The forecast is based off of weak signals provided by the La Niña, -PDO, -PNA, +AO and +NAO.
December is probably going to be messy temperature-wise, so confidence in the forecast is low. It has been a very frustrating forecast.
This is in STARK contrast to my forecast from mid-October:
(forecast anomalies based on the 1981-2010 normals)
So why the big difference? Blocking… or rather the lack thereof. A lot of forecasts called for the return of a -AO/-NAO blocking pattern in the Atlantic, which would pull cold air down from Canada into the U.S. This blocking was supposed to be set up by above normal October Siberian snowfall and stratospheric warming (with some persistence thrown in as well), but neither of these ended up occurring, resulting in a progressive +AO/+NAO pattern. This allows the Pacific to have more of an influence on the forecast as well, which with a -PDO and -PNA pattern puts a trough out in the western U.S. and ridging over the eastern U.S.
The big question for December is “how stable is the pattern?” and the answer is “not very.” Because of the uncertainty in the forecast, a lot of the CONUS is near normal (+/- 1F anomalies), with cooler temperatures in the western U.S. to reflect the -PDO/-PNA pattern and aboves along the East Coast to reflect the +AO/+NAO pattern.