Tuesday, July 23, 2013

Last Night's Rains Set Up Today's Flash Flood Watch

Doppler radar estimate of rainfall yesterday evening and last night. Yellow shade yields 2"+. Orange yields 2.5"+.
Rainfall in parts of the region last night topped two inches -- especially along I-295 through South Jersey, in the city (2.49" at the Airport in three hours), across parts of Berks and Lehigh Counties, and along I-195 in Central New Jersey (where Jackson Township in Ocean County picked up three inches of rain in an hour).  The heavy thunderstorms yesterday that extended into last night were due to an atmospheric disturbance that's pushing into the region. A meso low developed at the surface in Southern Pennsylvania and helped kick those thunderstorms that dumped as much as six inches of rain in Lebanon and Lancaster Counties east towards and through Philadelphia during the late night hours.

The fact some areas got prolific rainfall last night and the prospect for more thunderstorm activity later on today yields a Flash Flood Watch across most of the region -- exceptions being the Shore counties in South Jersey plus the southern two-thirds of Delaware, places where thunderstorms were minimal in impact yesterday and last night compared to the city and points north.

Thunderstorms, as our daycast earlier mentions, will be scattered around from midday into the afternoon hours. Modeling has downplayed some of the risk for thunderstorms in the morning runs compared to runs from yesterday. Downplay does not mean completely removed, however...and the higher resolution models as well as the GFS and Euro both show scattered showers and storms popping in the midday and afternoon hours across the region.

These do not look as widespread, especially north of the city, as what transpired yesterday.  However, because the atmosphere is still full of moisture the prospect of heavy rains and localized flooding in any thunderstorm that does pop does exist.  Atmospheric PWAT's (index of how much "juice" or moisture is in the atmosphere overall) stay above 1.75" through the course of the day but do trend lower as we work through the afternoon as the disturbance responsible for last night's stuff pushes offshore.  To me, it suggests a scattered storm threat this afternoon with the prospect of some additional localized flooding...just not on the order of the heavy rains that a chunk of the region picked up.

More: Current Weather Page