Today is the final day of a week-long severe weather outbreak that's produced hundreds of reports of large hail, damaging winds, and a couple of tornadoes across the central United States. The threat for severe thunderstorms is shifting east into more heavily populated areas. A few tornadoes are possible today from western Tennessee through the Washington D.C./Baltimore metro areas.
An enhanced risk for severe weather—a three on a scale from zero to five—exists across the Mid-South from Mississippi through Kentucky, with the greatest risk for severe weather across an area from around Nashville north into central Kentucky. A slight risk for severe thunderstorms (two out of five) stretches into the Mid-Atlantic, with the bustling Washington D.C., Baltimore, and Philadelphia metro areas under the gun for strong thunderstorms tonight.
Let's break the risk down by individual threats.
Strong southwesterly winds at the surface are dragging warm, unstable air into the Mid-South and Ohio Valley, and the combination of instability and wind shear will allow thunderstorms to easily turn severe and possibly produce tornadoes. The Storm Prediction Center is concerned enough that they've issued a 10% risk for tornadoes for central parts of Tennessee and Kentucky, meaning that there's a 10% chance of seeing at least two tornadoes within 25 miles of any point in the shaded area. 10% doesn't sound like much, but that's a pretty beefy risk.
Areas shaded in dark green are under a 2% risk for tornadoes, which is marginal, but the threat can't be ruled out in any of the stronger storms that fire up across this zone.
As such, several tornado watches are in effect along and east of the banks of the Mississippi and Ohio Rivers. The watches are in effect for counties shaded in red on the above map. According to the SPC, there is a high likelihood of at least two tornadoes in the westernmost watch, and a moderate likelihood of at least one tornado that produces EF-2+ damage in both watches.
The agency also warns of the risk for tornadoes from the suburbs of Washington D.C. through southern New Jersey, where a 5% risk for twisters is in place today. Instability across the Mid-Atlantic is pretty low this afternoon thanks to this cloud cover, but any thunderstorms that are able to get going will be able to tap into an ample amount of wind shear and turn severe in a hurry.
If your location goes under a tornado warning, go to the lowest level of the building (preferably a basement) and take cover in an interior room; you want to put as many walls between you and the outside as possible to protect you from flying debris. If you're in your car and a tornado is coming your way, seek shelter in a sturdy building nearby or, as a last resort, drive away from the tornado perpendicular to its forward motion.
The severe thunderstorms we've seen over the past couple of days have been prolific hail producers, with 344 reports of large hail since Tuesday. Many of the reports were for hail larger than golf balls. There was even a report of hail to the size of a grapefruit (4.00 inches in diameter) yesterday in Independence, Kansas.
Large hail is a pretty common sight in the early spring months when there's still enough cold air aloft that thunderstorms have an easy time producing hailstones. Today is no different, and there is a 30% risk for hail from the northeast corner of Louisiana up through central Kentucky. I wouldn't be surprised if we got a couple of reports of hail the size of golf balls, but very large hail needs ample instability (it takes a strong updraft to keep chunks of ice suspended in a storm) and there's still some pretty thick cloud cover across the region.
The risk for damaging winds in excess of 60 MPH is standard during bouts of severe thunderstorms, and today is no different. It doesn't take much to knock down trees and power lines, and wind gusts stronger than 60 MPH can cause building damage. The combination of high winds and large hail can shatter windows and injure those indoors, so even if the thunderstorm is warned for the default 60 MPH winds and quarter-size hail, you should stay away from the windows and ride it out.
Once these thunderstorms blow through tonight, cooler-than-normal air and breezy conditions will take hold for the weekend before giving way to warmer conditions next week. Today will be the last day we see organized severe weather until next week. You can keep up with severe weather watches at the SPC's website, and warnings on individual storms issued by your local National Weather Service office.
[Images: author, GREarth | Updated at 5:01 PM EDT to include the second tornado watch.]