Increased Snow Chances / Amounts for Tomorrow

Well.  That was sudden.

As a frontal boundary approaches our area, it has been clear for a few days that the 21st would be cold, windy and that there was a chance that snow would overspread the area due to a small wave of low pressure riding along the front.

Our big snow producers typically come from the Gulf of Mexico and ride up the coast.  It takes quite a potent little low to produce a lot of precipitation if it has terrestrial origins. 

For the last couple of days, it looked like this would be a minor event.  An "inch-or-less" type of event.  Then came yesterday evening's model run.

The models run at 18z (1 PM locally) but since the upper air data from the day isn't available yet, they basically run from "inertia" or, more accurately, a "boatload of physics calculations" using the morning data.  Thus, they are used to get a general idea of the forecast.  They are rarely used for accuracy. 

In any case... the 12Z GFS which is based on upper air data had started to suggest the idea of a stronger and just as importantly, slower moving system.  The 18Z NAM then spit this clown map out and the meteorologists of the mid-Atlantic woke up from their nap and said, "hm!":

This shows a narrow area of significant snowfall in our area and especially points south and east.  Some of those totals are quite high but it's less about accuracy in the 18Z run and more about the overall suggestion:  a stronger, slower moving storm.

Overall, the storm has the look of a "clipper" system that then moves along the coast and intensifies rapidly. 

Overnight, the trend in the models continues to be stronger - except for the Euro.  Last night's 00Z Euro run showed this:

which still shows snow but in lighter amounts.

One this is certain:  it will be cold.  Precipitation type doesn't enter the equation like usual.  I expect high temperatures in the 20's which will result in high snow rations - perhaps as high as 15:1.  The map above shows 10:1 ratios so even taking 1.5 times the above snow produces a forecast for more snow than we've seen around here in 2-3 years.

This morning's data continues to come in and it all points to an increasingly significant storm for our area.  The RGEM, for example, calls for 7-10 inches:

The morning GFS is also in that range:

There is a Winter Storm Watch for our area and with the models trending stronger, this looks like it's trending towards a major event for our region.  For now, I'd say expect at least 4 inches of snow but the chances of more (maybe in the 8-10 inch range) are not out of the question.

There's unusually little variance with this storm as far as forecasting it goes.  I don't see any indications that it could end up producing nothing at all - but with the way winter forecasts go around here, I'm going to keep watching.

I'll update again later.