For most of the day, I've been trying to figure out how this forecast could end up completely wrong. We do live in the DC area, after all, and if there was ever a place where snowfall forecasts go to die, it's here.
But I can't think of one.
The cold air will be in place, the upper-level energy is where it should be right now and the front is heading this way which will knock a good 25-30 degrees off our high temperatures today by the time the snow starts.
I suppose the only thing that could go wrong is if the storm gets its act together later than forecast by the models which would mean a less intense storm as it passes by. At the moment, this seems unlikely.
The GFS model shows the storm as it leaves the coast tomorrow and rapidly intensifies over the Atlantic. The trough of low pressure is expected to switch from a positive to a negative tilt which almost always means a more intense and slower-moving storm.
As the storm starts to influence our region, it will encounter the sharply-colder air and that should improve the accumulation numbers relative to the amount of precipitation produced by the storm. The rule-of-thumb ratio is 10 inches of snow per 1 inch of liquid precipitation. I think we do better than that tomorrow.
This will produce fluffy snow that will blow around in the strong gusty winds and that will be easier to shovel. It will, however, be complete crap for making snowballs or snowmen.
The snow could be quite heavy in the morning - especially if banding sets up (as it should). Forecasting the precise location of the banding is next to impossible so that's the usual reason for the range of forecast numbers. In this case, the National Weather Service is predicting 4-6" widespread throughout the DC area.
Another metric to consider is vertical velocity - a measure of the movement of the air. In the case of storms, higher vertical velocities imply more intense storms and heavier precipitation.
You can see from the graphic above that vertical velocities should be fairly high during the storm, suggestive of moderate to heavy precipitation.
Some snow amount considerations:
The NAM, which is a pretty good short-range model, shows the general thinking of most mets in the DC area - albeit with slightly higher values. The map above shows 0.5-0.6 total liquid precipitation which, converted to snowfall, produces numbers in the 6-8 inch range.
The GFS forecasts a similar range, maybe even a bit higher than the NAM - more in the 7-10 inch range for the DC area.
I feel that the NWS mets are being a bit conservative. It seems to me that there's potential for more than 6 inches of snow with this storm. Not crippling by any means but certainly more than we've seen around here in a while.
The evening models will be coming soon and I'll check those out as well. The only impacts to this forecast that I can see are the time of storm intensification which is something the models usually handle pretty well. My confidence in this forecast is, therefore, rather high - always considering, of course, that it's a snow forecast and it's for the DC area.
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