Flooding, thunderstorm risk for Wed. a.m. rush

The timing of the big, bad cold front to huff and puff and blow through the region tomorrow could not be worse. At the moment, the frontal passage looks to occur between 5-10 a.m. from west to east across the metro region.

So what exactly can we expect?

There is fairly high confidence we'll have heavy rain, with model output showing the highest amounts over 2" west of D.C. Likely amounts fall to less than 1" southeast of the District. A Flash Flood Watch is in effect for most of the region owing to the potential for torrential rain over a short time period. Rainfall tomorrow would need to exceed 1.9" at Reagan National and 2.02" at Dulles (both from 1974) to set a daily rainfall record.

What about the severe thunderstorm threat?

The powerful squall line that ripped through the region around midnight two weeks ago served as an important reminder that damaging storms can and do occur at "odd" hours even in their "off" season. You just need the right dynamics in the atmosphere.

The front passing through has some of the same dynamics that ignited the storms of November 17:

1) It's powerful-- driven by even a stronger temperature contrast than 11/17 front. However, while the air is colder behind the front, it is not as warm ahead of the front.
2) The storms will be fed by a strong low level jet -- as high as around 75 knots or about 85 mph - supplying moist, unstable air from the south.
3) There is some turning of the wind with height, or shear, which will help lift the air and may cause a few storms to rotate.

On the other hand, the upper level dynamics don't seem as impressive. There aren't the same, strong upper level winds to suck up the air. In more technical terms, the National Weather Service Office in Sterling writes in its latest discussion

BEST FORCING FOR ASCENT WILL LAG BEHIND SURFACE BOUNDARY TONIGHT AS UPPER WAVE BECOMES NEGATIVELY TILTED AND LIFTS ACROSS APPALACHIANS.

Also, while there are impressive low level dynamics (i.e. the strong low level jet mentioned above), there won't be much instability due to lack of sunlight and heating of the surface. The 11/17 event was able to overcome the lack of instability, but I'm not as convinced tomorrow's event will be able to do so.

Nonetheless, a few thunderstorm downdrafts could flush strong low level winds to the ground sufficient to produce some damaging gusts, especially east and southeast of the District. Also, the turning of the wind could trigger an isolated tornado or two, though the highest risk is well south of the District from Richmond into South Carolina.

The bottom line: expect heavy, wind-driven rain for tomorrow morning's commute and the slight possibility of flooding and severe thunderstorms with damaging winds.