My call yesterday was for light precipitation this afternoon, mainly rain, mixing w/ pockets of sleet. Instead...counties north and west of D.C. are being treated to an afternoon of light snowfall, and we will likely see 1"-2" of accumulation before the storm pushes out between 5-6 pm tonight.
I believe the NWS was also taken by surprise! They were in "reaction" mode, rushing out winter weather advisories as the flakes began to fall.
First, the surface weather map (below) shows a weak region of low pressure approaching from the west. A stationary front separates mild air and rain to the south, and cold air with snow to the north:
Alas, the early morning weather balloon launch at Dulles revealed very dry air in the lowest 20,000 feet of atmosphere, with temperatures hovering right around freezing at 5,000 feet. When precipitation did begin to form in the cloud layer - perhaps as a mix of rain and snow at that level - it evaporated into the dry air. Evaporation extracts heat from the air. This chilled the air layer at the critical snow-making level (5,000 feet) several degrees below freezing. The cooling continued, in spite of southerly winds at 5,000 feet pushing warmer air in from the south. In the parlace of Meteorology 101: Evaporative cooling won out over warm air advection. Snow, not rain, was able to efficiently form in the cloud layer, and remain as snow all the way to the ground.
I've highlighted this pocket of evaporatively chilled air in the diagram below - showing isotherms of temperature at 5,000 feet. Isotherms at or colder than freezing are shown in blue, those above freezing are shown in red:
|Modified from NOAA|