5:04 p.m. update: Severe thunderstorm warnings have been issued for eastern Prince William, most of Fairfax County, northwest D.C., and and central and eastern Montgomery County as individual cells ahead of the main batch of rain may produce some damaging winds through around 5:45-6 p.m. These storms are also producing extremely heavy rain.
From 3:47 p.m. A plume of deep tropical moisture surging northward ahead of a slow-moving front promises to unleash wave after wave of heavy rain, beginning early this evening and continuing through Friday morning. Along and west of I-95, 2 to 4 inches of rain are likely, with locally heavier amounts possible. East of I-95, 1-3 inches will likely be more common.
Both this evening’s commute and Friday morning’s commute will likely be adversely affected by the rain.
The amount of rain forecast could easily result in flooding of creeks, streams, and low-lying areas. As the ground is wet and saturated from the heavy rain event two weeks ago (on April 28-30), as little as half an inch of rain in an hour – which is pretty much inevitable for parts of the region – could cause flash flooding.
Please remember never to cross a flooded road in your vehicle: Turn around, don’t drown. Pass this information to family and friends. During the flood event on April 30, multiple swift water rescues were carried out by emergency management due to motorists whose cars were swept away.
The first wave of torrential rain is possible during this evening’s commute, arriving between roughly 5 p.m. and 8 p.m. from southwest to northeast. Following this initial wave, in which severe thunderstorms with damaging winds may be embedded, additional waves are likely overnight. Heavy rain bands may “train” or move repeatedly over the same areas, especially west of I-95 and towards the I-81 corridor.
The National Weather Service Weather Prediction Center has declared a moderate risk of flash flooding, and indicates moisture levels are more than two standard deviations above normal.
The heaviest rains move off between 6 a.m. and 8 a.m. Friday morning from west to east, but the morning commute – especially early – may be complicated by standing water and flooding streams and creeks from the rain overnight. Lingering showers could continue through midday. By early-to-mid afternoon, skies should brighten and late afternoon and early evening hours should be completely dry.
The rise in water on local areas creeks and streams will eventually work its way down into the area rivers, and river flooding is a “significant” possibility during the day on Friday on into Saturday according to the Middle Atlantic River Forecast Center.
In tidal regions along the Potomac and Chesapeake Bay, minor coastal flooding is possible around the time of high tides starting tonight…so places like Old Town, Washington Channel and Annapolis should expect a bit of flooding, similar to the event two weeks ago. Refer to the coastal flood advisories from the National Weather Service for more information.
Models are in reasonably good agreement that 2-4 inches of rain will fall along and west of I-95, with somewhat lesser amounts to the east. These model forecasts often do not capture highly localized heavy rain bands which can produce even higher totals.
NAM model (high resolution)