22nd June - lack of storm longevity around SE WY / Western NB

[Cammie Czuchnicki]

Hi all,

I am out here on my first chasecation from England. We have been here for just over 2 weeks now and seen some amazing weather. We've tracked a number of discrete supercells and watched as they've upscaled into huge MCSs.

However yesterday we were on the cell that fired north of Cheyenne and tracked it east toward the NB panhandle. It wasn't long before it started dissipating at a time when we were expecting it to really get going. It's top just seemed to lower and it blew itself out. This seemed to be the case for anything emerging from this area of SE Wyoming. We even bumped into a couple of other chasers from the Scottsbluff area who were also at a bit of a loss about what to do and informed us that obviously something was missing.

We decided to call it a day around 6 o'clock local mountain time. Whilst we were travelling east along the I80 to prepare for today's set up we saw another incredible cell develop to our South, around Hayes county. The rate at which it grew was astounding and we deviated to go and have a look. Again, before long, it's top just fell. We could even see areas near the top of the storm where the clouds looked as if they were falling under their own weight.

This type of scenario is something we haven't experienced much in our time out here and I was just wondering if anyone had any ideas what was missing yesterday around that area. I'm sure it's not a straightforward answer and there could be a number of reasons, but I thought I'd see if anyone had an idea why this may have happened.

Many thanks in advance,
Cammie

 

[Dean Baron]

At first I would say maybe they were firing above a low level cap, but if their development was explosive then they were probably tapping in to low level moisture/instability. Although I wasn't paying attention to the dynamics or storms out there yesterday, I would say upper level winds were probably lacking causing the downdraft to fall back on to the updraft. But that is just a guess. One other thing that just popped in to my head is maybe the terrain? Storms fired off higher terrain but then moved off the higher terrain making it more difficult to tap in to surface moisture? Again these are just guesses...

 

[Matthew Grzych]

It was probably due to cap strength and lack of forcing. The initial storm went up in WY on high terrain and within a well mixed environment where the cap was broken. However, as it moved off the high terrain and into NE where the cap was stronger, with a lack of strong surface convergence and/or upper level forcing the cap eventually pinched off the updraft. I think the southern storm later in the evening went up on a boundary (warm front/outflow). Once it moved off of the boundary, again due to the lack of low level and upper level forcing, the cap won out.

There was plenty of shear and instability out there yesterday to support supercells. There was just a lack of large scale forcing. If there had been stronger upper level forcing and low level upslope flow, storms would have lasted longer.

 

[Andrew Stoller]

Watching the storms yesterday and the day before, the shear profile I think wasn't quite conducive to really good updraft ventilation, and the storms quickly became outflow dominant. You could see on the radar that once they got going, an outflow gust front quickly showed up on radar moving out ahead from the storms. Sometime too much directional shear and not enough speed shear will promote HP/outflow dominant storm modes.

When you see a nice balanced classic supercell on radar, you'll hardly see much of a forward flank gust front moving too far ahead of the storm, and the RFD gust front will tightly trail the southern flank of the storm and won't move out ahead of the storm while the updraft has good ventilation and it's not going outflow dominant.

 

[Matthew Grzych]

Watching the storms yesterday and the day before, the shear profile I think wasn't quite conducive to really good updraft ventilation, and the storms quickly became outflow dominant. You could see on the radar that once they got going, an outflow gust front quickly showed up on radar moving out ahead from the storms. Sometime too much directional shear and not enough speed shear will promote HP/outflow dominant storm modes.

When you see a nice balanced classic supercell on radar, you'll hardly see much of a forward flank gust front moving too far ahead of the storm, and the RFD gust front will tightly trail the southern flank of the storm and won't move out ahead of the storm while the updraft has good ventilation and it's not going outflow dominant.

Take a look at the 00 UTC LBF sounding. There's good turning from the surface (ESE) to 500mb (SW at 50 kt). It actually looks like a great profile for classic supercells. 50 kt at 500mb & 90-100kt at jet level. That's not typically an HP wind profile. Even with storm motions of 30 kt, rain should have been lofted downstream quite a ways. Strong surface flow was lacking out there however. One thing that likely happened is that the storm fired in a relatively dry environment in WY where lots of evaporatively cooled downdraft air was generated initially. I did see a decent outflow just ahead of the storm, however, the storm's updraft kept up with the outflow. Typically this would cause storms to grow upscale from a classic supercell to HP, and then bow structure (as the strong outflow just keeps forcing renewed convection along the surging boundary). But yesterday there was just too much cap (the LBF sounding had -125 CIN) and no real sign of a low level jet to force storms either. Here in GXY the outflow/cold front surged through just before dark and we had a surface observation of 80/61, but not even a cloud formed along the boundary convergence. The cap was just overly strong yesterday.

 

[Cammie Czuchnicki]

Cheers guys - your help is much appreciated.