Monday, February 27, 2012

Updating Wednesday's Event

We're closing in on Wednesday's event and the models are coming into a bit of focus on the specifics regarding what falls from the sky and how that sets up...however there are still some differences and how those differences play out can mean a different outcome for those of you in the Lehigh Valley, Poconos, and even in the hillier parts of the suburban counties north and west.  For the most part, Philly (city) should experience rain although a tiny bit of snow and sleet can't be ruled out on the very front end of this event as some evaporation takes place from precipitation falling into the initially dry airmass.   It doesn't look significant at this point and it shouldn't cause significant impacts for the city other than the "ooo" and "aww" factor for the snowstarved.

For the suburbs, any front end burst of snow and sleet will transition to rain pretty quickly, with the upper half of the suburban counties seeing any frozen part of this lingering a little bit longer.  Best case scenario for the snow crowd is that snow and/or sleet could accumulate to an inch or two on the highest hills of Upper Bucks and Upper Montgomery Counties and maybe a coating on lawns and cars north of the Pennsylvania Turnpike and Interstate 195 in lower elevations before washing away when the transition to rain takes hold.  The Euro is your model of choice for any front end snow in the Pennsylvania suburbs. However, for reference the early morning runs of the NAM (above) don't bring snow south of PA Turnpike at all and barely gets it into Berks County at 10 AM on Wednesday.  The GFS is in concert with the 6z (early AM) NAM in terms of being a wholly liquid event for the immediate suburbs.

Farther north and west is where you see snow hang a bit longer...and the farther north one is the more snow you'll see.  At some point the transition from snow to non snow in the Lehigh Valley will take place but not before you pick up a couple of inches (1-3" perhaps in Allentown), with the transition to non snow taking place last in the Poconos.  For the Lehigh Valley, you should transition to rain at some point on Wednesday afternoon, with the Poconos transitioning to some mix of icy rain, sleet, drizzle, and snow.

There are some things that can throw a wrench in the works up north.  First, how heavy the precipitation thump is on Wednesday.  A lighter precipitation event from a dynamic perspective will yield less snow as colder air above any warm air push will not be able to completely mix down.  The Euro, for instance, was the coldest of guidance in yesterday midday's run but has moderated a bit in last night's run as its precipitation shield was a bit lighter in last night's run.  Lighter precipitation would allow for milder air aloft to get in quicker.  Temperatures won't necessarily warm much at the surface in a lighter precipitation event compared to heavier...but the heavier precipitation shield would yield more (or at least in the suburbs, some) snow.  If the front end thumping of precipitation is lighter, we can expect to see not only less snow but a quicker transition from frozen to non-frozen for precipitation type.

Best case scenario for the Poconos is more than six inches of snow.  Worst case would be one to three with lots of mixing of sleet and rain.  The trend for the Poconos from a modeling perspective is away from significant icing up here and towards a thump of snow ending as a slopfest of drizzle and sleet pellets.

Precipitation should start in the morning hours on Wednesday, generally between 6 and 10 AM from west to east.

Regardless of what falls on Wednesday, it will be a raw day as temperatures in many locations hang out in the upper 30's or lower 40's.  It won't be a nice day as meteorological winter ends on a bit of a lousy weather note.