Tuesday, November 06, 2012

Snow Forecast For Coastal

A buffet of advisories and warnings are out for the region for tomorrow with the approaching and developing coastal storm. This storm will bring rain to the Delaware Valley, with rain transitioning over to snow as precipitation intensifies and lifts into a marginally cold airmass away from the coast.  Winter Weather Advisories are out for the I-95 corridor and northwards into the Poconos, with Wind Advisories along I-95 and High Wind Warnings along the coast as winds at the Shore could reach 50 mph or higher in gusts.  The strongest winds probably occur north of Atlantic City and then along Long Island -- and up on Long Island, gusts to 60 mph are possible.

The storm will intensify rapidly over the next 24 hours, tracking offshore by about 150 to 200 miles.  However, it will throw a swath of moisture to its northwest and into our region, bringing precipitation from late morning tomorrow through tomorrow evening.

This is a very tricky forecast.  We have precipitation moving in, probably as rain on the front end.  However, as precipitation intensifies the potential for rain to transition to snow increases.  Modeling has suggested that precipitation will be a bit heavier along I-95 and the odds favor a transition from rain to snow at some point in the afternoon and evening.  IF precipitation is not heavy enough, this transition to snow does not take place.  However, given that models are suggesting a bit of a stronger low pressure center at this point we need to account for the potential for a full changeover to snow in the afternoon tomorrow.  So, yeah, tomorrow evening's commute could be pretty crappy.  The NAM (below) for 4 PM shows that transition taking place across the Philly burbs and points north.  Other models show the transition taking place earlier -- it's going to be a dicey forecasting day tomorrow and potentially a dicey afternoon commute.

The worst impacts for this storm will be at the coast in terms of wind -- gusts to 50 mph or higher are possible, sustained winds will likely break 30 mph in some spots.  However, in terms of snow it will probably be the place with the lowest accumulations -- coating accumulations are possible as rain mixes in thanks to the ocean being in the 50's for temperatures.

Inland, and away from that ocean, the snowfall potential increases.  We've adjusted our snowfall potential and are accounting for a changeover to snow for more locations and are accounting for some moderate to perhaps heavier snow on Wednesday evening in spots.  We're projecting 2-4" along I-95, 1-3" farther west.  Some spots will exceed 4" in the 2-4 swath, probably to the north of the city in the higher terrain.

One of the runs of the GFS earlier today was suggesting as much as 8" on a 10:1 (ten inches of snow in an inch of liquid) ratio for Philadelphia.  Keep in mind that given the snow that's upcoming will be dense, wet, and not on the traditional 10:1 ratio this type of outcome may be a bit excessive.  If we cut those totals by half and go with a 5:1 ratio, 4" would be a reasonable high end total for Philly.

The forecast is low confidence but if things play out on changeover in the afternoon, this looks like a reasonable outcome.  Most of the snow sticks on grassy surfaces at the onset of stickiness but in heavier bursts the roads will get some slush and snow cover on them, especially after sunset when the sun's rays aren't around to mitigate the falling snow.

We'll update this a bit more tomorrow morning.