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Posted at 10:26 AM ET, 01/03/2012

Warmest December since 2001, also wet


2011 tied for 6th warmest December on record, and 5th warmest November-December on record
Following on the heels of a top-ten warm November (9th warmest), we also recorded a top-ten warm December. The month tied December, 1923 for sixth place with an average temperature of 45 degrees. The warmest Decembers (1889 and 1984) were only 0.6F warmer than this year.

Taking November and December together, the two-month period tied for 5th warmest on record.

Despite the record warm month, Washington Reagan National Airport reported absolutely no record temperatures, amazingly enough. But Dulles managed two records (Dec 20th record high minimum of 43 degrees broke the record of 42 set in 1998 and Dec 21st record high of 61 which broke 1998’s 60).


Percentage of days with highs and lows in different 10-degree categories at Reagan National Airport (DCA)
Most of our days during December produced highs in the 50s (or warmer) and lows in the 30s. And as reported earlier, we had no highs in the 30s for the first time since records have been kept.

Precipitation

Precipitation was interesting in that it was the first one without an inch of snow since 2008, and more notably, we saw the wettest one-day rain total in all of the winter months (3.1” on Dec 10th). December’s overall rainfall tally of 4.9” ranks it as the 16th wettest December since the late 1800s. The wettest December in recent memory was also the second wettest on record: 2009’s 6.79”, but much of that fell in the frozen form!

Overall pattern


Departures from normal in pressures or heights at about 18,000 feet (500 mb) during December. Areas shaded in yellow, orange and red tended to run warm whereas areas shaded in blue and purple tended cold.
Why was December so much different than the last few years? Well, both 2009 and 2010 saw significant high-latitude “blocking patterns”. This meant that strong high pressure (ridging) patterns over Greenland and over the Arctic Circle in 2009 and 2010 were able to redistribute cold air southward into the U.S. more easily. This year, we saw a complete reversal due to differences in the tropical Pacific to Indian Ocean (known as the Madden-Julian Oscillation) and a quiet stratosphere. The result was no blocking and cold air locked in the far northern latitudes. A warm-dominated jet stream pattern prevailed.

And January 2012?

The lack of blocking patterns continues here in early January despite our brief return of frigid weather today and tomorrow. However, we are finally seeing signs of possible changes in the pattern by mid-January due to some changes in the tropics as well as in the stratosphere. The result could mean better chances of intermittent cold pushes and even snow by the second half of this month. At the very least, it could prevent January from following in the footsteps of Nov-Dec 2011 with being in the top ten warmest rankings. You can see the National Weather Service’s final January outlook (updated Saturday) for temperature (leaning warm) and precipitation here: January outlook

See also Wes Junker’s outlook for warmer than average conditions during January and below average snow.

Further information

The National Weather Service publishes nice monthly assessments usually within a week of the close of each month (should be available shortly):

You can click on your closest airport location here:

Reagan National (DCA)
Dulles Airport (IAD)

Baltimore-Washington (BWI)

Historical Washington, DC data provided by NOAA.

See also: Ian’s excellent rundown on December climatology

By  |  10:26 AM ET, 01/03/2012

Categories:  Recaps, Local Climate, Latest

 
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