December felt more like an extension of autumn rather than the start of winter, and, indeed, Washington’s average temperature during the month was only 0.3 degrees colder than November.
The mild December fit into a recent tendency for this month to be warm. Five of the 10 warmest Decembers on record have occurred since 2000.
The toasty December helped 2021 close as the fifth-warmest year on record in Washington.
It was warm, and it was dry. Little rain fell in the first half of December, and most precipitation held off until the month’s tail end. The monthly rainfall total of 0.63 inches was the driest since 2017′s half-inch and ranked as the sixth driest on record. The overall amount was 2.78 inches below normal. No measurable snow fell.
The month featured several warm periods with only a few small tastes of winter. And even the few chillier-than-normal days still reached at least 40 degrees. Meanwhile, 10 of the 31 days reached at least 60 degrees.
Numerous high-temperature records were set during the month.
On Dec. 11:
- Washington set a record high of 70, topping 68 from 1897.
- Dulles set a record warm low of 46, topping 44 from 1965.
- Baltimore set a record high of 70, topping 69 from 1979.
On Dec. 17:
- Washington set a record high of 65, topping 64 from 1939 and 1984.
- Washington set a record warm low of 54, topping 48 from 1984 and 2012.
- Dulles set a record warm low of 53, topping 49 from 1984.
- Baltimore set a record warm low of 52, topping 47 from 1984 and 2012.
On Dec. 19:
- Washington set a record warm low of 50, topping 49 from 1984.
The prevailing December weather pattern featured a big chunk of cold air trapped in Western Canada as a warm zone of high pressure frequently resided over much of the Lower 48.
It was unusually warm not only in Washington but also in much of the eastern two-thirds of the Lower 48. But that cold air that was locked over Canada is finally getting unleashed as we kick off January.
2021, overall, was a warm year. It was the fifth-warmest year on record in Washington, with only 2020, 2019, 2017 and 2012, the warmest year, milder.
Here is how 2021′s temperature compares with the warmest and coldest years on record and recent 10-year and 30-year averages:
From the standpoint of precipitation, the dry December wasn’t enough to keep 2021 from being wetter than the most recent 30-year average, but it was considerably drier than last year:
How was our December forecast?
At the end of November, we correctly called for a mild and drier-than-normal month; however, it was even milder and drier than we expected. We predicted temperatures to be one to three degrees above normal instead of the nearly six-degree departure. We called for December precipitation to be one-half to two inches drier than normal, whereas there was actually a deficit of almost three inches. Overall, we’d grade our outlook a B or B-minus.