Wednesday, February 6, 2013

Feb 6, 2013: Light Icing, Then Rain, For Thursday Morning Rush

Winter continues with a succession of mainly weak areas of low pressure, containing limited moisture, in frozen form.

This has been our pattern for a couple weeks, because the jet stream at 30,000 feet is racing across the U.S. in a largely west-to-east configuration.   When the jet stream flow in straight configuration, without significant meanders or bends, storm systems at the surface remain weak and fast-moving.

There are a few elements expected to come together Thursday morning, bringing the threat of a light wintry mix.  These are shown in the forecast map below:

Surface forecast chart, Wednesday evening.
First is a blast of chilly, sub-freezing air from the northeast, as strong high pressure (the 1038 mb anticyclone sitting over Ontario, above) circulates cold air southward on clockwise winds.   The cold, dense air hangs near the surface and piles up against the Appalachians.  As the dome of chilly air builds over the Piedmont and metro areas, precipitation (forming as rain 5,000 feet above the surface) falling through the sub-freezing air layer will freeze into sleet (ice grains) or freezing rain (glaze ice - freezing on contact with the surface).  The dome of cold, terrain-trapped air is called the cold wedge or cold air damming.

The source of moisture aloft is the second element shown above - a developing coastal low pressure system over the southeast U.S.   The system will slide toward the northeast on Thursday morning, to a location off Cape Hatteras.

Now, let's advance the forecast map by 12 hours:

Surface forecast chart, Thursday morning.
The coastal low has drawn closer to us, overspreading limited moisture.  Light precipitation falls as a wintry mix.  However, as the day progresses, two things happen:  (1) the strong anticyclone supplying cold air retreats to the northeast;  and (2) the coastal low pulls in milder air off the Atlantic, warming surface temperatures above freezing.  These two changes erode the cold wedge, and precipitation changes to rain.  Precipitation amounts are expected to be light, since the coastal low stays largely to our south and east of our region.

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