Wednesday, March 20, 2013

March 20, 2013: A Storm Looking More Certain Sunday Night

A consensus is emerging that a complex coastal storm will develop off the Eastern Seaboard this Sunday into Monday.

However, most computer models suggest that there will be two pieces of energy - a primary system over the Tennessee Valley, and a secondary or coastal piece of energy off Cape Hatteras. 

There are significant differences in the models on which storm will predominate, and in particular their tracks and intensities. 

For now, a wintry mix is on the table for the D.C. - Baltimore metro region, with elevation-dependent snow (possibly heavy) over the mountains to our north and west. 

Once the storm departs Monday morning, a reinforcing blast of chilly arctic air returns to start the first week of April.

Here is the best guess concerning positions and intensities of the two storms, from the NOAA Weather Prediction Center:

Forecast Map, Sunday Morning
In this map, note the two areas of low pressure (Tennessee Valley and Georgia).   A weak wedge of cold air damming remains in place between the two systems, over the Mid Atlantic (blue "H" over NY-PA).

Forecast Map, Monday Morning
Note how the two systems remain separate. Eventually the primary storm over the Tennessee Valley weakens, drifting northeast, while the secondary of coastal system deepens as it moves northeastward away from Cape Hatteras.  Neither storm is very intense.  This is a bit of an "outside track" for a coastal low to clobber us with heavy snow.  However, with the western piece of energy and enough cold air in place, we could still pick up accumulating snow in some parts of the region.

Bottom line:  I expect a rain-sleet-snow mix here overnight Sunday, with a low chance of a major snowstorm. 

Stay tuned as the details of this complex forecast continue to get better resolved.

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