Tuesday, December 25, 2012

Second Verse, Snowier Than First In Poconos & Central PA

Tomorrow's storm system for us is today's problem in the Southern US. Severe weather along the Gulf Coast plus snow in the Ozarks and the Mid Mississippi Valley to name two issues with the storm as it moves east.  This storm is a pretty tricky one to forecast and there's a fair spread of model uncertainty even within 36 hours of the storm's arrival tomorrow afternoon.  The uncertainty is around the amount of low level cold air that holds in place across the Lehigh Valley and northern and western suburbs.  It will mean the difference between sleet/icy rain and simply cold rain.  Some of the modeling waffles on the amount of surface cold available, others are suggesting a stronger icy rain signal for this event.

One of the models that is strongly suggesting some icy potential in parts of the region is the Canadian model -- which is placing Southeastern and Eastern Pennsylvania in a swath of icy rain or sleet for part of the storm tomorrow afternoon and evening.  The storm would initially start as snow, similar to the system that moved through last night, in the city and the immediate burbs.  However, because this storm is stronger than last night's there will be a greater push of milder air aloft.  This will help transition any snow to a mix of icy rain and sleet for the city, north and west.  The city's transition to rain will be quick but it won't be as one pushes away from the city.

If the Canadian is to be believed, the potential for significant sleet or icy rain accumulation does exist.  However, even there the storm drags in enough warm air to change things over to rain...eventually.

However, if this mild push aloft is delayed a bit, you will get more snow.  The Lehigh Valley will probably have the most bust-prone of the forecasts in this storm and it won't take much to put you into a snowier solution.  Be forewarned about the possibility.

Modeling shows that the Poconos should remain all snow -- and the possibility that a foot of snow falls in some of the higher elevations up there.  Some areas along the Susquehanna River may warm above 32 in Central PA and change the valleys over to a cold rain or icy rain but the track of this storm system should keep enough cold air in for the mountainous areas to give them a snowstorm.  The GFS and Euro are pretty consistent in showing the Poconos as being the winners on this storm.  Some nudging on the snow/mix line may still take place but as of now it looks more likely the Poconos cash in the most on this storm.

So, how much ultimately falls?  We pointed out the problem with the Lehigh Valley and that the forecast there could bust but given past historical trends the valley should probably be prepared for a fair amount of sleet or icy rain before you transition above 32.  If a colder solution verifies, you guys probably end up with closer to five or six inches of snow.  The gradient probably will vary a fair amount from Lehighton to I-78.  This part of the forecast may change up here.

The north and west burbs should probably get a couple of inches of front end snow, transition to sleet for a time, and then to a cold rain.  The city and I-95/I-295 corridors get a front end coating, perhaps an inch of snow, which will mix briefly with sleet and rain and then become a cold rain.  The Poconos will get between six and ten inches in general, with some one foot totals possible.

In terms of rain, which we haven't discussed much so far, modeling indicates an inch to two inches of rain will be common throughout the region during the duration of the storm.  Some localized flooding and ponding of water on roads will be possible in the rain-soaked parts of the region.

Timing-wise, the first part of the storm should move in around Noon south of the city, between Noon and 2 in the city, and after 2 PM north. The transition to rain occurs first along I-95 and then inches northward as the storm progresses.  For those below I-295 in NJ or I-95 in DE you may start briefly as snow or sleet (or a combo of the two) but not see it stick before transitions to rain take place.