Thursday, March 29, 2012

Rainy Friday Night & Saturday Morning

Our next storm system is poised to move through the region on Friday night into Saturday as low pressure tracks either overhead or just to our south.  While in either case the track of the low means rain for the region, the bulk of precipitation with the storm system will fall on the northern side of the low...the biggest difference between "south of city" and "overhead" meaning we might get a decent rainfall for the first time in a while (our last half inch plus event at Philadelphia was back on February 29th) or, more likely, we continue to get spits and fits of lighter rainfall.   A more northern track may also yield some snowfall for areas north of I-80 that get in on the band of precipitation tracking through a marginally cold enough atmosphere and surface.   It shouldn't result in any snow for anyone close to Philadelphia regardless of outcome but regardless Friday night and Saturday morning aren't going to be tropical style rains nor tropical style temperatures locally!

The NAM is the "overhead" (in fact, a couple of runs of the NAM take the low north of city) model and was also the generally less rainy model -- a couple of tenths of an inch for Philly on north, with a third of an inch plus for the Poconos and points north in runs early this morning (see data below), although the model has become a bit more robust on rainfall in its midday run.  The GFS has been traditionally the more rainy model -- suggesting between a third of an inch for AC and as much as over eight tenths of an inch in Mount Pocono.  It also is a southern track system -- taking the low south of Philadelphia but is quite a bit more aggressive on rainfall compared to the Euro, which is also tracking the low south of the city.

The Euro's rainfall, not listed, is generally between a quarter and a half inch for many locations.  The Euro comes through as a less dynamic version of the GFS in terms of track -- a bit NAM-like in precipitation output and not as stacked in the atmosphere.   It still paints a halfway decent event through the region but with far less "moderate" rainfall through the region than the GFS does.  In terms of reality and how modeling has performed with rainfall events of late, projecting the lesser totals has made a bit more sense but ultimately the final track of the low will help determine just who sees what in rainfall.    A general quarter to half inch type rainfall isn't out of the question for the region.

If we do pick up more than three tenths of an inch of rain with this event, it will be our largest rain event of the month, beating the 0.30" event between the 2nd and 3rd.

Rainfall should move in after 8 PM on Friday west of the city and be out of town by mid to late morning on Saturday, with the duration of the day generally cloudy although some sun can't be ruled out in the afternoon.