Wednesday, January 18, 2012

Saturday's Snow Chances Aren't Great, But...

...there's at least a chance that some snow will fall near the region on Saturday with the second of two upcoming systems. The first one is a weak clipper front that crosses the region on Thursday night and early Friday morning, which will result in flurries or a few snow showers for the region and might bring a coating of snow to folks north of the Pennsylvania Turnpike and I-195 in New Jersey. It's not a huge system by any stretch but merely a quick moving wave of energy.

The Saturday system has a little more pop to it and will work into what was initially a colder atmosphere.  However, the low pressure system is poised to track near or just south of Philadelphia.  With east winds coming in off the Atlantic we'll probably see the colder air aloft retreat as the storm moves in.    However, precipitation will be moving in as the cold we may start as some snow or sleet in the city on Saturday morning before precipitation nudges over quickly to rain, similar to what happened with this last system that moved through but with a bit more precipitation.

Farther north/west of Philadelphia, temperatures aloft will gradually become an issue for most everyone despite a likely start to the event as some snow or snow/sleet mixed.   Temperatures will moderate above the ground, resulting in a transition of snow over to sleet and/or rain from south to north...with some accumulations of snow and/or sleet possible as one travels north of Philadelphia during the front portions of the event.  The GFS shows the potential for some icy rain/sleet across the Pennsylvania suburban counties as temperatures at the surface would be below freezing even though temperatures aloft will be above freezing.   Since we're three days out on this event, this forecast is far from set in stone...and I tend to think in the end the icy rain potential will be a bit higher farther north (Poconos, generally between I-78 and I-80) than where the GFS is setting up shop now.   For what it's worth, the Euro computer model (above) is a bit milder than the GFS and doesn't have the icy rain chances locally as it confines them to the mountains in Central PA -- around here it's a snow to rain type event, with some minor accumulations of snow possible north of the PA Turnpike.

It doesn't mean that it's going to be sunshine and lollipops for weather on Saturday in Philadelphia or the suburban counties north/west -- the early portions of the event might be a bit slick as temperatures will start out near/below freezing but we should see conditions improve as the day wares on in most of the suburbs.  The system's timing suggests a morning start and a finish during the early to mid afternoon from west to east.

It's too early to speculate on specific accumulation amounts but the best chances of seeing anything more than an inch will be generally north of a Trenton-Pottstown-Reading line.  A few inches of snow are possible in the Poconos if the system stays slightly "colder" aloft but I fear even there some mixing of sleet and even rain can't be ruled out.