The atmosphere is primed to spawn strong thunderstorms in the Mid-Atlantic this afternoon and evening, some of which could be severe with large hail, damaging winds, and even a couple of tornadoes possible. Since the storms will occur in heavily populated areas around rush hour, even a small, weak tornado could be dangerous.
The Storm Prediction Center has issued a tornado watch for parts of Pennsylvania, West Virginia, Maryland, and Virginia through late this evening. The watch consists of the area in and around the shaded red box on the map at the top of this post.
A tornado watch means that conditions are favorable for the development of tornadoes. The SPC is giving the area a 40% chance of seeing at least two tornadoes in and around the watch area.
The dynamics aren't all that great — definitely on the low end of the scales — but supercells are possible, and some of them could produce tornadoes. Not every tornado outbreaks has off-the-charts instability and rotation like we've seen in the central U.S. a few times this year.
Check out the Energy Helicity Index (EHI) for around 4PM Eastern today. The EHI is an index that takes into account both the instability and about of rotation that a storm can tap into. Higher EHI values indicate a more conducive environment to tornadic supercells. Values between 1 and 2 are sufficient for storms to produce tornadoes, and anything higher than means the environment can sustain strong tornadoes.
One weather model, the Rapid Refresh Model, shows EHI values between 1 and 2, showing that the environment can indeed sustain tornadoes today if the storms are able to tap into the energy and rotation available to them.
(Caution: video contains strong language)
Suffice it to say, it doesn't take a strong tornado to make a mess of things, especially in a densely populated area like the D.C. metro and even more so during rush hour traffic. A similar situation happened a few years ago on I-95 in northern Virginia near Washington D.C. A tornado crossed the interstate during rush hour and pelted gridlocked cars with debris from the nearby woods.
Tornadoes won't be the only threat today. Large hail (larger than quarters) and damaging winds in excess of 60 MPH will be a more common sight than tornadoes, and the two threats are dangerous in and of themselves.
Keep an eye on the weather this evening. Think about your route home and try to find an alternate route if your commute usually leaves you stuck in a traffic jam (let's face it, around D.C., of course it does).
[Images via Gibson Ridge and College of DuPage]