Outlook: December could be chilliest in three years with near-average temperatures expected.

Computer model forecasts have trended a little colder

'In November, it was warm enough at times for shorts and short sleeves. But skip ahead one month, and we may find ourselves frequently in hats, gloves and scarves.

Dec. 1 marks the start of meteorological winter, and computer models suggest that November’s abnormally mild weather won’t stick around. As the last two Decembers were warmer than average, this one has a chance to be the coldest since 2017.

We’re projecting near-average precipitation to go along with average temperatures, meaning we may even get some shots at snow this month.

The forecast
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.

Temperatures during the first half of December are forecast to run very close to average, based on the latest Canadian, European and American model outlooks as shown here (from left to right):

Conditions during the second half of the month are more of a wild card.

Monday morning’s latest 16-to-20-day outlook from the National Oceanic and Atmospheric Administration’s CFS model offers intrigue for Washington’s winter lovers, as it projects slightly lower-than-average temperatures along with above-average precipitation, a favorable combination for snowfall. Confidence is always super-low on long-term extended forecasts, but the week starting Dec. 14 will be key to watch.

The influences
Two key issues seem to be affecting the forecast for December: a weakening polar vortex and a moderate La Niña event in the tropical Pacific Ocean.

A strong, stable vortex over the North Pole helped bottle up cold air over the Arctic in November, much as we frequently saw last winter. The vortex may now be weakening, which would increase the chance for shots of cold air into our area in the weeks ahead.

In addition, the intensity of the La Niña pattern continues to increase and has reached moderate strength. Dating from 1950, moderate La Niñas in December have favored somewhat colder- and wetter-than-average conditions, on balance, though our most recent moderate case in 2011 was quite warm.'>>>

https://www.washingtonpost.com/weather/2020/11/30/outlook-december-could-be-chilliest-three-years-with-near-average-temperatures-expected/?