Southern Plains Summer Subtropical Ridge / Cap / Pulse Diurnal Convection

[wxdoofus]

I have a question for those familiar with the patterns of the southern plains.

 

Having lived all my life in the southeast U.S., forecasting in the summer here is fairly straight forward. Partly cloudy with isolated to scattered afternoon thunderstorms. Even when the subtropical ridge is centered directly overhead, we still typically experience at least very isolated pulse diurnal convection (about 20% coverage on an average day when the ridge is overhead). I had always assumed that this was the norm for a subtropical ridge. That's why this graphic of July average thunderstorms came as a shock.

 

 

It was no shock that the thunderstorm numbers were a bit higher within 75 miles or so of the coastal areas due to seabreeze enhancement, but even farther inland in places like Birmingham, AL and Tupelo, MS (too far inland for the seabreeze to really be an issue), there's still isolated to scattered convection each afternoon with any particular location getting caught under one some 10 to 12 days each July on average. I suspected similar results in the southern plains and was surprised to see that much of Texas only averages 2 to 4 thunder days per July.

 

Doing a bit of research, I've noticed that there's often extended stretches of zero POP in Texas and Oklahoma when directly under the subtropical ridge, as opposed to places like Alabama that keep around a 20% POP when they're directly under the subtropical ridge.

 

I first assumed that vegitation may help keep dewpoints lower in some areas and this may be part of the reason, but obviously not the only reason. There's still clearly a much lower number of storms into forested areas of eastern Oklahoma and even up into southwest Missouri where there's not only forested areas but also hilly terrain from the Ozarks (high dewpoints and orographic lifting).

 

Researching further, I took a look at archived soundings and it seems the capping inversion aloft tends to be much more pronounced when the ridge is directly over the southern plains than it is when it's directly over the southeast. I'm assuming this is likely the prime reason for lack of daily diurnal convection in the southern plains?

 

If I'm on the right track here, then I'm curious as to why the capping inversion would be stronger over the southern plains than it is in the southeast when given similar strength subtropical ridges? Is the stronger capping the result of an EML from the Rockies and/or Mexican highlands that gets weaker the farther east away from the highlands it gets (thus a stronger EML over TX/OK under the ridge and weaker EML when that ridge builds eastward into the southeast U.S.)?

 

Would love to hear some insight on this from those experienced with these areas.

 

[OKwx2K4]

I can not really contribute intelligent sounding overly technical knowledge to your question. However having been from Oklahoma my whole life (1/2 20 miles south of OKC and 1/2 in eastern Oklahoma) I'm aware of this and have been since I was a young teenager. We refer to it around here as the "Death Ridge". My best analysis of it would be that although it is most often a humid ridge, (largely dependant upon preexisting conditions, I've seen some of the driest summers of my lifetime beneath it as well) it becomes more of a semi-permanent feature until mid-late August because of surface feedbacks. More or less it transitions to a heat dome and remains so until the western monsoon is over for the most part. It can also remain as long as the jet stream is active over the North-central US and Canada. If you look back to the summers of the mid to late 1990s and early 2000s as well as 2010-2012 it should provide some greater insight to what you are looking for. Www.mesonet.org has detailed daily info on every weather station in Oklahoma dating back to 1994 and you can find it under weather>past data and files>station summaries, if I remember correctly. Good luck with your search! I hope I was of any help at all and more experienced and technical meteorologists on here are more than welcome to correct my mistakes here.

[H2Otown_WX]

I remember having a discussion about this in one of my college meteorology classes. I think it's as simple as the source region of air masses. When you have southerly flow ahead of a low in most of Texas, the air is coming from Mexico and is often very dry. I believe the category for this air mass would be continental tropical (cT). Here is a link explaining the definition: http://www.theweatherprediction.com/habyhints/90/Farther east where the Gulf of Mexico is the source region, the air mass is often maritime tropical (mT) which obviously fuels thunderstorms and would explain why even though places like Birmingham are quite far inland they still receive more thunderstorm days than most of Texas.