This time of year, researchers, students, tourists and thrillseekers converge on the Great Plains to chase tornadoes. God seems to love them, since not one has been killed yet. The big skies, gridded road networks, flat terrain and photogenic qualities of Plains thunderstorms beckon.
As of midnight CST, the Storm Prediction Center in Norman, Oklahoma predicts a slight risk of severe thunderstorms over the lower Ohio and Tennessee River valleys.
This diary, hopefully, will explain some of the conditions that favor severe summer weather.
One condition that forecasters look for is what we call a steep lapse rate; temperature falls rapidly with altitude. A parcel near the ground nudged upward would thus continue to rise and accelerate. Steep lapse rates over Wisconsin and northern Illinois on Wednesday, for example, triggered severe thunderstorms and hail even though the air near the surface was fairly dry and chilly.
Another favoring factor is the presence of moisture. Moist air is less dense than dry air, assuming equal pressure and temperature, so a parcel of moist air surrounded by dry air would likely rise. More importantly, though, water vapor in the air condenses, thus releasing the energy that it absorbed in the process of changing from liquid to gas. This is called "latent heat release" and it frees energy to perform work (blow things around?) or raise temperatures.
A third favoring factor is vertical shear; the air far above the surface must be moving faster than it is near the surface. What goes up must come down; if you want to maintain a supercell the warm air going up must not be cooled by rain or snow coming down. With enough shear, downdrafts fall ahead of updrafts. Without enough shear, rain cools updrafts, killing them.
Speaking of killing updrafts, one way to do so is called capping. If temperatures rise with increasing height, warm parcels from the ground will not rise. Most nights, the temperature near the ground is cooler than it is a mile up, this is a cap that makes predawn tornadoes rare on the Plains. In some cases, warm air from over the Rockies spreads over the Plains 1-2 miles above ground; this is also a cap.
Typically, daytime heating and perhaps other processes (dry lines, fronts, old boundaries still churning from yesterday's storms) combine to bust the cap; loads of warm moist air rush upward and turn liquid or solid. A storm is born.
[Here my knowledge of just what goes on in the cloud gets sketchy. I know what the circulations within a supercell are, but can't yet explain their causes to an educated lay audience.]
One indicator sometimes used to judge the likelihood of severe thunderstorms is called CAPE, or convectively available potential energy. CAPE combines temperature and moisture at varying levels of the atmosphere into a convenient number that shows some skill at predicting severe weather. In rough terms, you follow a parcel from the ground upward, cooling it in accordance with the laws of physics. At every point, you notice whether your parcel is warmer or cooler than its surroundings. If your parcel is warmer, it's pushed upward and you add to CAPE. If not, you don't. The higher the CAPE, the worse the storm. (An opposing indicator is called CIN, or convective inhibition. The presence of a cap, for instance, is reflected by CIN).
Armed with this knowledge and eighth-grade science (storms near fronts), let's look at a couple maps. The 00Z NAM analysis shows where our front is: the ND/MN border to Central Alabama. (Please note that west of 100 W, 850 mb temperatures are not reliable an indicator of what's happening far enough above the surface to ignore diurnal heating/cooling.) At 7 pm CST, the anticipated CAPE over southern Missouri is tremendous, up to 5800 J/kg. The College of DuPage site (weather.cod.edu) shows 1 pm CST capes maximized at 3000 near Branson, MO The ideal for severe weather is perhaps a bit less -- the 4000 near Cape Girardeau would be better. All the CAPE in the world won't help you build a storm if a cap is in place.
700 mb temperatures in western Missouri as of now are around +4 C, 700 mb temperatures in western Kentucky are around -4 C. My guestimate (taken by following the isobars clockwise about 35*24 or 840 miles) would show 700 mb temperatures around 10 C at 7 pm over W Missouri and -4 over western Kentucky. This means capping is more likely in western Missouri than it is in Kentucky.
So if I were to predict where convection would start, my guess would be the southwest suburbs of St. Louis. Your mileage may vary and be better.