Monday, March 04, 2013

First Snowfall Map For Wednesday

We've been doing weather here for several years -- it'll be eight in July.  In my time forecasting, busting, and succeeding in forecasts I haven't come across a time where the modeling for a storm less than 48 hours out is as divergent as it is now.  February 5th, 2010 and Boxing Day 2011 were two storms that had some pretty epic modeling swings in the home stretch but we haven't seen a modeled storm varying as wildly within 48 hours among the "big three" of modeling as this one has.  It also doesn't help that it's March and precipitation intensity will be the driver for precipitation type and how much snow ultimately falls.

What we're wiling to concede on is that more individuals will get impacted to the city's north than we thought before.  We can't deny that precipitation will move north than our initial thoughts.  We also think that the NAM's late afternoon solution is a clear outlier to the rest of the models that are out there.

Some basic trends:

1) Snow will be elevation dependent initially.  If you live on a hill, you will get more than a valley.  If you live in the Poconos, you probably end up with a bit more snow (although less overall precipitation) than your friends in Allentown.  If you live in Western Chester County, you could probably see the most snow in the immediate region, although more snow will fall farther to the west.  Again, with this storm the major winner from snow will be the mountains of Virginia and West Virginia.

2) Rain and snow will fall through the region on Wednesday, starting early in the day south and advancing northeast through the early morning hours.  Rain will gradually transition over to snow late in the afternoon or at night as the storm intensifies in the Atlantic, with some wrap around snowfall likely on Wednesday night through the Delaware Valley and down to the coast.  More on this later...

3) The coast will get pummeled, badly.  Winds gusting to 60 mph, sustained to 45 mph.  An inch or more of rain seems likely, perhaps more than that.  Tidal flooding and beach erosion are likely Wednesday afternoon and again early Thursday morning.  We're probably NOT looking at Ash Wednesday coastal flooding levels at this point, thankfully, but we will come within a foot of those levels being reached with the Thursday morning high tide (eight feet or so at Cape May and Lewes).  This will be a bad storm there, similar to Ash Wednesday of '62 in impact but probably not as high a flood.

Midday modeling from the GFS today suggested a general 2-4" swath of snow across higher elevations to the city's northwest, perhaps more than that in a few spots.

The Euro suggested the swath of snow sets up along I-95, with three inches or more possible along I-95 mostly from backlash on Wednesday night.  The backlash part of the storm will be the toughest to predict from an intensity of precipitation standpoint.  This could be the part where it actually *does* snow in Philadelphia and stick a bit as a heavier band of precipitation may work through.  This is where precipitation intensity matters...and if precipitation falls heavily enough, it will be able to overcome the modest warming at the surface and transition a rain/snow mix over to snow.

For NOW, we're projecting one to three inches in Philadelphia and along I-95, with two to five inches across the far western suburbs into Chester County, through the Turnpike corridor out to Harrisburg.  Lancaster, York, Baltimore and DC's northern and western suburbs could see between five and ten inches of snow.  A red shaded area along and just east of I-95 could see more than the projected one to three if precipitation banding is heavy enough Wednesday evening...this is a wildcard zone where some spots could make a run higher than three inches if everything comes together right.  High risk, high reward.

Farther northeast, two to five in the Poconos from elevation bonus snow as the system up there should be mostly if not all snow.  For most of the one to three zone rain will likely mix in at times -- it may fall as snow early, change to rain, change back to snow.

At the Shore, less than two inches is fact, it may not stick at all if it does transition to snow on Wednesday night.

A lot of things can go wrong either way -- if precipitation is heavier than projected, we probably see more in the way of snowfall.  If the Euro track holds serve or even retreats, we will miss on snowfall in the Poconos. Don't get your hopes up for much more than a few inches and be pleasantly surprised when things in deed do go beyond that.

We will look at everything tomorrow morning and revise our thoughts late tomorrow afternoon if needed.