The Washington Post
December may be warmer and drier than normal
With snow flurries fluttering about at the end of a colder-thannormal November, one might wonder if wintry weather is set to continue and even tighten its grip in the coming weeks. However, the current weather pattern is evolving into a milder one that favors above-normal temperatures and below-normal precipitation for 2021’s final month.
By Thursday, temperatures are forecast to surge into the 60s for the first time in two weeks. Pulses of cooler weather late this weekend and late next week offer only bite-size bits of winter between prevailing mild periods.
That may be the story of the month — a roller coaster temperature-wise, but more ups than downs.
For December overall, we favor temperatures to run 1 to 3 degrees warmer than the 30-year normal of 41.7 degrees, with precipitation running 0.5 to 2 inches drier than the 30-year normal of 3.41 inches. And, yes, snowfall should also be below the normal of 1.7 inches, probably between a trace and one inch.
Note that our winter outlook issued in early November called for a colder-than-normal December.
But the evolving weather pattern and latest data now support a milder outlook.
Computer models agree on a mild and dry December
Three long-range models are in lockstep agreement that December will be milder than average, not only across the Washington region but also in most of the Lower 48 states.
All of the models project the warmest weather with respect to normal over the Central United States and Rockies, but mild weather oozes into the eastern part of the country as well.
These same models also show a dry outlook for the east.
The main influence
The main forecast driver for the December outlook is La Niña, indicated by colder-than-normal waters in the tropical Pacific Ocean.
La Niñas tend to favor mild and dry weather over all but the northern tier of the Lower 48.
For Washington, La Niña Decembers often feature a lot of temperature variability but end up warmer than normal. La Niña conditions were prevalent last December, and we experienced days with highs ranging from the 30s to 60s, but the month was warmer than normal overall.
La Niña Decembers also tend to be dry. Last December was soggy despite La Niña, but this year’s La Niña is more mature and seems to have contributed to the ongoing dry conditions that prevailed through November.
What can go wrong?
There is plenty of data to follow in the weeks ahead that could offer twists and turns in the December outlook. Changes in tropical influences from the Indian Ocean to the Pacific Ocean could bring about changes in the forecast and more volatile weather, especially toward the second half of the month. Variations in the jet stream pattern in the Arctic may also influence how much cold air transport is available at times.
However, for now, we favor the warmer days to outweigh the colder ones, with more dry days than not.