* Flash flood watch tonight into Wednesday night *



Water vapor image of eastern U.S. storm system, 1:15 p.m. Tuesday (National Weather Service)

The intense storm system which has spawned swarms of tornadoes in the Midwest and South gradually shifts its vast supply of energy towards the East Coast tonight and Wednesday. Extremely heavy rain may fall over the Washington, D.C. metro area late tonight into Wednesday night. There is also a slight risk of severe thunderstorms and even a few tornadoes Wednesday, especially in the afternoon and evening.

How much rain?

Computer models continue to simulate the potential for 2 to 5 inches of rain across the metro region between today and Thursday morning.

GFS model: 4-5 inches



GFS rainfall simulation through Thursday morning (WeatherBell.com)

High resolution NAM model: 1.5-4 inches



High resolution NAM rainfall simulation through Thursday morning (WeatherBell.com)

Canadian model: 2.5-4 inches



Canadian model rainfall simulation through Thursday morning (WeatherBell.com)

Rainfall this afternoon and this evening should generally be showery, falling at a light to moderate intensity. This rain is simply the primer for the more significant rain to follow.

The period of heaviest rainfall is likely to occur between very late tonight and late Wednesday night.

The rain is likely to cycle through the region in waves, with possible embedded thunderstorms. There is some potential for “training” where heavy rain cells track over the same areas repeatedly. Those are the areas where dangerous flash flooding could occur and localized amounts could even exceed 5 inches. It’s also possible that some areas get passed over by the heaviest rain and see totals closer to 1-2 inches, dodging the flash flood bullet (most likely east of I-95).

The National Weather Service Weather Prediction Center has declared the region under a moderate risk of excessive rainfall Wednesday, meaning there is a 10-15 percent chance of flash flooding in any location.



(National Weather Service)

Here’s a short technical discussion from the National Weather Service discussing the flood risk:

AS WARMER AND MORE MOIST AIR SPREADS NORTHWARD TONIGHT INTO WEDNESDAY…THE THREAT OF HEAVY RAIN INCREASES MARKEDLY. PRECIPITABLE WATER VALUES ARE PROJECTED TO INCREASE TO AROUND 1.75 INCHES…WHICH IS 2-3 STANDARD DEVIATIONS ABOVE THE NORMAL. CONVECTIVE ELEMENTS [THUNDERSTORMS] WILL ALSO BE PRESENT…ACCENTUATING THE HEAVY RAIN THREAT. . . . THIS TRANSLATES TO A FLASH FLOOD THREAT…A STREAM FLOODING THREAT…AND ULTIMATELY EVEN A MAINSTEM RIVER FLOODING THREAT. . . . ALTHOUGH FLOOD POTENTIAL IS HIGH…THE MAGNITUDE OF THE FLOODING IS STILL UNCERTAIN.



GFS model shows extreme quantities of moisture (precipitable water) being drawn up the East Coast Wednesday (WeatherBell.com)

Severe storm and tornado risk

As moist, tropical air streams into the area overnight and Wednesday along a northward progressing warm front, it will gradually dislodge the cooler air presently wedged into the region, increasing the atmosphere’s instability. At the same time, as a strong westerly jet stream enters the region, the amount of wind shear – or change in the wind direction with altitude- will increase. The combination of increasing instability and wind shear may create a favorable environment for strong to severe thunderstorms and tornadoes.

“The Mid-Atlantic needs to prepare for a tornado outbreak tomorrow, with the most likely region extending from central Va. into the Carolinas,” says Jeff Halverson, Capital Weather Gang’s severe storms expert. “Depending on the northward progress of the warm front, a higher threat could extend through the DC-Baltimore corridor. Tornadic activity could focus, in particular, along the warm frontal boundary itself.”

The National Weather Service Storm Prediction Center has placed the region under a slight risk of severe thunderstorms. The D.C. area is on the edge of severe storm and tornado risk as it’s not entirely clear whether enough warm, unstable air will arrive to support their formation.



Risk of severe thunderstorms Wednesday. Yellow shaded area is “slight.”

A model forecast of the Significant Tornado Parameter, a measure of available tornado ingredients, exceeds one over much of the D.C. area – with increasing numbers to the south – suggesting the risk for tornadoes in the region needs to be closely monitored.



Significant tornado parameter forecast at 5 p.m. Wednesday. Black dashed line indicates region where average forecast is greater than one and supportive of tornado formation. Color contours indicate probability of equaling or exceeding one. (National Weather Service)

If it rains for much of the day in D.C. and temperatures do not rise, instability will likely be inadequate for severe storms and tornadoes. But if the warm front successfully punches through and there is a break in the rain and intervals of sunshine, the atmosphere could become extremely volatile.