Our forecast favors December to run warmer than normal (the 30-year average temperature is 39.7 degrees, but the 10-year normal is 42.5) with above-average precipitation, too. We favor more rain than snow and lean toward below-normal snowfall overall during the month.
Over the past decade, Washington has experienced a cold December just three times. The most recent one was two years ago, and it was just a few decimal places colder than average. Otherwise, warmer-than-normal Decembers have been our default, despite two colder-than-normal Novembers in a row.
December is typically our wettest month of meteorological winter (December through February) at 3.05 inches of rain and melted snow. Last year was very wet, with more than six inches. The alternating pattern of dry and wet months since the fall (dry September, wet October, dry November) should continue because we lean toward a wetter-than-average December.
Our normal December snowfall is 2.3 inches, but we have not reached or exceeded that value since the super snowy December of 2009.
Weather modeling is in fairly good agreement that the first week of December will carry forward the chilly pattern that persisted for much of November, but then a moderating warming pattern returns for the second week. That warming should prevail through at least the middle third of the month and maybe into the holidays, as it did last year.
Here is the forecast temperature difference from normal through Friday from the American modeling system indicating somewhat chilly conditions:
And here is the flip toward milder conditions predicted by the same model between Friday and Dec. 13:
A large area of tropical forcing in the Indian Ocean is forecast to ripple eastward over time toward Southeast Asia, which tends to be a warm signal for eastern North American weather in December:
We need to watch two issues closely for some possible late-month cold and wintry mischief:
- Tropical forcing could reach deeper into the Pacific later in the month, which is frequently connected to colder impacts in late December.
- Powerful warming in the stratosphere toward the North Pole could disrupt the polar vortex by the holidays and deliver colder air. This stratospheric warming event is occurring much faster than the big one last year that delivered some colder weather at times in January and February:
This winter has the potential to bring bouts of colder and snowier weather in the months ahead, but for now it seems like at least middle December will offer a hiatus from more significant wintry concerns.