After significant model shifts last night towards a much drier storm system next week, recent runs of the NAM, GFS, UKMET, and Canadian models have all shifted their thinking once again. Taken at face value, the GFS would develop a 4-8 inch snowstorm across the Mid Atlantic Monday morning into Tuesday.

However, we must first navigate our way through this first low pressure system, already beginning to throw virga our way. One thing I would like to make clear to everyone is that this first system is basically set in stone save the minor timing differences between models. The second, larger system is what's causing forecasters the big problems.

Storm A: Sunday morning-Sunday evening:

Between midnight and sunrise Sunday, snow showers are expected to break out across the entire region for a few hours, before rapidly changing over to rain by mid morning. Hence, no snowfall accumulation is anticipated at this time.

Storm B: Monday morning-Tuesday:
Now here's the tricky part: Late last night, computer models shifted their forecast tracks south and eastward, which would significantly limit the amount of precipitation the metro area would see. However, as recently as 30 minutes ago, the morning run of the North American Mesoscale Model began shifting its low pressure track even further northwest, which would point towards a major winter weather event early next week.

Given the recent alignment in model thinking, forecast confidence is somewhat higher. Precipitation looks like it will move into the District between 6 and 10 AM Monday. One major concern I have is the potential for low level insulation given the time at which precip is expected to enter the Mid Atlantic--if it moves in later than 12PM or so, low level temps will have warmed into the mid to upper 30s, decreasing potential for accumulating snowfall.

While model differences still run rampant, things are coming together for at least a moderate snowstorm in the Mid Atlantic.

Snowfall Accumulation Probability Map:

The chart at left a rough estimation of my current thoughts with respect to snowfall accumulation through Tuesday evening. Stay tuned to DCweather, Capitalweather.com, or the National Weather Service for more updates on this developing winter storm.

(click for larger image)