The first day of a two-part severe thunderstorm event is unfolding across the middle of the country, with an enhanced risk for severe weather—three on a scale from zero to five—possible in Oklahoma, Arkansas, and Missouri. Storms could produce very large hail, damaging winds, and a few tornadoes.
The Storm Prediction Center just issued its fifth severe weather watch of 2015, which is unprecedented in the 45 years the agency has kept records of its severe thunderstorm and tornado watches. We usually see more than 50 watches issued across the United States by this point in the year, a testament to the cool, stable airmass and unfavorable tracks storms have taken as they've trekked across the country.
Counties shaded in blue are under a severe thunderstorm watch as of 3:00 PM CDT, and they'll remain under the watch until severe thunderstorms exit the region and the danger has diminished. The risk for severe weather isn't limited to the watch area—if you live around the watch, you should keep an eye on the weather, as well.
Why are we seeing severe weather today? As shown in the poorly-annotated map above, a strengthening low pressure system is getting its act together over southern Kansas this afternoon, dragging warm, humid air into the central Plains from the south behind a warm front. The clouds cleared out of the region from south to north this afternoon, allowing temperatures to rise into the upper 70s and low 80s across the area at risk for severe weather. The combination of daytime heating, moisture, and lift along the cold front will spark some thunderstorms along and east of the I-30 corridor.
There's enough wind shear (wind changing speed and direction with height) that any storms that do form across the risk area have the potential to turn into supercells. The High Resolution Rapid Refresh (HRRR) model does a good job of showing a squall line with embedded supercells as it moves across Oklahoma, Missouri, and Arkansas. The model simulated radar begins at 2:00 PM CDT and ends around 9:00 PM CDT.
The agency issued the enhanced risk for severe weather this afternoon as a result of the 30% probability for hail that exists in cities like Joplin, Springfield, and Fayetteville. Hail could reach two inches in diameter (larger than a golf ball) in the strongest storms.
A 15% risk for damaging winds (58+ MPH) exists over roughly the same area. Some of the storms in and around the severe thunderstorm watch may produce isolated wind gusts up to 70 MPH, which could easily damage trees, power lines, and toss loose objects scattered around your house.
It's also worth mentioning that there's a marginal risk for tornadoes in any of the storms that form across the risk area. There's enough moisture present that any storm that gets some good rotation on it could produce a brief tornado. Even a small, weak tornado can pose a threat to life and property, so pay attention to the weather as you go about your evening.
A similar situation will unfold a bit farther to the southwest during the day on Wednesday, with large hail again serving as the dominant type of severe weather. Much of central Oklahoma will see the risk for hail two inches in diameter or larger, which is the size of a hen egg, or between a golf ball (1.75") and a tennis ball (2.50").
The National Weather Service issues warnings for individual thunderstorms, while the Storm Prediction Center handles the overall risk for severe weather. You can keep tabs on your local radar using Wunderground's great radar tool.
[watch/analysis/model maps: GREarth | risk maps: author]