While the forecast did call for snow and/or rain in the early morning hours, I don't think anyone was quite prepared for the accumulations - up to 3 inches in some spots - that we woke up to this morning.

Prof. Jeff Halverson, at the University of Maryland Baltimore County, has offered an explanation. It's called a "cold air damming" - a wedge of cold, dense air that had settled in hung east of the mountains despite advancing warm, wet air from the South, and wouldn't let go. It was enough to make more of the predicted overnight precipitation fall as snow:

"[T]hat became entrenched east of the Blue Ridge overnight, and this kept the morning precip falling as a frozen mixture.  "The Wedge" is notoriously hard to predict.  The warm air push from the south was not strong enough to scour out this dense air mass.

From here, he says, "Expect a quick, hard hit of heavy precip returning around 1 pm through about 9-10 pm tonight, then a quick cutoff.  Most forecasters are predicting 4"-8" but there will be embedded bands that are very narrow, and extremely hard to predict more than 1-2 hours in advance where these will set up.  These are the "thundersnow" corridors. 

"Timing will be ugly - visibility could drop to near zero in the heart of the evening rush, not so much from wind-blown snow, but big flakes falling at 1"-3"/hour.  Interestingly, the [NWS meso-scale model] is predicting a narrow swath of heavy ice accumulation just north of  the I-95 corridor and mainly rain along and east of I-95. If you buy this model, the heavy snow stays across our far west and north burbs."

Here's the official forecast for BWI-Marshall from Sterling. Here's the Winter Storm Warning, posted for the northern and western counties.