Dire California atmospheric river storm predictions now seem less likely, computer models show

While Central and Southern California are still expected to be walloped by another atmospheric river storm that will arrive Sunday and last through Tuesday, the computer models that initially predicted a potentially catastrophic amount of rainfall have eased off somewhat.

European computer models suggested Thursday that as much as 12 inches of rain could fall through early next week in the mountains near Santa Barbara, but newer models have eased off slightly, with 7-9 inches forecast to fall on the Sierra Madre, San Gabriel and Santa Ynez Mountains.

In a message posted to X, UCLA climate scientist Daniel Swain, who held an online briefing with reporters Friday, noted that the European climate models “have backed off slightly from the dramatic SoCal rainfall predictions from yesterday,” but warned that there was still “widespread/significant flood risk.”

The National Weather Service also continues to warn residents in the Los Angeles area to prepare for the “potential for damaging, life-threatening flooding” as well as for the possibility of power outages caused by the storm.

A lot more rain set to fall

Los Angeles normally receives between 10-15 inches of rain in a year, and could see a third of that or more between Sunday and Tuesday.

Swain said that the area of California that would be hardest hit was the coast between Monterey and Los Angeles, but that the Sierra Nevada Mountains will be “hammered” with as much as 4-6 feet of snow, and heavy winds are also expected to start on Sunday in the Bay Area and to extend south as far as Santa Barbara.

The first of two back-to-back storms came ashore in California on Wednesday, setting a new daily record for rainfall in Long Beach, which received 2.45 inches, and Los Angeles, which got 2.49 inches in a single day.

Intense thunderstorms have accompanied recent atmospheric river events in California, bringing torrential downpours like the ones that inundated Ventura in December and San Diego in January. This week’s first storm also saw torrential downpours, with over an inch of rain falling in an hour in Long Beach on Wednesday and that same rate in San Francisco on Thursday.

Unlike the first round of moisture, Sunday’s storm is expected to be a slow-moving system that will dump more rain on already saturated soil. Los Angeles is expected to see as much as 5 inches of rain, and more in the surrounding mountains.

"This is the type of rain that they cannot handle," FOX Weather Meteorologist Britta Merwin said. "This is a guaranteed flood setup. There's no way around it. We know it's going to be bad, and there's going to be huge impacts."

Why are these storms so volatile?

Swain said that one reason that California was seeing such severe weather is that the ocean temperatures have been “three to five degrees Fahrenheit above average.”

“Much of the ocean between California and Hawaii is experiencing a significant marine heat wave and has been for months,” Swain noted, adding, “the warmer the surface ocean is, the more potential evaporation there is off of it, and, in general, the more moisture there is in the lower atmosphere.”

The cause of that warmer-than-average ocean, which gives rise to more atmospheric instability, Swain said, was a “combination of El Nino and global warming.”