Are atmospheric river storms coming to California soon? Here’s the forecast

California’s weather pattern this December is set to be a mixed bag of rain and shine.

The first week of December is set to bring some wet conditions, primarily impacting the northern region of the state. Weather models and recent trends with El Niño are favoring equal chances of above- or below-average temperatures and precipitation through the end of the month.

While low pressure systems are expected to frequently march toward the West Coast in early December, the bulk of their rain and snow is more likely to impact the Pacific Northwest. That’s because the wavy nature of the jet stream, the fast-moving ribbon of winds that travels high above the ground, will act like a maestro guiding this month’s orchestra of storm systems.

Just as a conductor dictates the movement and intensity of music, the jet stream directs the path and strength of storms. The American and European weather models indicate that the path of the jet stream will favor a northerly path during the first half of December. 

Low-pressure systems traveling toward the West Coast during the first two weeks of the month are expected to bring California only light rain and snow. The forecast calls for intermittent rounds of showers reaching the North Coast, Bay Area and Sacramento Valley and little to no rain for residents in Southern California.

This calm start to December seems a bit counterintuitive to what’s expected during an El Niño pattern. But this pattern typically doesn’t bring a spike in storm activity to California, and particularly Southern California, until January or February. So, even though El Niño eventually may play a role in shaping winter weather, it’s not expected to have a strong impact on rain and snow impacts this month.

Come mid-December, things change. The medium-range models show the jet stream taking a path aimed directly at the Golden State sometime after Dec. 15. A ridge of high pressure that often blocks storm systems from arriving in California will begin to break down. 

A similar subtropical ridge was in place during the winters of 2020 and 2021 and led to extreme drought during those subsequent summers. This time, we expect a much weaker ridge, which should allow the jet stream to flow more freely and bring rounds of storms across all of California.

Reach Gerry Díaz: gerry.diaz@sfchronicle.com; Twitter: @geravitywave