Bumper-to-bumper traffic makes its way into downtown Los Angeles as an atmospheric river sagged southward from northern California earlier this year. A similar weather setup is on tap this week. (Richard Vogel/AP)

Californians proudly regard themselves as early adopters and trendsetters. So, of course, they’ll be the first to experience an aberrant weather pattern that is expected in the continental United States over the coming days.

A jet stream sagging into the mid-latitudes is forecast to drive into the Golden State some unseasonably late precipitation — and a lot of it — starting Wednesday and continuing into the weekend.

“All in all, ensemble forecasts project rainfall over the next 5-7 days will potentially exceed 200% of normal for the entire month of May across much of the region,” said the National Weather Service’s forecast office in Monterey.

Although precipitation records might fall, flooding isn’t expected despite deep snowpack and full reservoirs.

That same jet stream is then expected to interact violently over the Plains and Midwest with warm, humid air oozing northward from the Gulf of Mexico — the combination needed for extreme and dangerous weather.

California’s wet weather will be fed by three “atmospheric rivers,” fire hoses of moisture from the tropics and subtropics responsible for an amazingly wet winter over much of the West. Since Jan. 1, downtown Los Angeles’s rain gauge has taken in 13.78 inches, about one-third more than normal.

It’s not rare for an atmospheric river to strike the state in May, said Marty Ralph, director of the Center for Western Weather and Water Extremes at the University of California at San Diego’s Scripps Institution of Oceanography. Going back to 2012, of those seven years, four saw atmospheric rivers make landfall in California.

The first of the three will come ashore in northern California at a moderate intensity but with a quick passage.

According to the Weather Service office in Hanford, “Orographic lift under a southwesterly flow along with difluence will support precipitation production on Wednesday. Vort-max energy … along with the cold frontal passage early Thursday morning will add a third and fourth lift component to the equation of even better precipitation production.”


A freeway underpass was flooded in San Francisco recently after an atmospheric river moved through. (Jeff Chiu/AP)

Rainfall in the coastal areas is expected to max out at two inches, with the Bay Area getting about half that, which is notable because the climatological average for May at San Francisco’s airport is 0.47 inches.

In the Sierra Nevada range, a winter storm warning is in effect through Friday morning, with forecasts calling for snow accumulations of six to 12 inches, with localized amounts up to two feet.

The system will sag southward, soaking the Los Angeles area Thursday with a half-inch of rain, while San Diego could get a quarter-inch — again, totals that are about double their May averages.

Because this system is coming in cold, instability could cause thunderstorms, while daily high temperatures will fall 10 to 12 degrees short of daily averages.

After the system exits late Thursday into Friday, there will be a day’s worth of warming before the second atmospheric-river-fed storm arrives Saturday into Sunday. Ralph said uncertainty still surrounds the forecast, but model runs have trended stronger over time.

The third storm is set to strike Tuesday, although the location and strength have yet to come into focus.


Seven-day rainfall forecast from National Weather Service. (PivotalWeather.com)