12:30AM Tuesday update: The late trends are still all for a major Wednesday storm. But there is some hint that the secondary storm may form a bit more to the north than it looked earlier Monday. If this trend continues, this would mean more rain for D.C. before any change to snow and the change would be later in the day Wednesday. A late ensemble for the storm and precipitation around it Wednesday morning.
With the current trend, I think by Tuesday most forecasts will be more about rain Wednesday ending as snow in D.C. with the lower 3" amount more likely (60% chance) but still a major snow storm for the Blue Ridge and Shenandoah where 6-12" is still likely
It's been more than two years since D.C. has even had 1" of snow and almost 15 years since a "big" March snowstorm. The storm coming our way is still on track to be a big winter storm here, but the impacts are now likely to begin by Tuesday evening. Here is the late satellite image showing the clouds and general weak storm in the middle part of the country.
All the simulations of what will happen continue to converge that the primary storm to our west will reform near the Virginia Capes Tuesday night and then "wrap around" moisture will pull warm Atlantic air over cold air at the surface and produce very big (12" or more) snow in the Blue Ridge and Shenandoah area west of the D.C. area. Here is the latest ensemble showing the storm and precipitation track from Tuesday at 10PM (upper left) thru Wednesday afternoon (lower right) from data from the great PSU weather wall.
The dashed orange line is roughly were the rain snow line will be. It now is likely (70% chance) that the storm will begin as rain for the Tuesday evening rush. This is a problem for road crews since pretreatments will likely be washed away. Overnight Tuesday a wet snow continues well west of D.C. and then as the Wednesday morning rush approaches, the rain changes to a wet snow in the metro area. (lower left image above). Heavy wet snow is likely Wednesday afternoon (70% chance) and Wednesday evening rush is likely to be a slippery, slushy mess. Here is our latest "odds" outlook for snowfall in the immediate D.C. area.
Expect more than that far western suburbs and it is likely (60% chance) that the high elevations of northern Montgomery-Frederick County Maryland and south along the Blue Ridge and the Shenandoah Valley will see 8-12"+ of wet snow. Here's our latest storm total map
The risk continues to be tree damage from the weight of the snow and power outages. A nice discussion of the analogies of this storm is here with the Capital Weather Gang. We'll update later this evening and include a detailed timeline.