but of course would never discount days when dynamics are the stronger player because they can be interesting too.

Of course, Glen is correct when he states that these are not the only variables to be so concerned with when thinking about a chase, and putting faith in the models' ability to accurately depict either can also be tricky. But when it comes to raising my excitement and adrenaline level in just thinking about the possibilities, I'd have to say that the high instability days of late May and early June are the ones that get my blood pumping.

 

[David Draun]

The Plainfield F5 on August 28, 1990 was a high CAPE/low shear event, if I remember correctly.

 

[Jeremy Lemanski]

I believe the Jarrell, TX event was a high instability/low dynamics day

Correct: http://www.srh.noaa.gov/srh/topics/attach/...ml/ssd97-31.htm

Second link is an interesting read i found while searching for high cape/low shear tornadoes: http://members.cox.net/jondavies2/nonmesot...casestudies.htm

 

[Gabe Garfield]

One thing that you can't forget is that shear is storm-relative. Ambient shear might be quite weak, but if you get a baroclinic boundary or some other mechanism of enhancing shear (i.e. deviant storm motion), good things can result given sufficient instability. For this reason, I'd almost always opt for the days w/ higher instability and at least the hope of a slightly stronger shear environment.

Gabe

 

[Glen Romine]

Well, if we are going to try and extract every possible caveat - then heck let's just say any environment regardless of forecast CAPE or shear might support a significant event (if mesoscale or microscale conditions make the storm environment vastly different than the synoptic environment). Or, we could answer the question as asked, instead of trying to find every possible exception. Florida routinely sees 15 knots of shear during the summer, high instability and boundaries owing to sea breezes - but I don't hear of anyone planning their chase vacations around it. Maybe it's not as simple as just having instability and a boundary and trusting they will interact to make something interesting. Hmm.....

Glen

 

[Anthony Silver]

I tend to favor the high instability scenario (especially during the spring months). The higher the CAPE values, the better chances you'll have at explosive thunderstorm development, especially if there's a lifting mechanism nearby (surface boundary, outflow boundary, low pressure system, etc). Obviously, if there's a strong cap in place above the aforementioned setup, the chances of initiation will be little to none, but i'd still target the area. However, I wouldnt turn my back on the low instability, strong shear setups, especially during the summer and fall months. I've seen some awesome storms with this scenario in the past, particularly in Mid-September/Early October.

 

[Shane Adams]

Well, if we are going to try and extract every possible caveat - then heck let's just say any environment regardless of forecast CAPE or shear might support a significant event (if mesoscale or microscale conditions make the storm environment vastly different than the synoptic environment). Or, we could answer the question as asked, instead of trying to find every possible exception. Florida routinely sees 15 knots of shear during the summer, high instability and boundaries owing to sea breezes - but I don't hear of anyone planning their chase vacations around it. Maybe it's not as simple as just having instability and a boundary and trusting they will interact to make something interesting. Hmm.....

Glen

I can't comment on Glen's post because I'm not a meteorologist. Basically, if the choice is higher shear/lower CAPE or vice versa, I'll take the higher CAPE days. I can't even begin to explain it in mesocale or synoptic scale terms, but I can find the tornado, which is all I'm worried about.

It's not easy for some people to discuss (in person or on paper) how we chase, how we know where to go, and how we come to the preferences we have. I understand there's more to it than a simple shear/CAPE question, but don't ever expect me to write about it.

 

[Steve Miller TX]

Definitely the high CAPE days. Not 2500-4000 though with only 15-20 knots of shear. Gimme 5000 or more and I'm elated because I know I'll see some severe storms if they do form...no doubts about it. If there is a boundary around relatively parallel to the mean flow, then I'm taking the day off work. I love severe thunderstorms whether they produce a tornado or not. If I'm going to spend the time and money to chase, I want a good storm and a high CAPE day pretty much guarantees that (if the cap busts...lol!)

I've had MORE bust days with low CAPE and high shear simply because the updrafts tend to get "decapitated" if the CAPE is too weak. I can remember several high risk days that went "pffft" because of this...even with SREH of 500+ and favorable mid/upper dynamics. Also, high shear to me means storms hauling ass that are difficult if not nearly impossible to stay with for any period of time. I just think the odds of getting a good storm are much much less.

As far as days that were supposedly only 500 CAPE and strong shear, I'd be willing to bet that some localized/mesoscale CAPE of 1000 or more was around....either surfaced based or mean layer. Just my suspicion. ;-)

As was mentioned, I can count many high CAPE (5000+) days with only 15-20 knots of 0-6km shear......but favorable *storm relative* shear/winds (read some of Rich Thompson's case studies on this). High CAPE days will see storms usually moving more devient to the mean flow or even becoming stationary. Anybody remember one of the events in early June in SW OK? I can't remember the exact day, but I'm driving into Kansas for the next day's chase while watching that beauty on radar barely inching southward. You could pulled up a lawn chair and watched it. Of course, Jarrel is an example...so was the awesome Lake Whitney tornado a few years ago. Those are just a few prime examples. And, gosh, how many outrageous Nebraska tornadic supercells have we seen with high CAPE and low shear? I've seen enough to know that I'd like to retire there....LOL!!

Basically, I believe that the "ratio" for impressive supercells with high CAPE/low shear is much higher than that for low CAPE/high shear days.