Friday, July 12, 2013

Humidity Front Dictates Placement Of Heaviest Rains

A good chunk of the Philadelphia metro is under a Flash Flood Watch today as an upper level disturbance, in concert with a "humidity front" (my term), team up for showers and storms around the Mid Atlantic.  Down in the DC area, heavy rain and thunder last night produced flash flooding and some spots picked up nearly six inches of rain.   Dulles Airport, just west of Washington, picked up four inches just since 9 PM last night.

Rains down there are slow moving as an upper level disturbance is helping fire up the rainfall.  A frontal boundary of sorts -- can't call it a warm front as much as it's a humidity front -- separates tolerable humidity overhead (dew points this morning near 60) from tropical air that lurks from Dover and Atlantic City on south.  This humidity front will act as a focal point for shower and storm activity later today, producing some localized heavy downpours.

Modeling is struggling to focus on exact areas for heavy rainfall today due to the small scale nature of this system but odds favor south of the city where heavier storms will develop.

While we're under a flash flood watch, guidance on flooding is relatively high outside of a few pockets in Chester County and Southwest Sussex County, which isn't a flood watch.  Most of us need two to three inches of rain in an hour's time to produce flooding...three to four inches of rain in three hours' time will do the trick.  Given storms are slow moving, the risk for flooding does exist but I think said risk is relatively low given the relatively higher flood guidance.

That said, with the front acting as a mechanism to trigger showers and storms through the day, some spots in South Jersey and Delaware could pick up two inches of rain by day's end.  We'll keep an eye on it through the day today and update as necessary.