[Latest NEXRAD radar image of the Northeast US; courtesy University of Wisconsin]
Bands of heavy rain have pushed into the DC metro region and are on the doorstep of Philly as an unusually strong upper level trough of low pressure heads southeastward towards the Mid-Atlantic region. The heavy rain bands have prompted flash flood watches and warnings to be issued by the National Weather Service in the DC metro region and they very likely will be extended up the I-95 corridor as the day progresses (at least they should be). While the New York City metro region could receive heavy rainfall by late today, it is more likely in the overnight hours as an occluded frontal system and secondary low pressure system head that way. All of the big city metro regions in the Mid-Atlantic from DC-to-Baltimore-to-Philly-to-New York City have the potential to receive as much as 5 inches of rain by the time all is said and done from this vigorous weather system.
One of the interesting aspects of this unfolding weather event is the chill in the air for mid-August. A cool ocean flow of air has developed ahead of the developing low pressure system and that combined with cloud cover has kept temperatures way below normal for this time of year. Temperatures at the noon hour were 69 degrees at Dulles Airport in Virginia, 74 degrees at Reagan National Airport (DC) and only 77 degrees in Central Park, NY.
Warmer air is, in fact, trying to advance from the south and this process will increase the risk for severe storms later today and early tonight; especially, in the region between DC and Philly. It remains to be seen if any of the I-95 corridor region breaks into the "wwarm sector" air mass and experiences a surge in temperatures. The best chance for that to happen would be from the DC metro region to the Delmarva Peninsula. The heavy rain will not continue unabated through the day in the I-95 corridor. In fact, there is very likely to be a break in the action between today’s heavy rain and tonight’s heavy rain.