But as this storm moved closer to the Atlantic coast, it began drawing in relatively mild air off the Atlantic ocean. At the critical 5,000 foot level of the atmosphere (the level in the cloud layers that determines precipitation type - rain or snow), temperatures warmed rapidly from several degrees below freezing, to several degrees above freezing. By late morning, rain - not snow - formed in the clouds. But air in the lowest few thousand feet still remained below freezing (courtesy the cold air wedge). So rain drops froze into sleet grains - and sleet began to accumulate on all surfaces.
The 5,000 foot analysis map shows how the 0 C isotherm has pushed north and west of the metro centers (compare with the map from early this morning) - coincident with a changeover from snow to sleet:
|Early Morning - Cold Air Damming; Early Afternoon - Warm Air Invades from Atlantic. Adapted from NOAA|
|Morning: Snow-Sleet Line south of D.C. - Baltimore. Afternoon: Snow-Sleet Line North of D.C.- Baltimore. Adapted from WeatherTap|
This progressive transition from an air layer deep enough and cold enough for all snow, to an intermediate structure supporting sleet, finally to an atmosphere warm enough for all rain, took place over 12 hours. The warming of this layer proceeded from top-down. The final graphic below illustrates how meteorologists quantify the rate of temperature rise at 5,000 feet - a process called warm air advection. The map shows a very strong pocket of warm air advection (solid red colors) over VA-MD early this afternoon. Note the southeasterly winds, up to 75 mph, transporting mild air inland off the Atlantic. D.C. and Baltimore lie within the northern edge of this intense, warming pocket.
|Warm air advection early this afternoon changes the region over to all rain. Adapted from NOAA|