Friday, December 28, 2012

Brief Update Into The Evening On Tomorrow's Snow

The good news is we're not making changes to our forecast of 2-4 inches for the city for tomorrow's event.  That said, an important rule of thumb with any snowfall forecast is...and we say it all the time...that a forecast is a range. Two to four inches means that anywhere from two to four may fall and that snowstarved expectations should be focused on the low number and not necessarily on the high, especially if you are close to a zone that has a lower snowfall predicted not too far away.  In those cases you should assume you are on the lower end of the range.  In the case of our map, Philly is more in line for two inches than four since the one inch zone is about 15 miles to your southeast, with four more likely north and west of the city.  Hopefully that all makes sense and mutes any inflated expectations.

The bad news is that it's possible not all of tomorrow's precipitation along I-95 (or at least at the Airport) falls as snow.  It may end as rain or a snizzly hybrid of drizzle and snow grains.  However, the front end of this precip event will get in early enough and precipitate with enough frozen pop to produce accumulating snows at the Airport before any snizzle (rainy/snowy mess) takes over at the tail end of precipitation.   In other words, snow for a good chunk of the evnet...ending as some drizzle as precipitation winds down...but the bulk and mothership of the event in the city itself will be frozen.

North and west, you have nothing to worry about.  You'll do fine with this event.  Between I-95 and I-295, the snizzly end looks likely...not because of any major shifts in track but modeling indicates some above freezing air nudging in the lower atmosphere.  It's enough to wreak havoc on an all snow solution at the Airport or in South Jersey's suburbs.  Below I-295, it looks more likely that you'll go from snow to rain at some point as enough milder air aloft melts any frozen precipitation aloft (temperatures are below zero in almost all of the atmosphere except for the bottom layers of the atmosphere in South Jersey as the event progresses).  You get some snow on the front end but precipitation is inclined to change to rain at some point between I-295 and Hammonton, with mostly rain below that.

A good indication of how close the rain/snow line is on the models is with the Canadian model (below).  It has the snow line along 295 through the event and has the event as all frozen in the snizzle on the end.  Other guidance, including the Euro, suggest a drizzly end to precipitation but snow to begin and for most of the event.

We'll post another update in the morning and, if needed, tweak our forecast....but as of now it looks like things are in line for the city's first official accumulation of the winter!