Several atmospheric rivers have soaked California this month, and another one is about to drench the Golden State. A days-long spate of active weather will bring downpours and possible flooding to California, as well as significant mountain snow, the chance of mudslides and even the risk of an isolated tornado.

Flood watches blanket much of state’s coast and Central Valley, with a widespread 3 to 5 inches of rain are expected. The National Weather Service predicts a moderate risk of flash flooding and excessive rainfall on Sunday — level 3 out of 4 on the risk scale — for portions of the Santa Lucia and Santa Ynez mountain ranges in central and Southern California, where there’s a 70 percent chance that rainfall rates will top 0.75 inches per hour.

“This should be enough to result in some areas of significant flooding,” the Weather Service wrote.

Advertisement

The elevated risk for excessive rainfall won’t diminish until Wednesday.

Wind advisories and high-wind warnings also span the entire coast, with gusts of 45 to 55 mph anticipated near the shore and at least 60 mph in the mountains.

The Weather Service’s Storm Prediction Center has even taken the unusual step of drawing up a level 2 out of 5 risk for severe thunderstorms that could produce tornadoes in the Sacramento Valley on Monday, warning of a likelihood for rotating thunderstorms during the late morning and early afternoon. Sacramento, Stockton and Modesto are in the zone to watch.

While the initial atmospheric river should subside by late Monday into early Tuesday, a potent upper-air disturbance will keep additional rain in the forecast through late Wednesday.

The drenching comes barely 10 days after a top-tier atmospheric river dumped 7 inches of rain — nearly two months’ worth — on Los Angeles in just two calendar days, and may have dropped over a foot of rain on the UCLA campus in 24 hours.

The setup

A low-pressure system is present several hundred miles offshore of Northern California. It will weaken gradually over the next several days as it ambles toward Oregon.

Advertisement

The low, which is spinning counterclockwise, is dragging a strip of robust tropical moisture — with origins as far away as Hawaii — into California. Like a fire hose, it will remain pointed at the state for days while slowly moving down the coast. By early Wednesday, the moisture stream will pinch off as it reaches Southern California.

Simultaneously, the parent upper-air disturbance — a pocket of high-altitude cold air, low pressure and spin — will approach California. That frigid air aloft will generate thunderstorms and downpours that will pinwheel into California, bringing some additional rainfall and the risk of funnel clouds or an isolated tornado.

The entire event will wind down Wednesday.

Hazards

Heavy rainfall

With the initial atmospheric river, most locations in central and Northern California will see about 12 hours of rain. It will arrive in northern regions late Sunday, around midnight, then reach about as far south as Santa Barbara by late morning or lunchtime.

Advertisement

The coastal mountain ranges will generally see 3 to 5 inches of rain, although 6 to 8 inches may fall in the mountains between Monterey Bay and San Luis Obispo in Pfeiffer Big Sur State Park.

The Central Valley will generally receive 2 inches of rain, with closer to an inch south of Fresno. A few localized greater totals are expected in the Sacramento Valley.

The Sierra Foothills below the freezing line might also pick up between 4 and 7 inches of rain. A few landslides or mudslides can’t be ruled out.

In Los Angeles, about 2 to 3 inches of rain is expected, mainly Monday into Tuesday. The National Weather Service is warning that, despite rain totals being less than the previous storm, heavier downpours brought about by thunderstorms will exacerbate any ongoing flood risk.

“Debris flows, mudslides, landslides, and swift water rescues could happen just about anywhere,” the agency cautioned.

Advertisement

Like Los Angeles, San Diego is under a flood watch. It will see its heaviest rain Tuesday into Wednesday morning.

Heavy snow

Snow will fall in the higher terrain of the Sierra Nevada, mainly above 6,000 feet, all day Monday. Summits above 7,500 feet might see 3 to as much as 5 feet of snow, while areas at elevations between 6,000 and 7,000 feet will receive 8 to 12 inches. By Tuesday, the snow will taper to isolated snow showers.

The mountains of Southern California could see 1 to 3 feet of snow, mainly above 7,500 feet, the Weather Service said.

Strong winds

Monday will be the main day for any gusty winds, with gusts of 40 mph in the Central Valley from Modesto all the way to Shasta-Trinity National Forest. Gusts of 45 to 55 mph are expected at the coast, primarily north of San Luis Obispo. In the mountains, gusts topping 60 mph are possible, while gusts over 80 mph may buffet the Sierra Ridge.

Advertisement

In Southern California, the Weather Service office in Los Angeles wrote that winds could reach 40 to 60 mph in the mountains and 20 to 40 mph elsewhere.

Tornado risk

It’s rare to see the Storm Prediction Center forecast a risk of tornadoes in California, but weather models have been consistent in highlighting the San Joaquin Valley as a likely location for a few rotating thunderstorms.

Cold air in the upper atmosphere will contrast with slightly milder air near the surface to allow a few pockets of ground-level air to rise and form thunderstorms. The thunderstorms won’t be overly tall, but they’ll exist in an environment of strong wind shear — or a change of wind speed and/or direction with height — particularly with the jet stream slicing overhead.

ncG1vNJzZmivp6x7uK3SoaCnn6Sku7G70q1lnKedZMSmrdOhnKtnYmV%2FdXuPa2ZqcF%2BWwa670qmfnqqZmHqztdWeqWarpKS%2FrnnCmqOinp%2Bnu6qtjJ%2BjqKeUqHw%3D