Thursday, January 24, 2013

Winter Weather Advisories For Most Of You

Winter Weather Advisories are out for tomorrow evening for most of Southeastern Pennsylvania, South Jersey, and all of Delaware as a couple of inches of snow move on through the region tomorrow afternoon and evening.

You'll notice on the graphic to my left that halves of the suburban counties are included in an advisory but not the whole of Chester, Montgomery, and Bucks.  Advisory criteria in the counties is split in half -- three inches in the northwest halves of the counties, two inches in the southeast halves.  Philadelphia and Delaware County, and those south and east of those two, have historically been at two inches for advisory criteria snowfall, with the outer suburban counties running at three.  That changed this winter when the NWS split the county forecasts in half...they also changed the advisory criteria for the lower halves of the suburban counties.

Confused?  Considering everyone is getting the same amount of snow, it shouldn't be but the advisories are only out for those areas because their criteria is low enough for that.

So how much...and when?

How much is still a general one to three inch snowfall -- best chance at three in Delaware, best chance at one (if that) north of 195 in New Jersey.  The atmosphere will be dry, which means that the radar will suggest snow is falling for a time before it reaches the ground.  However, once it starts because temperatures aloft and at the surface will be cold snow will stick to the ground quickly and snow ratios will be a bit higher than your usual snowfall (powdery snow ahead).  We should be able to generally coax that one to three outside of north of 195.

When?  Expect flakes to start after 2 PM west of the city, around the PM rush hour in the city and immediate suburbs, and after 6 PM east of the Delaware River.  The higher resolution NAM (above) shows the potential for some dry air eating away at the snowfall as it moves in but the storm's energy should still be able to coax out enough snow over a four to six hour period as it moves through.  That means it'll end by 6-8 PM in Western Chester County, by 10 to Midnight at the Shore.  Since it's starting during the PM rush hour, travel impacts could be a bit higher than usual and it could be a slow slog home on Friday evening as the flakes fly.

This is not a big storm...hasn't been modeled as one since early Tuesday...but it's still snow and should make most of the snowstarved happy with the upcoming snowfall.  It's a fast moving system so it won't snow for terribly long but it should make for a pretty evening for a number of places.