Thursday, November 29, 2012

Washington's 7th Driest November;
Second Biggest October/November Relative Decrease

November 30, 5 PM Update: Added 2012 monthly data and charts.

Original post:
With the mere 0.06" of light rain on Tuesday, November 2012 will close out tomorrow with only 0.60" of precipitation at Washington. That ties it with 1908 as the 7th driest November in 143 years of climate history. The driest was in 1981, when 0.29" was reported.

This exceptionally dry month, following the 5.82" drenching in October, represents slightly less than a 90% month-to-month decrease, second only to the 92% in 1981. In absolute terms, the 5.22" rainfall reduction is the 5th largest on record. The largest absolute October-November decrease was 7.49" in 2005, following the all-time wettest October.

Following wetter-than-average months in September and October, this is now the 9th month of 2012 with below-average precipitation. The November departure of 81% below average is followed by March with a 71% deficit.

The 29.42" total precipitation through the first 11 months is over 7" below the average amount to date. If December rainfall is equal to climatology, the annual amount (marked "Climo" on the chart) will remain the second driest since 2000, behind the 29.95" in 2001. Of the 13 years so far this century, only 4 others have been below average.

November was very dry throughout the Mid-Atlantic region, with nearly all areas below 50% of normal. Large areas from Washington eastward and southward have been below 25% of normal. The 0.71" at Baltimore is less than 22% of normal, and the 1.12" at Dulles is 33% of normal. Richmond's 0.27" is 8% of normal and the 5th driest in records dating back to 1872.

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