Wednesday, December 07, 2011

Timing Tonight's Changeover

Rains continue across the region this evening -- temperatures are dropping though slowly -- we're into the 40's now except in the immediate city and points southeast.   It's still 66 in Sussex County, Delaware -- they were in the 70's earlier this afternoon believe it or not.   Colder air does lurk though -- 30's in Central Pennsylvania and in the Poconos, with snow now reported in Williamsport and points west and north of that.  Flood warnings or advisories are out for a number of counties in the region and many of us have already received an inch or more of rain so far from today's rain, with more to come over the next six to ten hours.  Radar shows the heaviest slug of moisture working northeast through Washington now, with a squall line of wind/heavy rain extending south from Central Virginia.  Those storms will lift northeast through the Delmarva over the next few hours and possibly graze Cape May.

Heavy rain should gradually nudge towards a steady thump of snow over the next few hours, starting in the Poconos and transitioning towards lower elevations as we work towards the 11 PM - 2 AM timeframe.

It is mild this afternoon locally -- temperatures are slowly sliding though and that slide will continue over the duration of the evening.   Snow is currently falling over Western Pennsylvania's higher elevations and the snow/non-snow line will continue to march east this evening towards the Delaware Valley.  As you can see with temperatures in the 50's at 3 PM, it's going to take quite a while to get to the point where snow is flying locally.  It will happen though, likely in the 11 PM - 2 AM timeframe in the region, earlier in the Poconos where a few inches of snow is expected.

Temperatures will being to slide into the 40's in the early evening hours, likely dropping into the 30's west of the city towards 10 PM, with 30's into Philly by Midnight or 1 AM.  Higher resolution modeling (shown below) suggests a changeover from rain to snow to occur into Lancaster and Berks Counties, as well as the Poconos, by 11 PM (if not a touch earlier in the Poconos).  The change from rain to snow could occur into Philadelphia by 1 or 2 AM (particularly the higher elevated parts of the city). A two to three hour shot of snow is expected across the Philadelphia suburbs in Pennsylvania, as well as north of I-195 in New Jersey.  That shot of snow may not last quite as long across the city and South Jersey but flakes will fly as far south as the Airport and it's possible a coating could accumulate into the heart of the city.

Our accumulation forecast for the region still holds -- lower elevations north and west of the city will see an inch or two, higher elevations will pick up more.   The city itself will see a coating of snow, with maybe an inch in Mount Airy.

We'll provide another update later this evening regarding progress on the colder temperatures.

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