Monday, November 26, 2012

Winter Weather Advisories North & West of City

Winter Weather Advisories are out for tomorrow across the northern and western suburbs of Philadelphia, Central Pennsylvania, and the Lehigh Valley.  The potential for a couple of inches of snow tomorrow continues to be in the offing for those of you outside of Philadelphia, with the city and I-95 corridor  likely dealing with a mix of rain and snow as precipitation does not look to be as heavy and temperatures at the surface and in the lower atmosphere just warm enough to prevent an all snow solution for the city.

More on the accumulation part of this system in a bit -- first, the timing aspect.  This is mainly a morning and midday event, with precipitation moving into Philadelphia during the 4-6 AM timeframe.  Radar from the higher resolution modeling suggests that precipitation begins to move in just before the start of rush hour and continues into the afternoon before ending from west to east.

The NAM continues to be the aggressive model in terms of moisture, painting a several inch swath of snow across the outer parts of Bucks, Montgomery, and Chester Counties.  This is not reflected in other modeling -- both the GFS and Euro are much less aggressive for reasons we pointed out earlier today.  Consensus short of the NAM is a modest snow event overall.  IF the NAM's more aggressive snowfall output verifies, we're going to be off by a bit.  However, the NAM has a tendency of overdoing precipitation rates and also overdoing mesoscale features so we tend to discount the "how much" factor at this point but do buy the placement and timing factor.  By and large, expect precipitation as you head out the door tomorrow morning and expect precipitation to end in the early afternoon.

In terms of precipitation types, if you're north and west of Route 1 in PA and NJ you're likely seeing mostly snow for the bulk of the event.  A mix with rain can't be ruled out close to Route 1 but the odds favor a bit more snow than not.  Below Route 1, expect at least a mix with rain if not a total changeover to it at some point during the course of the event.  Odds favor little more than a coating, if that, along I-95.  Farther south and east, this should mostly be a rain event...some snow on the very front end can't be ruled out but as the day progresses temperatures will rise sufficiently above freezing to make it a liquid event.

For accumulations, in general we're expecting an inch or two inches across the northern and western burbs.  Some three inch totals are possible in isolated spots -- probably most likely between Allentown, Doylestown, Reading, and Pottstown.  If the NAM's aggressive look is right, snowfall would be a bit higher than our projections across the outer burbs but we're not banking on that as of yet.  It likely does not change the end game for Philly either way, though.