Temperatures jumped around quite a lot and, though we got drenched several times, some dry spells were prolonged. Twelve days ran colder than normal, and 17 days were completely dry.
Temperatures ranged from highs in the cold 30s to the warm 60s with lows in the icy 20s to the mild 40s. Washington record a trace of snow, while Dulles and Baltimore picked up 2.4 and 1.6 inches.
Washington and Baltimore registered no temperature or precipitation records during the month, but Dulles reported two of the latter:
- On Dec. 14, Dulles’s daily rainfall of 1.24 inches broke the record of 0.96 inches from 1999.
- On Dec. 16, Dulles’s daily rainfall of 1.30 inches broke the record of 0.99 inches from 1974.
Nationally, temperatures were warmer than normal, as they were in Washington, thanks to an active Pacific jet stream pattern that drew in mild air off the ocean.
Even so, it was cold and stormy at times, with some rapid swings in the weather. This volatility is a hallmark of La Niña winters and will probably continue through February.
The warm and wet December helped 2020 rank among both the top 10 warmest and wettest years on record in Washington.
Temperatures averaged over the year tied for the third warmest with 2019; note that the five warmest years on record have all occurred since 2012:
Rainfall was the seventh-highest on record, and 2020 became the second year in the last three to rank in the top 10:
How was our December forecast?
At the end of November, we predicted the following for December:
We lean toward average temperatures, between 39 and 41 degrees (the 30-year average is 39.7 degrees). Monthly precipitation should be between 2.8 and 3.25 inches, which is also around the 30-year average of 3.05 inches. We expect snowfall from zero to three inches, which is near the average of 2.3 inches.https://www.washingtonpost.com/weather/2020/11/30/outlook-december-could-be-chilliest-three-years-with-near-average-temperatures-expected/
The actual December average temperature of 41.4 degrees missed our predicted range by less than one-half degree, but rainfall was nearly two inches more than we expected. Reagan National Airport received a trace of snow, while other parts of the area saw about one to three inches — so our snow outlook was on target. Overall, we grade our outlook a B-.