Tuesday, November 27, 2012

Winter Weather Advisories Remain Out

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Winter weather advisories are out for the counties to the north and northwest of Philadelphia for today.  Our "storm" is coming in from the west and southwest early this morning, bringing a couple of waves of precipitation along with it for the ride.  The first should move through during and morning rush, with a second wave projected to move in later this morning and towards midday, enhanced by a wave of energy over the Ohio Valley.  This low is modest and pretty flat in its energy -- not exactly a powerhouse storm by any stretch.  Precipitation will be light to perhaps moderate at times, but nothing that's going to be suggesting any significant impacts in the city itself.  Off and on periods of precipitation, frozen farther north and west, mixing and oscillating between rain and snow in the immediate north/west burbs, mixing between rain and snow but probably more rain right along the river, and mainly rain south and east of the city.  Rainfall or liquid equivalent precipitation amounts today should be in the quarter inch to a third inch ballpark for the most part.

Precipitation may start as rain initially in the Philly suburbs but as it moves in, cooler air aloft gets pulled down to the surface and helps transition that rain to snow to the city's north and west...and transitions it to a mix in and close to the city.

We've slightly tweaked our snowfall projections for today based on where things stand as of this morning with surface and low level temperatures.  There hasn't been a huge shift except for immediate suburbs to our northwest, which have been nudged into the "coating" department and out of the 1-2" projection we offered last night.  Low level temperatures are a touch warmer than modeled early this morning, which lends to a potential mixing issue for the immediate burbs.  Even if snow does ultimately fall, it could be a situation where it has a harder time sticking thanks to precipitation being light and temperatures struggling to fall in response to lighter precipitation.  Even as the steadier swaths of precip move through, the low levels of the atmosphere appear to be just warm enough to prevent much more than a coating.

We're added a stripe of 3" along Blue Mountain and across the Southern Poconos -- it's possible a few spots right along the mountain or within 10-15 miles of there get to this level but the majority of folks to our northwest shouldn't expect more than an inch, perhaps two of snow out of today's event.