Tuesday, December 06, 2011

Snowy Update For Wednesday Night

We're starting to get more consensus around the Wednesday night event regarding what'll happen and how it will happen. In some respects, the event will be a less snowy repeat of Snowtober in the sense that those with elevation will benefit, whereas those near the coast and south/east of 295 will probably struggle to get much in the way of snow.  We're also dealing with the atmosphere cooling during the event as precipitation moves in, with rain transitioning to snow for those colder, hillier locations more quickly while those who are near the coast will see less snow.

There are a lot of differences as well -- one, this will not be as wet or heavy a snow as Snowtober.  Second, the dynamics aren't quite as strong with this wave of low pressure as they were with Snowtober although there will be precipitation and moisture that will provide the necessary "pop" to help cool the atmosphere to lead rain to transition to snow.  Third, this is a nighttime event so snow may have a bit of an easier time sticking in those places where the transition to snow takes place a bit later.

Timing looks to focus on rain with the low pressure wave arriving Wednesday afternoon into evening and with temperatures in the 40's at the onset of the storm it will start as rain across the region.   Rain spreads northeast into the region and as low pressure along the frontal boundary intensifies, precipitation will gradually become more intense and cooler air will draw down from the north.   Rain will gradually transition to snow from west to east through the later evening hours and after Midnight.   If we used the Euro as a guide, that transition takes place generally between Midnight and 3 AM across the western burbs.   The map below shows snowfall accumulation between 11 PM and 2 AM, with the rain-snow line by that point generally from Easton to Norristown to Concordville, with rain falling east (in the black shaded areas), and snow across Central into Northeast PA and the far outer suburbs.

That transition shifts east and south after 2 AM but precipitation also will begin to wind down in the same time frame from southwest to northeast, meaning the changeover to snow *could* be brief and only provide a quick coating to an inch on the tail end of the storm in the immediate Philly metro, with coating type accumulations possible down into South Jersey.   The timing of the changeover rain to snow will be the toughest part of this storm, similar to Snowtober...meaning if we get heavier precipitation as colder air works in the changeover could take over a bit earlier than what the models are speculating.  However, all three main models (GFS, Euro, NAM) show the changeover time in the region ranging from 11 PM - 2 AM west of the city to around 2-4 AM in Philly Wednesday night.  Precipitation will end generally across the region by 6 AM, with a dry but chilled day on Thursday.

In terms of accumulations, the highest snowfall potential is in the Poconos where six inches can't be ruled out. Accumulations in the higher hills north/west of Philly -- Bucks, Chester, Upper Montgomery County into Berks County -- could hit three or four inches, with 1-3" a reasonable call across the rest of the northern and western suburbs.   Philly and the I-95 corridor will see some snow at the end of the event but accumulations along the river will be coating type from this at the tail end of the storm as the majority of the storm for I-95 will be rain.  Mount Airy and the higher elevations in the city might pick up an inch or two from this system.  South and east will see coating type accumulations at best...likely just a few flakes at the end of the storm.

We'll have an updated forecast map this evening and some updates through the day via twitter and Facebook.