Winter Finally Arrives?

I'm watching a storm system that has become significantly more interesting in the past 24 hours.  A low pressure system tracking from the southeast had previously been forecast to move quite far to the south, making any adverse effect on our region unlikely.  In a "winter" full of near misses and no cold air, this didn't seem especially surprising.

However, in the last 24 hours, models have shifted their forecasts significantly northward.  So much so, that we're approaching the "sweet spot" of the storm where cold air and moisture are co-located.

The result?  Snow seems more likely now - and it could be a nice shot of snow during the daytime hours tomorrow.

The NWS office in Sterling has raised a Winter Storm Watch.  The official forecast now is for 2-5 inches in the advisory area which covers most of the DC area.  Because of the storm track, the totals actually increase to the south and areas around Fredericksburg could see more like 4-8 inches of snow.

If the storm track continues to shift north, some of that heavier snow could fall in the DC area.

Let's look at the maps:

Here's the GFS total forecast precipitation for the storm.  This is from last night's model run...we're still waiting for the data from this morning.

You can see the precipitation is heaviest in central Virginia but this is a significant northward shift from previous runs of the GFS.  That trend has continued and those darker areas which represent heavier snow are inching this way.

We should wait for the data from the morning model runs and the ones later this evening before making any solid forecasts.  For now, let's say the chance of accumulating snow tomorrow is better than 75%, the chance of more than an inch is well over 50% and it's safe to assume 2 inches or so will fall in the DC area with more certainly possible depending on storm track...
 
I'll update when more data comes in.

For what it's worth, here's the current National Weather Service Snow Total Forecast Map.  Our local NWS is well known for being conservative with snow storm forecasts so it's a bit of a surprise that they're really bullish about this storm.  Maybe they've seen model data I haven't yet.