Things are going to get interesting in a hurry across the central part of the country this evening as severe thunderstorms rapidly develop in the moist, unstable air pumping in from the tropics. These dangerous thunderstorms even have the potential to produce a few tornadoes, some of which could be strong and stay on the ground for a while.
The Storm Prediction Center has issued a moderate risk for severe weather—a four on a scale from zero to five—for a chunk of the central Plains from northern Oklahoma to central Missouri. A three out of five (enhanced) risk for severe weather encompasses a larger area from the northern suburbs of Oklahoma City north to Kansas City and east to central Indiana.
The greatest hazards this afternoon are very large hail and tornadoes, some of which could be "significant." A significant tornado is one that produces damage that rates an EF-2 or stronger. Hail could easily grow larger than golf balls in the strongest storms. Above is the probability of tornadoes this afternoon; a 5% risk is cause for concern, so a 15% risk for tornadoes (significant ones, at that) is something that should give residents pause and reason enough to pay close attention to the forecasts for watches and warnings.
The strongest storms also have the potential to produce hail larger than baseballs, so that could also pose a couple of problems.
Today's severe weather will be a pretty standard severe weather event for April. We see warm, moist air pumping in from the Gulf of Mexico, a center of low pressure near the surface with a dry line and a cold front approaching from the west. A dry line is a boundary that separates moist air to the east and dry air to the west—they can serve as the focus for potent thunderstorms, especially in the southern Plains.
You can almost swim through the air in some places. It's not quite July gross, but it's getting up there, especially considering that this is one of the first bouts of sticky weather we've seen this year. Here are the latest dew points, showing the sharp cutoff between humid to the south and drier to the north.
The atmosphere is quickly warming up (and destabilizing) north of Oklahoma City where sunlight is breaking through the cloud cover, and this is crucial for the development of thunderstorms. Strong instability needs to exist for powerful updrafts to form and develop severe thunderstorms, and it's (almost) in place across the area at greatest risk for dangerous thunderstorms.
Above is a model-generated SKEW-T chart for southern Kansas right around now. SKEW-T charts use data collected from weather balloons (or, in this case, from a weather model) to show the temperature and moisture profile of the atmosphere. SKEW-Ts are immensely useful in severe weather situations, since they allow you to analyze how much instability (measured with Convective Available Potential Energy, or "CAPE") there is in the atmosphere. CAPE is like jet fuel that feeds the engines (updrafts) that power thunderstorms.
There's currently a capping inversion (the nerdy term is "convective inhibition," or CIN) in place across much of the risk area—the cap on this particular SKEW-T is circled in yellow. The atmosphere usually gets cooler with height, but an inversion occurs when a layer of air above the surface abruptly warms up. This warmer layer serves as a cap or a lid, preventing air from rising through it, which keeps thunderstorms at bay. Once the air near the surface warms up enough (or a forcing mechanism like a front approaches), the cap can break, allowing air to rapidly rise and thunderstorms to develop in just minutes.
The red-shaded zone on the SKEW-T shows the instability—the long, thick CAPE indicates the potential for powerful updrafts that can foster large hail and tornadoes once they can break through the cap in the mid-levels.
We've seen dozens of reports of severe weather today across areas from Missouri to West Virginia. The greatest risk zone is by no means the only area seeing severe weather today, it's just the most volatile region (and the most interesting to talk about). Even the most marginal severe thunderstorm is dangerous if a tree falls on you or you get caught outside when hail starts to fall, so even if your location isn't expected to see honkin' supercells like those getting ready to blow up over Oklahoma and Kansas, you still need to pay attention and take action if you're threatened by ugly conditions.
It's a classic week of screwed-up April weather. It's interesting in a nerdy sort of way, but dangerous whether you're interested in it or not. Almost everyone between the Mississippi River and the Atlantic Ocean will have to deal with a threat of severe weather through Friday evening, with the threat shifting east each day. Hail, winds, and tornadoes are all possible, so you'll have to watch the weather like a hawk through the weekend. The Storm Prediction Center issues severe weather watches and forecasts, while your local National Weather Service offices issue warnings for individual storms. You can keep track of storms near you by looking at your local Doppler radar; Wunderground's radar services are among the best available for free online.
[Images: author, HPC, GREarth, Twister Data]