• Update: Will La Niña emerge? The forecast is starting to shift
NOAA predicts equal chances of above- and below-normal precipitation across much of California, with a slight tilt in the odds toward drier-than-normal conditions in the desert regions of Southern California from December through February.

Regarding temperatures, neither cooler- nor warmer-than-normal conditions are favored in Northern California, with equal chances. Above-normal temperatures are favored in Southern California.

Predictability is lower this winter due to weak signals from climate patterns that influence California weather, Jon Gottschalk, NOAA’s chief of operational prediction, said at a news conference Thursday. 

“A lot of the variability that will come from the West Coast precipitation could be related to within-season or week-to-week variability of the storm systems,” Gottschalk said. “It’s a lower predictability winter than last winter with the strong El Niño.”

Earlier this year, La Niña was forecast to emerge in the tropical Pacific Ocean by early fall. But La Niña has not yet been declared, and recent forecasts indicate a weak event at best, according to NOAA. 
Still, a weak La Niña can influence West Coast weather. Cooler and wetter conditions are favored in the Pacific Northwest, which is good news for the winter snowpack.

Other less predictable factors also influence California winter weather beyond La Niña. A large pool of warm ocean water between Japan and the Aleutian Islands, more than 10 degrees warmer than normal, could alter the intensity and path of Pacific storms. 

“Warmer temperatures in the higher latitudes, especially in the Pacific, may raise the likelihood of more wetter storms impacting the West Coast,” Gottschalk said.
Forecasters often mull over historical archives to make their seasonal predictions. Of the 10 most recent years with La Niña and warm northern Pacific ocean temperatures, California had seven dry years and three wet years, said Andrew Schwartz, director of the UC Berkeley Central Sierra Snow Lab.

The wet years combined for 30% more precipitation than normal, while the dry years combined for 30% less than normal, he said. “So the only thing we see as possible is that regardless of whether it’s wet or dry, it’ll likely be a year of extremes,” Schwartz said.

Climate “normals” are based on average temperatures and precipitation between 1991 and 2020. San Francisco’s normal December through February rainfall is 13.5 inches, while Los Angeles averages 9.4 inches. 

Seasonal forecasts are inherently probabilistic, and NOAA does not make long-range snowfall predictions. Science is not yet advanced enough to predict individual storms beyond two or three weeks. However, climatologists can identify patterns in the ocean and atmosphere that influence weather patterns throughout the world. 

One of the most understood and well-documented climate patterns is the El Niño Southern Oscillation. Variations in sea surface temperatures in the eastern equatorial Pacific can alter the storm track over the United States.

During El Niño, or periods of warmer-than-normal tropical ocean waters, the West Coast tends to be warmer, with the storm track enhanced over Southern California. La Niña pushes the storm track northward, favoring wetter weather in the Pacific Northwest and generally drier weather in Southern California, while precipitation in Northern California can vary considerably. A stronger La Niña and El Niño tends to result in more pronounced impacts on West Coast weather.