From the ABC 7 Weather team

Southerly winds to bring in cooler temperatures by Wednesday

November 28, 2011 - 12:23 PM
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After near-record warmth, the D.C. area will see an odd cool-down Tuesday night into Wednesday.

Typically the D.C. area will cool off behind a cold front, when northwesterly winds help usher in a cooler airmass due to the pressure gradient force associated with an area of low pressure. This isn't always the case though as sometimes even southerly winds can bring in cooler air. This doesn't happen often but it will this week as a cut-off low to the south will do just that Tuesday night into Wednesday morning.

 
Forecast 850mb temperatures Tuesday night
Above is the cut-off low at 850mb with the black lines being constant lines of pressure, the wind barbs, and the colors being temperatures at that level. If you look at the D.C. area, it shows southwesterly winds at 850mb around 35 knots. As this system approaches the east coast, these southerly winds will actually bring in cooler air.
If you zoom out on our Interactive Radar, you can see temperatures in northern Alabama and Mississippi are only in the upper 30s, and areas of sleet have been reported with the cold core low currently over that region. It is this area of cold air that will move into our region Tuesday night into Wednesday.
GFS Forecast Surface Map for Tuesday night
Above is the forecast surface map for the same time period. It appears that snow will be possible to the west over the mountains but the remainder of the precipitation should be long gone around the immediate D.C. area. Lows should fall back into the 30s by Wednesday morning. Cooler temperatures are expected for the remainder of the work week with highs back near normal in the lower 50s.
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