Tuesday, December 11, 2012

Stormier Time Ahead, But Not Terribly Colder Yet

The next two storms are likely to be predominately rain for Philadelphia and the immediate vicinity.  The first, on Sunday, looks to track just to our north (as of now) and odds keep frozen precipitation confined to the Poconos and points north and east.

The one thing we will keep an eye on for Sunday's system is a slight potential for a bit of a cold air wedge to set up across the Lehigh Valley and perhaps upper suburban counties in case some lower level colder/dry air seeps down on Saturday ahead of the storm system.  Computer modeling is hinting at a backdoor front moving in on Friday...while Saturday will be a nicer day, dew point values may tank a bit and allow for some evaporational cooling as rains moves in.  Milder air aloft (notice the freezing line at 5000' (the lighter blue line) is north of Philadelphia) may prevent snow...but if some of that colder air over Northeast Pennsylvania and Upstate New York can slide down, we may have some freezing rain or a brief period of sleet on Sunday morning down towards Allentown.  Note we said odds are slight on this occurring - we're outlining the possibility - but we do think that by and large if you're not in the Poconos this event should be liquid and temperatures above freezing enough.

A second, possibly more potent, storm system lurks for the middle of next week.  Computer model varies in track -- the Euro (typically better) suggests a track into the Great Lakes before the low energy transfers east along an occluded frontal boundary to secondary low pressure development in Pennsylvania.  This would be a mainly rain track (as of now) although the potential for some front end icy rain or snow is possible in the Poconos on this particular track.  Milder air aloft and at the surface would flood our region -- and bring rain.

The GFS disagrees with this setup and brings a singular low up the coast with snow for Philadelphia.  We're nine days out and both of the models will probably fine tune track and details but the Euro has been very consistent with track on this storm for the past three days.  That said, don't get hung up on details yet for next week since the intensity and track of the storm are surely going to waffle around a bit if you look at computer guidance with any frequency.  Just know there are two storm systems over the next ten days -- odds favoring rain over snow with both at this point.

What this storm, regardless of track, will do, is bringing colder air east for the weekend before Christmas and into Christmas Day before it retreats.  This does not look to be a locked, long lasting cold pattern but we should see a solid multi-day duration of colder-than-average temperatures around the 21st-25th.  Nothing arctic in intensity, brutal or nasty in chill, but temperatures would probably struggle to reach 40 for high temperatures, with lows probably in the 20's consistently through this time frame.