Wednesday, December 26, 2012

December 26, 2012: The Transition from Snow To Ice To Rain

Today's storm is a classic example of a winter storm starting off with a wedge of subfreezing air stuck up against the mountains - called Appalachian Cold Air Damming - creating several hours of snow and icefall across the region.

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
You can see the change in precipitation types of D.C.-Baltimore on the radar between morning and early afternoon - noting how the snow-sleet line shifted north and west of the metro centers:

Morning:  Snow-Sleet Line south of D.C. - Baltimore.  Afternoon:  Snow-Sleet Line North of D.C.- Baltimore.  Adapted from WeatherTap
Around noon, winds at the surface transitioned from cold northeasterly to milder easterly flow.   The wind direction shifted as a new center of low pressure began developing along the Virginia coastline, setting up more of an oceanic fetch of wind into our region.  The arrival of slightly milder air at the surface began scouring out the cold air dam in place in the lowest few thousand feet over Washington- Baltimore.   As surface temperatures warmed, rain that forming at 5,000 feet, stayed rain all the way to the surface.  As of 2 pm, precipitation area-wide became plain rain.

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

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