5:20 p.m. update: Some good news – radar shows a break in the rain south of I-70 in Md., covering much of the D.C. area. However, more rain lurks to the south around Richmond, which may stream north later this evening. This is the last update in this post. For the latest forecast, see: PM Update: The rain train continues into Friday; a square cloud???
4:20 p.m. update: A flash flood warning is in effect for western Frederick County until 7:15 p.m., and includes the city of Frederick and Thurmont. Around 1 inch has fallen in the warning area and another 1 to 2 inches is possible.
3:58 p.m. update: A flash flood warning has been issued for south central Frederick County, east central Loudoun County, and west central Montgomery County until 6:30 p.m. Around one inch of rain has fallen in this area and another 1 to 2 inches of rain is possible. Locations affected include Ashburn and Poolesville.
3:31 p.m. update: A flash flood warning has been issued for central Loudoun and southwest Frederick (Md.) counties until 6:30 p.m. About an inch of rain has already fallen in this area and an additional inch or two is possible. Leesburg is included in this warning. Remember never cross a flooded road in your car. Turn around, don’t drown.
Overview – from 2:04 p.m.: Good news first: The risk of damaging winds, large hail, and severe lightning is low this afternoon and evening. Unfortunately, however, waves of rain are likely to cycle through the region. Heavy downpours and localized flooding are, once again, possible.
A flash flood watch is in effect through this evening. From the National Weather Service:
A VERY MOIST AIRMASS HAS SETTLED OVER THE REGION TODAY. ADDITIONAL SHOWERS AND THUNDERSTORMS ARE LIKELY TO DEVELOP THIS AFTERNOON INTO THIS EVENING. SOME OF THE STORMS WILL CONTAIN VERY HEAVY DOWNPOURS WHICH MAY LEAD TO FLASH FLOODING ESPECIALLY OVER AREAS THAT RECEIVED HEAVY RAINFALL YESTERDAY. HOURLY RAINFALL RATES WITHIN THE STRONGER THUNDERSTORMS MAY EXCEED ONE TO TWO INCHES PER HOUR.
The areas most vulnerable to heavy rain are those which have been hammered the last two days around College Park and Potomac in northern Prince George’s and southern Montgomery counties. But, since the beginning of spring, it has been very wet across most of the region, so the amount of rain needed to cause flooding is less than it would be otherwise (except in our far southern areas, where less rain has fallen recently).
Average rainfall amounts will probably in the range of 0.5-1.5 inches, but localized amounts could easily be more or less. We don’t see this as a widespread flood threat as the bands of rain streaming in are relatively narrow. But, given the possibility of training – heavy rain cells moving over the same areas repeatedly – flooding could certainly materialize in small pockets.
The timing for rain extends from the current time through around 7-10 p.m. – after which the coverage and intensity of rain should diminish.
The driver for today’s heavy rain is very moist air flow from the south and southeast.
The area is wedged between the clockwise circulation of high pressure to our east and counterclockwise circulation of low pressure to our west. The two pressure areas together are acting like a moisture pump, transporting abundant moisture over the region.