Wednesday, February 06, 2013

Fine Line Between Snow & No

It's a very, very fine line regarding Friday's storm system between getting snow and not. We mentioned yesterday that in order to bring accumulating snow in Friday's storm system to the region you need a faster intensification process with a low that will organize over Texas today as it hits the Atlantic on Friday.  We said climatology is typically not on the side of this happening but it can happen...and the Euro continues to show signs of wanting to fire up snows into Philadelphia.

Friday's storm is still one of very low confidence for this area, not so much to the northeast over New England where it will snow on the order of one to two feet, with some localized totals to 30" not out of the realm of possibility.  It will be a very memorable storm for them and New York City could see significant accumulations as well...they are a bit more on the fence on this like we are and for them it's the difference between a foot of snow and 2".

For us, it's either a several inch snowfall in the city proper or a hybrid of rain and snow showers that might end as snow and coat lawns around town.  Snow accumulations are likely northeast regardless of scenario, but the intensification factor with that southern energy will determine the fate of Friday night's precipitation.

Precipitation will start in the pre-dawn hours on Friday morning north and west of the city with the northern piece of this storm, with light snow and freezing rain or drizzle moving in.  That will cross through the region as overrunning light rain and snow pushes up from the south on Friday morning.  That light rain and snow could change over to light rain for most of the region for a time on Friday before the southern storm intensifies.  How quick that intensification takes place determines the snow factor.  The Euro (above) is a stronger, more wrapped up storm that will cause a faster changeover back to snow and produce a heavy thump of snow from the city on northeast.  The GFS, showing a slower intensification, does not yield as quick a changeover from rain to snow in the region.  It does produce some snow, especially north of town, but the lack of explosive development yields a "March 5th, 2001" type snizzle fest that will pop New England pretty good but leave minor accumulations around town, at best.

Given the potential for rapid intensification with the southern low, it would not surprise me to see a sharp snowfall gradient from southwest to northeast through the region on Friday night, sharper than what the Euro is advertising.  There is often a very sharp cutoff on the south or southwest edge in storm systems that have rapid intensification associated with them, whether it be December 29th, 2000 (below left) or December 25th, 2002 (below right) -- two classic examples of storms that screwed some, rewarded others.

These two storms featured somewhat similar dynamics -- southern track storm system that fused with northern energy, developed quickly and produced heavy thumping of snow near parts of the region but screwed a chunk of the region out of snowfall.  If the EURO run is correct, that screw zone sets up southwest of  Philadelphia...with it farther northeast on the GFS (New York gets only a couple of inches of snow).

We'll provide more info and more updates as we close in, including an update later this afternoon on any improved consensus in the modeling.