Strong cap

[Michael Wilkinson]

This forecast sounding is from tonight in Central Arkansas. There was brief heavy rain and lightning.

SRH 0-1, SRH 0-3, wind shear, wind direction changing with height, Surface CAPE, ML CAPE, LCL heights- all looked favorable for a tornado, but even in the “Possible Hazard Type” box, it says “None”. Am I thinking right in thinking that the large cap was the factor that prevented tornado formation? Is it correct that CAPE above the cap is irrelevant if the cap never breaks?

When checking Storm Relative Velocity on RadarScope, there was no rotation anywhere looking at Tilt 1 very close to the radar site (within 10 miles). Does it make sense that we could have heavy rain and lightning without the cap being broken? or does thunderstorm formation absolutely indicate that the cap has been broken?

How far back into basic meteorology classes do I need to go to understand tonight’s setup?

 

[Mike Marz]

I am not very familiar with the particular convective setup and environment that occurred in your area on Friday as I was not really paying attention to that area of the country. That being said, the sounding that you posted above does indeed have a very strong inversion, or cap, to surface based parcels. In order to get a tornado, your storm will need to be ingesting unstable air from the surface. If that particular sounding was verified in your area, than a storm would not be surface based. Thunderstorms can form at different levels of the atmosphere, and are often elevated above the cap. This means that they are drawing their energy from above the inversion layer. So you can have thunderstorms with a capped environment, this just means that these storms are not ingesting energy from the layer nearest the ground, and they can therefore not take advantage of the wind shear in that sounding.

 

[Michael Wilkinson]

Thank you, Mike That helps. Now, I can see that all the wind barbs above the cap are pointing the same direction. It makes sense because that was the direction the storm was moving, and the Storm Relative Velocity on RadarScope was showing no rotation.

 

[Jeff Duda]

I wouldn't really consider the sounding as having a cap in the typical sense. A "cap" is usually considered to be an elevated warm layer above the top of the daytime PBL. While there is an elevated warm layer in that sounding, it is atop an extremely shallow surface layer, which likely does not represent a typical daytime PBL. The T/Td profile near the surface looks more like a nighttime radiational cooling signature, which, given the valid, time makes sense as that sounding is valid in the evening.

Regarding the possible hazard type issue, the SHARPPy program has an involved decision tree that is used to determine a classification. My guess is that the sheer amount of CIN (whether surface-based or mixed-layer) was high enough to consider severe weather largely unlikely. While it is not obvious from simple examination of the sounding, I can tell that there is elevated CAPE (i.e., parcels originating well above the surface - and above the warm layer - that possess substantial instability), so it does make sense that there were storms. However, those storms would not have been accessing as much of the instability or the shear below their respective source layers and thus much of the shear in that hodograph doesn't really represent the environment an elevated storm would have been subjected to. Thus I would not have expected too much in the way of severe weather from a storm occurring in such an environment (although it cannot be ruled out).