Thunderstorms Creating Their Own Wind Shear

Mike Z

EF1
Sep 11, 2017
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I read something that said the Jarrell, Texas tornado formed in an area with next to no shear. It said the updrafts were so strong ( extremely high CAPE that day ) that the storm created it's own shear.

How rare is that?
 

James K

EF2
Mar 26, 2019
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Colorado
I don't remember where it was, but somewhere I remember reading that strong storms can do that with the right conditions.
 
Jun 16, 2015
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I'm not an expert on low shear/high CAPE setups, but a few thoughts:

There seems to be a misconception that some of the most historic violent tornadoes have occurred in an environment with extreme instability and little to no shear. This is not completely true. It is fairly common to hear about strong/violent tornadoes in a "high CAPE/low shear" environment, but by low shear, we're still generally talking about at least marginally favorable shear for supercells.

I looked at NARR data for the Jarrell, TX tornado event and while shear was not overly impressive, it wasn't "next to no[ne]."

As a general rule, I like the benchmark of 30 knots of deep layer shear for supercells. It's hard to get them with less than 30 knots of shear and I would consider 30-35 knots marginally favorable. The NARR data suggests that 0-6km shear was in the 30-35 knot range around Jarrell between 18z 5/27/97 and 00z 5/28/97. We don't have the resolution to see if shear was even more substantial on the smaller scale. Note that mesoscale boundaries can and often do enhance shear on smaller scales than mesoscale.

Now, it is true that large buoyancy can make up for marginal shear, but storms can't make their own shear if there's little or no shear to work with, at least not beyond a brief pulse, meaning, not for an extended period of time. Think pulse/mult-cell convection. Long-track and long-lived supercells/tornadoes are usually associated with substantial deep layer shear, and/or a boundary that a storm can ride along. Pulse/multi-cell convection can occasionally produce briefly large hail, especially if there is strong/extreme instability in place.

Jarrell had a track length of approximately 5 miles. If we were talking about a much longer lived violent tornado in the presence of limited background shear, then it would be more noteworthy.

If you look at high resolution models, it is not uncommon for the HRRR, as an example, to blow up a small area of substantially stronger shear than the "area averaged" shear in proximity of a (forecast) supercell. I've noted this several times this year, especially across the High Plains and in areas in which directional shear is substantial, but deep layer shear is more marginal. This is why you can "cherry pick" a forecast sounding that is convectively contaminated and find parameters that are not representative of the larger scale environment. Even in these setups, high resolution models rarely predict sustained supercells in an environment where there is sub-marginal shear.

I'm not sure I really answered the question, as I'm not an expert in this area and I think I jumbled a bunch of ideas together...

Main points:
-Thunderstorms can briefly pulse up to severe levels in environments with little to no shear
-While this is true, storms in this situation are usually short-lived and often fairly disorganized
-Large buoyancy (CAPE) can offset marginal shear, but in order for organized, intense convection, you still need at least marginally favorable shear
-Small scale boundaries can locally enhance shear, which may help storms become severe in a larger scale environment that seems less favorable
 
Mar 8, 2016
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Bloomington, IL
I wouldn't say Jarrell "created its own shear". Jarrell moved extremely deviant to the 0-6km shear vector due to boundary interaction, which resulted in more effective shear than for a storm following the mean wind. Looking at modified FWD soundings for that day, there was actually a decent bit of effective shear already in place in the order of about 45kts or so which is plenty for Supercells, especially with such extreme instability in place. What was lacking was low level shear, which the deviant motion to the Southwest along the boundary likely made up for.
 
Agree with Devin - we have to consider the storm-relative flow, and the effects of boundaries and cold-pool generation can have the updraught moving at a highly deviant motion, which can result in quite large storm-relative shear.

On days like May 24th, 2016 ('Dodge Day') the effect of a storm 'tacking-on' to a boundary helped with what was, overall, not 'great' background shear to create the tornadofest. Similarly on the next day around Chapman, KS.

Big CAPE helps, of course, just down to the higher thermodynamically-induced vertical accelerations.
 
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Dave C

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Jun 5, 2013
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Looks like everyone covered the main point; relative shear is the main factor.

This is reason to pay attention to right moving storms for enhanced tornado potential. They are, with the typical S or SE low level winds and SW uppers, experiencing more relative turning with height when they turn right. Same is true for any storm that latches onto a boundary- it may be obtaining better shear or updraft maintenance relative to other local environs.

Simla, Dodge City, Jarrell, Chapman, and countless others are all great examples of the difference between no tornado, and monsters or cyclic producers.

Shear is one of the mechanisms for sustaining supercell updrafts and good turning with height often makes tornadoes likely, but it is good to remember we are still working on tornado formation. Focusing on shear, or CAPE or combinations of ingredients doesn't always tell us precisely how they come together in dynamic environments to create a tornado. Some people call theseJarrell type surprise storms mesoscale accidents- I attribute it with things we just don't know yet, or cannot observe due to resolution of our tools. Some of the supercomputer simulations by Leigh Orf have accurately modeled real storms and shown very interesting features and mechanisms at 30 down to 10 meter resolution, much more than forecast tools provide.
 

Dan Robinson

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The other factor to note is that non-supercell processes were at play in Jarrell and the May 19, 2012 Rago, Kansas event. In those two examples, deep-layer shear was on the weak side, but the vertical stretching on the boundary in an environment with strong 0-1km CAPE/low-level lapse rates was the main mechanism for tornadogenesis rather than the standard supercell/RFD process.

Jon Davies has a good writeup on this here:

 
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Jun 16, 2015
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The other factor to note is that non-supercell processes were at play in Jarrell and the May 19, 2012 Rago, Kansas event. In those two examples, deep-layer shear was on the weak side, but the vertical stretching on the boundary in an environment with strong 0-1km CAPE/low-level lapse rates was the main mechanism for tornadogenesis rather than the standard supercell/RFD process.

Jon Davies has a good writeup on this here:

Low-level lapse rates are important and often overlooked in severe weather setups.

An environment with large CAPE/high shear, but poor low-level lapse rates is less favorable for tornadoes than a large CAPE/MRGL shear environment with steep low-level lapse rates.

Today is a good example in the Dakotas, where there has been very little convection in an enhanced risk/10% tor area. Part of that is arguably due to marginal low-level lapse rates, despite severe weather parameters (like SCP) off the charts.
 
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Jeff Duda

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Oct 7, 2008
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I wouldn't say Jarrell "created its own shear". Jarrell moved extremely deviant to the 0-6km shear vector due to boundary interaction, which resulted in more effective shear than for a storm following the mean wind. Looking at modified FWD soundings for that day, there was actually a decent bit of effective shear already in place in the order of about 45kts or so which is plenty for Supercells, especially with such extreme instability in place. What was lacking was low level shear, which the deviant motion to the Southwest along the boundary likely made up for.
Agree with Devin - we have to consider the storm-relative flow, and the effects of boundaries and cold-pool generation can have the updraught moving at a highly deviant motion, which can result in quite large storm-relative shear.

On days like May 24th, 2016 ('Dodge Day') the effect of a storm 'tacking-on' to a boundary helped with what was, overall, not 'great' background shear to create the tornadofest. Similarly on the next day around Chapman, KS.

Big CAPE helps, of course, just down to the higher thermodynamically-induced vertical accelerations.
Be careful with the term "storm-relative shear" or "effective shear due to deviant motion". Technically, the only shear property that is dependent on storm motion is storm relative helicity, which is not the same thing as bulk shear. But I agree with the broader message that the deviant motion allowed for more significant vertical vorticity generation.

Shear is one of the mechanisms for sustaining supercell updrafts and good turning with height often makes tornadoes likely, but it is good to remember we are still working on tornado formation. Focusing on shear, or CAPE or combinations of ingredients doesn't always tell us precisely how they come together in dynamic environments to create a tornado. Some people call theseJarrell type surprise storms mesoscale accidents- I attribute it with things we just don't know yet, or cannot observe due to resolution of our tools. Some of the supercomputer simulations by Leigh Orf have accurately modeled real storms and shown very interesting features and mechanisms at 30 down to 10 meter resolution, much more than forecast tools provide.
Thank you! I agree very strongly and despise the use of the term "mesoscale accident". To me it's like attributing some other mysterious occurrence as an "act of god". No...it's just something you (or the entirety of humanity) has yet to figure out or explain.

The one thing I haven't seen mentioned here so far is the vorticity equation. It allows one to compute a budget to determine where the vorticity that makes any given vortex (whether a dust devil, tornado, mesocyclone, or synoptic scale cyclone) comes from. I agree that a given storm cannot create its own shear to create a monster tornado. The storm-scale outflow certainly can play a role in enhancing what's already there, but if there wasn't at least some degree of meso-gamma-scale vorticity present before the storm formed, then the storm wasn't going to generate that out of whatever other vorticity was present. If that storm didn't have enough shear to develop any sort of storm-scale rotation/mesocyclone, that tornado would likely not have formed because there would have been too complicated of air flows near the ground to promote an intense tornado, let alone any sustained tornado-scale vortex.