Original post from 9:45 a.m.
A record-setting stream of tropical air – for both its temperature and water content – has primed the atmosphere for heavy downpours that could lead to flash flooding today.
The National Weather Service issued a flash flood watch for metro D.C. and Baltimore and the adjacent I-95 corridor through this evening. It says a “quick inch” of rain could fall leading to flooding in urban areas (poor drainage areas most susceptible) as well as small streams and creeks.
Based on the latest modeling, it seems areas just east of I-95 have the best chance of getting deluged through the afternoon, especially areas of southern Maryland. In the immediate D.C. metro, showers and perhaps a gusty thunderstorm or two should be more scattered compared to Wednesday.
It’s not so much the total amount of rain that could cause problems but quick bursts of rain accumulating too fast for the ground to absorb – especially after Wednesday’s drenching, which saturated the soil.
On Wednesday, 0.5-2 inches of rain fell across the region, including 1.67 inches in D.C., a new record for the date (passing the previous record of 1.30 inches from 1907). BWI Airport registered 2.01 inches, also a new record (besting 1.04 inches from 1907). Lighter rain fell west of I-95. Dulles only logged 0.58 inches.
The intensity of the rain can be directly attributed to a historic influx of tropical moisture, which set a record for the highest level measured in any month between December and April.
The map below shows the plume of deep tropical moisture surging northeast out of the Gulf of Mexico up the East Coast through the Mid-Atlantic. The moisture levels, described by a measure known as precipitable water, are comparable to those in a humid July air mass.
The air will remain unusually warm and humid through the holiday weekend, with more chances for showers.