The outlook for California’s atmospheric river event is in flux

Forecasters use a series of weather models to look at forecasts several days in advance, and those models indicated Thursday that the storm will likely arrive later than originally expected. Models showed some disagreement about how much rain the storm would bring.

“The broader weather pattern that has dominated over the last few weeks is probably in the process of transitioning, and forecasting that transition into a new system, or a new weather pattern, is one of the hardest things to do as forecasters,” Marty Ralph said. Director of the Center for Western Weather and Water Extremes with the Scripps Institution of Oceanography in San Diego. “The two models show different behavior with respect to this transition.”

One model suggests a strong signal of an atmospheric river bringing heavy rain to California next week, while the other model shows the storm not quite bringing heavy rain, Ralph said.

Either way, an atmospheric river is heading into California next week, and rain is expected. It is expected to bring more rain to coastal California than the Sierra Nevada, potentially the largest rain event in the San Francisco and Los Angeles areas since Oct. 1, according to the National Weather Service.

The storm is expected to begin forming off the Northern California coast early Monday, said Jonathan Garner, a meteorologist at the Weather Service’s office in Eureka. Far Northern California could see some rain early Monday, but Garner said the area will receive the bulk of the rain Tuesday through Thursday. Rainfall amounts during those four days can range from 2 to 4 inches, perhaps as much as 5 or 6 in wetter locations.

“It looks like there’s going to be a storm, but how it develops, we’re not very confident right now,” Garner said.

Previous forecasts indicated the Bay Area would start getting rain on Tuesday, but Alexis Clouser, a meteorologist at the weather service office in Monterey, said the latest forecast shows the storm arriving in the region on Wednesday, with impacts continuing Thursday and Friday.

“The can is being kicked down the road, so to speak,” she said. “The timing is getting further and further behind.”

Rainfall totals in the Bay Area could range from 2 to 3.5 inches, but that is subject to change, the weather service said. “It will be over three days, which could be less or more, depending on how everything goes,” Clouser said.

It’s the same story in Southern California, where forecasters say confidence in the forecast is low.

“We’re still unclear on the exact details of rainfall amounts,” said Rich Thompson, a meteorologist at the Weather Service’s Los Angeles office. “Expectations are still all over the place.”

Thompson said the rainfall period in the area from San Luis Obispo to Los Angeles counties extends from Tuesday night into Saturday, with the best chance for rain on Wednesday and Thursday. “But this is subject to change,” Thompson said.

The best guess for estimated rainfall amounts is 1 to 3 inches, he said.

“There’s a 20% to 30% chance there will be more than that,” Thompson said. “There is a 10% chance it will be one inch or less.”