Wednesday, March 3, 2010

Winter 2009-2010 Temperature Review

Following a very warm November, Washington, DC temperatures remained above average into the first few days of meteorological winter 2009-10 (December-February). The peak of 13° above average was reached on December 3, when 65° was observed, the second-highest of the winter. Temperatures cooled toward the middle of the month; the coldest day was the 19th at 12° below average. The coldest minimum was 23° on the 30th. December overall was 1.6° below average, the coldest since 2005, but the average of 37.9° was well above the 31.8° in 2000.

A cold first half of January was counter-balanced by a warmer second half. Although temperatures turned cold again for the last 3 days, the month averaged 0.4° above the long-term mean. This made it the most nearly normal January in nearly a quarter of a century. The warmest day was the 25th at 22° above average. The day's high of 68° was the warmest of the winter, but 7° below the record for the date. The 3rd and the 30th were tied for coldest of the month at 14° below average. The lowest minimum was 16° on both the 3rd and 31st.

Cold temperatures persisted into February, which was the coldest month of the winter. Readings were at or below average every day through the 18th. Following a slightly warmer than normal spell ending the 24th, temperatures cooled again through the end of the month, averaging 34.2°, which was 3.9° below normal. This was the coldest February since 2007, when the average was 30.9°. The warmest days, the 19th and 24th, were each only 2° above average, and the coldest was the 7th, which was 12° below. The warmest maximum was 50° on the 21st, and the coldest minimum was 16° on the 7th.

In the longer term, given that both December and February were below average, the winter overall averaged 1.7° colder than the long-term mean. This was slightly colder than the -1.5° departure in 2004, but only half as much as the -3.5° in 2003. Including September and October, 4 out of the last 6 months since the beginning of fall have been below average.

Following last winter's minimal -0.2° departure, this was the second winter in a row to be colder than average, but only the 2nd out of the last 6. Of the 11 winters so far this century, 5 have been colder than normal, but the overall winter average from 2000-2010 is 0.7° above normal.

Images (click to enlarge): Washington, DC daily, monthly, and seasonal temperature departures from average for: December 2009, January 2010, February 2010, January 2009-February 2010, winter 2000-2010; CapitalClimate charts from National Weather Service data (permission to reproduce granted under Creative Commons License provided full attribution is made with linkage to this web page)

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