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P.M. Update: Low Temp Forecast Slightly Busted

Steve Scolnik @ 1:30 PM

It's pretty much an average mid-February day, with temperatures heading toward the mid-40's, close to the normal of 47 in Washington for Feb. 17. The average low for this date is 30, which was predicted for last night both here and by the NWS. In fact, the low at National was 36, and here on the Potomac weather patio it was 34. Why wasn't it colder? In a word, clouds. Instead of the usual clearing you would expect behind a cold front, we have had considerable cloud cover. If you remember Mr. Stockton's 8th-grade science class, there are several different ways heat can be transferred. The sun sends heat to the earth by radiation. The radiation heats the earth in different places by different amounts depending on the angle and duration of sunlight. During the long Arctic winter, the earth in northern latitudes radiates more heat than it receives. This cools the ground, and the overlying air is cooled by conduction. When that cold air moves southward, as it did yesterday behind the cold front which passed through our area in the afternoon, the temperature drops because of advection, or horizontal motion. As that air moved over the Great Lakes to get here, the lower levels were warmed by conduction from the water, and the less dense warmer air moved vertically by convection. The cooling of the air as it rose produced condensation in the form of clouds and lake-effect snow. At mid-day, radar was showing snow showers throughout western New York, Pennsyslvania, and West Virginia. There were even some flurries indicated just west of I-95 south of Washington, northeast of Richmond, and along the lower Delmarva. Keep an eye out and let us know if you see a stray flake or two in your area. The Ithaca and Syracuse areas in New York are forecasting a 70% chance of snow showers today and 90% tomorrow. Syracuse reported light snow every hour except one from 7pm last night through 9am this morning. Closer to home, Martinsburg, Parkersburg, and Wheeling in West Virginia, as well as Hagerstown in Maryland, reported light snow at 9am. The clouds from this activity moving over our area last night reduced the amount of cooling from radiation, so the temperatures were not as cold as they would have been with clear skies.

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