Monday, December 19, 2011

Zippin' Along With Lots of Weak Systems This Week

While Christmas may ultimately be brown in quite a few spots around the region, there is a chance that some folks could see some snow on Christmas Eve or Christmas Day. The best chances of this will be in the Poconos, the higher elevations up to our north, but computer modeling seems to suggest a chance for some of the northern fringes of our region to see accumulations from the third wave of what could be four waves of low pressure to track along over the next week through the Mid Atlantic.

Wave #1 is for Wednesday and will be rain...and mild.  We have that one pretty well established.  However, it could be cold enough at the surface in the Poconos for freezing rain at the onset late Tuesday night and early Wednesday morning.  Odds are pretty low that this occurs.

The second wave is for Thursday night into Friday.  Again, mild enough to support mainly rain except perhaps the highest hills in the Poconos, where snow or sleet could fall.  This is a relatively modest system in all likelihood -- it may scoot through with just a couple of tenths of an inch of rain, perhaps a half inch at most, but the odds are that this system is relatively weak as the pattern is forcing these lows to move quickly along and does not give them time to organize into stronger systems.

Questions are starting to crop up about the third wave of the train, for Christmas Eve (GFS) or Christmas Day (Euro).  This wave of low pressure will strengthen after it passes us by, enhanced by upper level energy that pivots through the Great Lakes.  There isn't much detail difference in result...just a difference in the day it impacts...the GFS being the faster and slightly weaker of the two, Euro the slower and slightly stronger.   Because the system lacks a significant amount of cold air to work with, the storm would need to explosively intensify in order to generate significant and widespread snowfall (and moisture as well).  Neither model has shown a consistent tendency to explode wave #3 but rain showers/flurries/snow showers look like they could be in the air on either day.  It wouldn't surprise me to see minor accumulations, again in the Poconos, perhaps in the Lehigh Valley, with the third wave.   It's also worth keeping an eye on down into Philadelphia but odds favor any snow with this third wave to be north of town.

The GFS' faster timing also supports a fourth wave moving off the Carolinas on Monday and Monday night of next week.  However, that system stays to our south in the GFS model.   The Euro does not show this fourth wave, consolidating a bit more energy into the third wave.