A nasty mix of atmospheric ingredients are likely to converge Wednesday and Thursday over the Mid-Atlantic, laying the groundwork for strong to severe thunderstorms in the region. We should be prepared for the possibility of more flash flooding, damaging winds, and perhaps a few tornadoes.
After a reasonably comfortable day today, a warmer, more humid air mass will bubble up over the region Wednesday. A warm front stretching all the way back into the Plains will rest on the northern edge of this heat dome, and thunderstorms will fire along it.
At higher altitudes, a rip-roaring jet stream will energize these storms and transport them from northwest to southeast at swift speeds.
Some storms are likely to develop independently in the most unstable, air developing in the Mid-Atlantic region in the afternoon. These storms could produce flash flooding, damaging winds, and perhaps a tornado or two.
Farther to the west along the front, in the Ohio Valley, storms firing along the front may coalesce into more organized storm complexes, says the National Weather Service Storm Prediction Center:
TENDENCY SHOULD BE FOR ACTIVITY TO GROW UPSCALE INTO CLUSTERS AND LINE SEGMENTS WITH POTENTIAL FOR SIGNIFICANT AND WIDESPREAD DAMAGING WIND AND LARGE HAIL AS ACTIVITY DEVELOPS EASTWARD AND SOUTHEASTWARD.
SPC says it’s possible some of the Ohio Valley storms “survive as they cross the mountains” but stops short of describing this grouping of storms as a derecho. A derecho-type system in the D.C. area still cannot be ruled out (more likely to the west), but it remains a low likelihood and difficult to predict scenario.
Derecho or not, SPC says potential exists in the Mid-Atlantic for “isolated damaging wind and large hail” from the Ohio Valley storms. If they make it, timing would likely be overnight Wednesday.
Overall, the SPC places the Mid-Atlantic in its slight risk zone for severe storms on Wednesday, while areas just to the west are under a moderate risk.
A risk of severe thunderstorms continues Thursday, as a very strong cold front for mid-June collides with the warm, humid air over the region.
Instability will be quite high, with levels of Convective Available Potential Energy likely exceeding 2,000 (see CAPE animation below).
(Note: CAPE levels exceeded 5,000 for the June 29 derecho last year)
Other favorable factors for severe weather include very strong winds aloft (see graphic below) which could be transported to the surface in storm downdrafts, and turning winds with height.
The possible mitigating factor is that storms may come through midday to mid-afternoon, before peak daytime heating (which would energize them further). Even so, ingredients are in place for vigorous thunderstorms with very strong winds, flooding rain, and possibly another chance for a few tornadoes.
Areas just southeast of the District like eastern Virginia, southern Maryland, and the Delmarva likely have the greatest risk of severe weather Thursday.
We will keep you posted with storm updates and any watches and warnings through both Wednesday and Thursday.