Tuesday, December 06, 2011

Elevation, December Style

Having had a chance to look at the midday guidance for Wednesday night's system -- a few minor changes have taken place from our discussion this morning.  First, the models have trended slightly stronger but also warmer with the storm as the low track is modeling a touch closer to the region.   This will not matter for those along the coast or in much of South Jersey -- this is modeled as a mainly rain event for you although some snow may mix in at the tail end.   Where this does matter a bit more is north/west of the city, where a slightly closer track could yield a slightly longer period of rain before the changeover to snow occurs.   Of course, in a stronger system the dynamic angle of the storm may be such that the strength could overcome the extra touch of warmth, resulting in about the same amount of snow as modeled earlier.

The key to this storm is elevation -- the higher in location you are, the better your chance of snow.   This storm will behave similarly to December 5th, 2009's elevation-dependent snowfall (only with a lot more rain) but will arrive at night as opposed to during the day, meaning that the potential for at least some snow to stick definitely exists for most everyone away from the coastline.   Our timing thoughts have not changed much -- modeling still shows a changeover of rain to snow in the city after 2 AM and towards the latter portions of the event, with the changeover to snow occurring around Midnight in the Pennsylvania suburbs and a bit earlier than that over the higher elevations in Central Pennsylvania.

Before the changeover to snow, a good bit of rain is likely across the region -- at least an inch, likely over two in spots (along and north/west of I-95) before the system pulls away early Thursday morning.   That resulted in flood watches being issued for the Delaware Valley as well as Central Jersey.  This rain will come in waves over the next 36 hours -- generally rain through tomorrow evening before the final low in the train is strong enough to enhance a changeover from rain to snow across most of the area.   Being that this is an elevation dependent storm, snowfall totals will vary quite a bit from mountain to valley.   We're pegging three to seven inches across the Poconos where the NWS has issued Winter Storm Watches, with lesser amounts farther south.  The Lehigh Valley, Berks County, and the outer edges of Bucks, Montgomery, and Chester Counties could see an inch in some of the lowest elevations, four inches in the higher hills, with the suburbs looking at an inch or two.   The city looks to pick up a coating of snow at the end, perhaps an inch or snow of slop in Mount Airy and the higher hills.   Snowflakes may fly at the end of the event at the Shore but accumulations there are not likely.

We'll provide another update tomorrow morning and, if needed, revise the forecast further tomorrow evening.   This isn't anything epic on the way in terms of winter weather but we reckon that it will be snowy for many, and a decent event for the north/west folks tomorrow night.