Strengthening atmospheric river poised to bring significant rainfall to Northern California

Updated forecast: Two atmospheric river-charged storms to bring rain and snow to Northern California

After several weeks with no precipitation, an atmospheric river is poised to bring significant rainfall to Northern California and the Bay Area on Friday.

• Live storm map: See where rain is hitting California and Bay Area

Here’s what we know

On Thursday, the ridge of high pressure responsible for the recent stretch of cold nights will begin to break down. At the same time, a strong low-pressure system will develop over British Columbia and move south toward the U.S. West Coast.

By Friday, the storm system is expected to stall over the Pacific Northwest, drawing in a plume of tropical moisture originating near Hawaii. This setup will tap into an atmospheric river aimed at the West Coast, bringing the first pulse of rain to Northern California.

Rainfall totals from this initial pulse will depend on the exact location of the narrow atmospheric river. There is still some uncertainty on the forecast details for now as weather forecast models come into agreement.  

By Friday morning, the main plume of moisture is likely to be concentrated near the Oregon-California border. Steady rain will develop along the Northern California coastline in areas such as Crescent City and Eureka, where rainfall totals could exceed an inch by Friday evening.

As the day progresses, and as the atmospheric river wobbles back and forth, light to moderate rain will spread south along the California coast toward the Bay Area. Much like the storms in November and December, there will be a sharp cutoff in precipitation south of the North Bay.

Cities such as Redding, Ukiah and Santa Rosa could receive a half inch or more of rain Friday evening. Rainfall totals for the Bay Area are less certain, but light rain showers are expected to extend far enough south to bring some precipitation to San Francisco, Oakland and San Jose.

Uncertainty over the weekend

By Saturday morning, the atmospheric river and its associated cold front will shift northward toward the California-Oregon border, along with the steadier rainfall. This shift will reduce rain chances for the Bay Area. But a second atmospheric river-charged storm could bring a second pulse of rain to parts of Northern California. 

Some forecast models suggest the atmospheric river’s northward retreat will be short-lived, with a new plume of moderate rainfall edging south toward the Bay Area late Saturday and into Sunday.

Other models keep the focus of the second rainmaker farther north, bringing another round of steady rain to Northern California on Sunday. More clarity on this forecast period is expected by Thursday.

Some Sierra and Mount Shasta snow

Unfortunately for skiers and snow lovers, this initial atmospheric river will bring a surge of warm air, limiting snow accumulation across Mount Shasta and the Sierra Nevada.

On Friday morning, snow levels will start at about 6,500-7,000 feet, with up to a foot of snow possible on Mount Shasta by the end of the day.

The Sierra, however, likely won’t be as fortunate. Warm air will continue filtering in at all atmospheric levels Friday, pushing snow levels above 8,000 feet by Saturday morning. Still, there will be time for a few inches of snow to accumulate at Tahoe-area resorts before the warm air takes over.

Reach Greg Porter: greg.porter@sfchronicle.com; Bluesky: @gporterwx.bsky.social.