Rainfall expected by the Weather Prediction Center from the storm through early Saturday.

UPDATE: The article below was last updated Wednesday afternoon. A new outlook on Friday’s storm potential, updated Thursday, is available here: Heavy rain to visit D.C. Friday afternoon and night with possible flooding. A few storms may be severe.

From Wednesday...

The D.C. area is facing a dual threat of strong to severe thunderstorms and heavy rain to close the workweek.

A risk of flooding and the threat for damaging winds from storms are the primary concerns for Friday into Friday night, but an isolated tornado can’t be ruled out. For now, the time of most weather trouble seems likely to focus from Friday afternoon through the evening, but the timing may shift a bit.

This is all thanks to an unusually strong storm system developing in the central U.S. The storm is predicted to produce severe weather in the Plains on Wednesday and the South on Thursday. It will become increasingly well organized as it heads toward the East Coast.

Much of the Washington region is under a “slight” risk for severe storms Friday, which is level two out of five in outlooks produced by the National Weather Service Storm Prediction Center. An “enhanced” risk (level three) is outlined for parts of Southeast Virginia and the eastern Carolinas, where a more widespread severe weather event is anticipated.


The Storm Prediction Center's severe weather outlook for Friday. (NWS)

“Bowing line segments capable of damaging wind gusts appear to be the primary severe threat although tornadoes will be possible as well,” the Storm Prediction Center writes in its latest discussion for Friday.

While the worst of this severe weather episode may remain south of Washington, this region can expect the potential for several rounds of storms.

Capital Weather Gang’s severe weather expert Jeff Halverson says everyone should closely monitor the forecast for Friday but also said certain factors could limit the extent of severe weather. Chief among them are limited instability because of cloud cover and relatively warm temperatures at high altitudes

It may be that heavy rain and some flooding ends up the bigger story locally, thanks in part to unusually high water content in the air Friday into Friday night.


The atmospheric water content, as indicated by a measure known as total precipitable water, is projected to be unusually high Friday afternoon. (WeatherBell.com)

The National Weather Service’s Weather Prediction Center writes atmospheric moisture levels will be extremely high (two to three standard deviations above normal).

Although it’s difficult to pinpoint where the heaviest rainfall will occur, “deep southerly flow . . . with a connection to the tropics” presents the potential for at least one to two inches of rain, the Weather Service office serving the Washington region says.

Halverson believes there’s a good chance of widespread one-plus-inch totals, but that a couple factors could cap how much rain ultimately falls. Strong thunderstorms focused to our southeast may impede the transfer of moisture north. In addition, the biggest rain events this time of year tend to have high pressure to the north, which stalls their forward progress allowing them to dump rain over an extended duration. While somewhat of a slow mover, this storm will not stall.

The prospect of another soaking rainstorm follows a waterlogged past year. Washington has seen somewhat of a rainfall reprieve so far this April with below-normal amounts. Even so, the ground remains wet. Any heavy rain falling in a short amount of time could cause pockets of flooding in poor drainage areas and streams. If we pick up several inches, the flooding would probably end up more widespread.


An area of extremely strong wind aloft moves by on Saturday. (Weatherbell)

Save for some lingering on-and-off showers into late Saturday, the bulk of the rain should be over by early Saturday morning. That’s when area weather geeks will eagerly await the Saturday weather balloon launch from the Weather Service

Winds over 140 mph are expected to be pushing through at an altitude of around 18,000 feet, which would challenge record highs for April and the warmer months of the year. These kinds of values are typically only recorded in the winter when the jet stream is most powerful.

With all that motion aloft, it will be windy down at the ground, too. Strong winds, gusting over 30 mph, are predicted to crank up late Friday and continue into Saturday.