Thursday, January 19, 2012

Projecting Saturday's Snow/Slop

The upcoming snow, sleet, and freezing rain....oh, and rain event for the region is a rather difficult forecast to pin down, especially if you are in the city and the immediate suburbs.  I think the farther north and west you are the easier this forecast is to pin down...and for the snow starved, the happier they will be.

Precipitation will move into the region overnight Friday into Saturday, likely in the 1-4 AM time frame.  Initially starting as snow, precipitation will gradually change over to a myriad of precipitation types based on location, surface temperature, and more.   Temperatures aloft will moderate above the freezing point in some locations and this moderation in temperatures should occur relatively early on especially from the PA Turnpike on south...when temperatures aloft rise above freezing a transition to icy rain or sleet may take place.  The question is how quickly that moderation in temperature takes place at the surface.  If surface temperatures remain below freezing we could see rain freeze on surfaces or have a bunch of sleet pinging around.   Given that there will be some subfreezing air out there to deal with at the surface you're going to have some issues with icing in parts of the region.

 The NAM computer model (above) shows the profile for 7 AM on Saturday.  It paints snow in blue, sleet in yellow, freezing rain in red, and rain in green.   We could see a two to three hour period of freezing rain or sleet in/around Philadelphia during Saturday morning before temperatures at the surface rise above 32.  We could also see the Airport/South Philly in better shape than Northeast Philly or Mount Airy with regards to icing.   You can see farther north that there's snow across most of the outer suburbs and into the Lehigh Valley and Poconos.  The Pennsylvania suburbs will change over a bit more slowly to sleet/icy rain -- might take until 9 or 10 AM and it might be on the tail end of the event where that transition takes place.   Precipitation looks to exit the region around Noon or slightly afterwards so the changeover north of the PA Turnpike likely results in a shorter period of icy rain.   There's some variance in the computer models at this point -- today's run of the Euro computer model shows the least ice in this event and is a general snow/no snow event, with the GFS and NAM showing at least some parts of the region getting ice.  The Euro has shown ice in prior runs, however.

South/east of I-295, you'll see the snow transition over to rain, with a shortened period of icy rain in between.  You should be rain by 7 AM in most of South Jersey although temperatures won't be much above freezing in this event.

In terms of accumulations, we're looking at a general swath of 1-3" of snow for most of the city, with some sleet and rain mixed in at some point during the storm.  Keep in mind that the best chances for 3" will be north...with the city probably more in line for an inch or two. North of the Turnpike in Pennsylvania and from Trenton on north in New Jersey we're anticipating 2-4" of snow that will mix with at least some sleet towards the tail end of the event.   The highest snowfall totals will be on the order of 3-6" and will fall across the Poconos and north of Route 222 in Berks County and the Lehigh Valley.  This is a first guestimate on snow and is subject to change...we'll take a look at information and continue to tweak as necessary over the next 24 hours.  We'll have another crack at this tomorrow evening.