Image above is the future-cast from 7PM this evening through 7AM Friday. Temperatures will be cold enough during the evening and overnight for the light snow to stick to untreated roadways making for a slick Friday morning rush hour, especially on secondary roadways.
Short Range Ensemble Forecast (SREF) for National Airport (green line on graph in the image below) predicts about a half and inch of snow with possibly one inch if the snow tonight sticks around a bit longer.
Weekend Storm *Should* Stay Well South and East of Washington:
Latest North American Model (image below) shows just what a nail biter the weekend forecast is as any shift in track closer to the coast will have a huge impact especially on our southern and eastern suburbs!
Latest Euro Ensemble mean snowfall forecast (image below) currently shows accumulating snow staying south and east of the District on Saturday. Southern Maryland and Fredericksburg could be right on the edge of some light snow, so will continue to monitor for any possible shift in track to the west.
Latest GFS Ensemble (image below) for Snowfall:
The GFS has been consistent in keeping the storm far enough south and east to spare the immediate DC Area from accumulating snow on Saturday. Track can still shift a bit west over the next 24 to 36 hours… so will need to monitor closely!
DC Storms 
The National Weather Service seems to be over-achieving in their advisory products, as they are forecasting 1 to 2 inches as far eas as parts of Loudoun and Frederick counties. What models are they looking at the Japanese model? The more snow the better but you never told us up to 2 inches could fall. They sometimes like to raise the flag too much. You do a great job!
Tonight’s event does show a coastal low not far from Hatteras, though not a strong one. It looks like it will be below freezing, so everything should stick on untreated surfaces. I would think that 1-3″ of relatively low water content snow would not be out of the question.
The 12Z NAM continues the trend of edging the snow into the DC area on Saturday. It shows a very nicely shaped, phased in 500 mb trough with the phased in height lines coming up from the SW passing well north of DC. That is not characteristic of a storm going out to sea to the south of us. I would say we are good for 1-3″. There could be significantly more if the storm ends up a little farther to the west.
Anything I say should be taken with a grain of salt, because I tend to be the classic case of someone who sees what they want to see and disregards the rest!