Tuesday, January 29, 2013

More About Wednesday Night's Wind & Rain

We're still looking at the potential of a very windy time of things on Wednesday evening and Wednesday night around the Delaware Valley as a potential winter-time squall line of wind and heavy rain fires up and moves east.  We're still not terribly convinced of the thunder end of this squall line although it certainly is possible.  However, odds favor the line being less convective (thundery) and more dynamic (windy) for most of the region. There's a lot of dynamic energy associated with this storm system and modeling shows this wind end of this really taking off aloft as the cold front approaches and then crosses the region.

Winds at 5000' above the surface are modeled to be over 90 mph across parts of New Jersey on Wednesday evening on the GFS model (seen below) and at 3500', the GFS is suggesting winds to 80 mph at Philadelphia around 7PM. Those high winds aloft won't completely transport down to the surface but they do support the possibility of 50-60 mph surface wind gusts should the warm front clear Philadelphia and up the coast, which the latter is likely by tomorrow evening.   Given the evolution in how winds are projected to increase with the storm as it moves through, odds favor the strongest wind gusts to be in New Jersey with the squall line as it moves through or just ahead of the line as it approaches.

That said, it will be rather windy for many locations.  Gusts to 40 mph possible west of I-95, gusts to 50 or 60 possible east of I-95.

There could be some showers around tonight into early tomorrow before the warm front pushes north, leaving us dry and breezy for a time into the midday and early afternoon.  Our best chance at 60 degree weather occurs between 3 and 10 PM tomorrow.  Rain moves into the region late in the afternoon or early evening, with a several hour period of rain possible before the heaviest of rains with the "squall" move on through.  Timing suggests the heaviest of rain moves through between 10 PM and 1 AM across the region from west to east. The high-res NAM (below) shows a pretty good approximation on the timing of the squall line (the graphic depiction below is for 10 PM tomorrow night) although there probably will be some rain showers ahead of the line across the region.

In terms of thunder potential, if there is any thunder to be had from this odds favor south of the city as there could be a bit better instability to support a rogue rumble or two.  It's not out of the realm of possibility to get some thunderstorm activity along the front but odds really don't favor it.

Rainfall will exceed an inch along and northwest of I-95, pushing two inches in parts of Eastern and Northeastern Pennsylvania.  Stream runoff is still an issue to keep an eye on around the usual suspect streams across Eastern Pennsylvania and perhaps around Wilmington.

Colder air will return on Thursday in the wake of the storm system -- essentially we go from 20's to around 60 to 30's in a three plus day window. Gotta love winter in Philadelphia!