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                   Saturday, February 19, 2011 - 12:00PM CST

Extreme dry air in the eastern Appalachian foothills

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A wind condition called downsloping, along with an already low-dewpoint airmass, is creating some significant dry conditions today along the eastern foothills of the Appalachian mountains. The map below shows some of the observations in the area (temperature readings are red, dewpoints green).

Feb 19, 2011 obs
Observations at 12:51PM EST

I don't think I have ever seen dewpoint depressions that low in my many years living in the region and watching the local weather. Of particular interest is the Pulaski, VA station reporting a dewpoint of -20°F, with a depression (temp/dewpoint difference) of 75°F and a relative humidity of 3%! That is some seriously dry air rarely seen in this part of the country, more typical of the desert southwest regions of Arizona and New Mexico. As expected, the wildfire danger with these conditions is high, and accordingly a Red Flag Warning is in effect for most of the foothills and VA/NC Piedmont.

When air flows over mountains, it will moisten and cool as it climbs in elevation, then warm and dry when it descends on the other side. The Appalachians see noticable upslope/downslope conditions whenever stronger winds are blowing across at or close to a perpendicular angle to them. During a more common northwest wind like today, places like Charleston and Clarksburg, WV and Pittsburgh are on the upslope side, with Wytheville and Roanoke, VA and Mount Airy and Winston Salem, NC on the downslope side. Upslope favors cooler weather with clouds and precip more likely, while downsloping features clearer skies with warmer and drier weather. In a more rare southeast wind configuration, the situation reverses, with the western foothills on the downslope side and the eastern areas seeing upslope.

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