[Current NEXRAD radar image; courtesy University of Wisconsin]
Showers and thunderstorms have been increasing in coverage and intensity during the past couple of hours and this trend will continue into the evening. The combination of an unstable and very humid air mass with a deep upper level low and associated jet streak is producing a heavy rain and severe weather threat from DC-to-Philly-to-NYC for this afternoon and evening and it will do so on Tuesday and Tuesday night as well. In fact, the picture gets even more complex on Tuesday as a strong and slow-moving cold front will approach the region from the northwest. Any shower that forms during this unstable period of weather from this afternoon through tomorrow night can produce heavy rainfall in a short period of time and any thunderstorm can reach strong-to-severe levels with damaging wind gusts, torrential rain and possibly even some large hail. The National Weather Service has posted a "severe thunderstorm" watch for the entire I-95 corridor through 8PM and another one is likely to be issued later tomorrow. In addition, much of the region is under a “flash flood” watch as upwards of three inches of rainfall can accumulate by the time cooler and less humid air pushes in behind the cold front later Wednesday.
The current NEXRAD radar image (above; courtesy University of Wisconsin) for the Northeast US shows numerous showers and thunderstorms over interior higher elevation Mid-Atlantic locations and these will roll eastward to the I-95 corridor over the next couple of hours. The simulated radar from NOAA’s WRF-ARW high-resolution computer forecast model for 7PM this evening shows predicted heavy rainfall across eastern Pennsylvania/western New Jersey as well as just to the north and west of the DC metro region (below; courtesy NOAA and Capital Weather Gang).
[Simulated radar image for 7PM from the WRF-ARW computer forecast model; courtesy NOAA, Capital Weather Gang]