A frozen Potomac on Jan. 1. (Andrew Pasko-Reader via Flickr)

The year 2017 was an abnormally warm year in Washington, but it certainly did not end that way.

A last-minute blast of punishing cold dragged December’s temperature into below-normal territory for the first time in seven years. The extreme cold that closed the month was an anomaly in a year that witnessed its warmest February and April on record, as well as its third-warmest October.

December proved to be an extremely volatile month. The first and third weeks ran warmer than normal while the second and final weeks were cold. In the end, the month averaged 0.5 degrees colder than normal.


The period between Christmas and New Year’s Eve was decidedly bitter, helping to establish some notably cold milestones for the month:

  • The final week was the coldest since 2000. It was the 16th-coldest on record, and fourth-coldest since the mid-1940s, when temperature measurements began at Reagan National Airport. The seven-day period was a remarkable 27.3 degrees colder than in December 2015. That 2015 week was the warmest on record in Washington.
  • The high of 23 on Dec. 31 was D.C.’s coldest high since Dec. 24, 1989, when it was also 23.
  • The two high temperatures of 25 or colder (Dec. 28 and 31) were the most since there were two in December 1993. (The record is eight in 1917).
  • Four days had highs of 32 or lower (three in the past week), the most since six in 2000.
  • The month’s four days with lows in the teens (all in the past week) was the most since seven in 2000. The low of 16 degrees on Dec. 28 and 31 was the coldest since 2004.
  • The windchill advisory on Dec. 31 was the first since mid-February in 2016.

The following cold temperature records were set during the month:

Dulles:

  • Dec. 15: record-low maximum temperature of 29 breaks 1968’s 30.
  • Dec. 28: record-low maximum temperature of 23 breaks 1993’s 24.

Baltimore:

  • Dec. 28: record-low maximum temperature of 24 ties 1993.
  • Dec. 28: record-low minimum of 10 ties 1950.

(None were set in Washington).

Washington joined a large area from the Midwest to the East Coast that endured colder-than-normal weather during December.


The cold air pouring into the eastern part of the nation resulted from a fairly persistent, bulging ridge of high pressure over Alaska, which sent the jet stream toppling south over the Lower 48. This is something we saw frequently in the cold 2013-2014 and 2014-2015 winters.  This pattern continues to drive colder weather into early January, too.


Cold but dry

It was cold and unusually dry in Washington, following on the heels of an autumn in which little rain fell.

The equivalent of only 0.5 inches of precipitation (melted snow and rain) accumulated, and drought crept back into the region. It was actually the driest month since September 2005 (when we logged only 0.11 inches) and the driest December since 1965 (0.47 inches).

Measurable snow managed to show up in D.C. three times for a total of 1.9 inches, which was 0.4 inches below normal. Those 1.9 inches still made it the snowiest December since 2010. The 4.0 and 2.8 inches that fell in Dulles and Baltimore on Dec. 9 were records for the date.

2017 ends drier and much warmer than normal

December’s slightly colder result helped make 2017’s average temperature about 0.7 degrees cooler than the hottest year on record (2012). But because of all of the other abnormally warm months, we eked out the second-warmest result:


The year 2017 ranked seventh-driest since 2000 as only two months during the calendar year — May and July — had above-normal rainfall.


December forecast review

In predicting what we thought would happen in December, we correctly called for a colder-than-normal month. We wrote: “We may end the month with the first colder-than-normal December since 2010.”

We also accurately forecast below-normal precipitation.

However, the specific ranges we predicted for temperature and precipitation were a little off. While we forecast an average temperature of 37 to 39 degrees, the actual average temperature was 39.2 degrees. Precipitation was a bigger mess as we called for two to three inches and a mere 0.5 inches fell.

For snow, we were on the money. We wrote: “We’ll go the ultimate ‘hedgecast’ of 1 to 3 inches for December, which is *still* snowier than last year’s mere trace outcome.” The actual snowfall was 1.9 inches.

Forecast grade: A-

What do you think? Tell us in the comments.