Southern California Faces ‘High Risk’ of Excessive Rain and Flooding

Parts of Southwest California on Sunday braced for heavy rains — potentially as much as an inch an hour — that could lead to life-threatening floods and “one of the most dramatic weather days in recent memory,” forecasters said.

An intensifying coastal storm will strengthen an atmospheric river that will stream warm tropical moisture into California. Rare forecasts have been issued for life-threatening flooding, hurricane-force winds, waterspouts, tornadoes and heavy snowfall across California from Sunday into Monday.

“This major storm will bring a multitude of dangerous weather conditions to the area,” the National Weather Service in Los Angeles said on Sunday morning.

The Weather Prediction Center issued a rare “high risk” prediction of excessive rain in an area that includes Santa Barbara, Ventura and Oxnard, saying eight or more inches of rain could fall in a 24-hour period.

Over the past decade, some of the deadliest and most destructive floods have occurred in areas that forecasters said were at this level of risk, which is a category they rarely use.

About half the time a high risk is issued, there is at least one fatality or injury, and about two out of every three times, there is at least $1 million in damage, according to data from the Weather Prediction Center.

On Saturday night, the National Weather Service said “rain totals still look to be very impressive” — of at least three to six inches across coastal areas and six to 12 inches for the foothills and mountains.

The rain had already begun falling across California on Saturday night. It was only expected to intensify through the day, with near-continuous rainfall lasting over the next 48 hours as the storm system slowed across Southern California.

The storm is also expected to bring heavy mountain snowfall and widespread gusty winds, Eric Schoening, an emergency response specialist with the National Weather Service, said during a briefing on Saturday.

“The most significant rainfall totals for this event are expected across the central and southern coastline, including the Los Angeles and San Diego metro areas,” Mr. Schoening said.

“This damaging flooding will be a threat to lives and property,” he added. He said that the conditions were stemming from an atmospheric river event.

The governor’s office said it was preparing for the storm with 21 swift water rescue teams placed on standby and having the California State Guard ready for rapid deployment if called upon.

The Santa Barbara Unified School District announced on Friday that all schools and the district office will be closed on Monday because of the storm. Santa Barbara City College said on its website that its campuses will have remote instruction on Monday.

The National Weather Service posted a high wind warning for parts of Southwest California from Saturday evening through Sunday evening 10 p.m. local time.

Winds were expected to be 30 to 45 miles per hour, with damaging gusts of up to 65 m.p.h. Forecasters predicted that they could take down trees and utility lines, leading to power failures.

Winds could become more robust in the mountains and over the coastal waters. A rarely used hurricane-force wind warning was issued for some coastal waters along the central coast of California, where winds could reach hurricane strength, 74 m.p.h. or higher.

For many Californians, the days of officials standing on the edge of nearly empty reservoirs pleading with residents to save water or to let their lawns die may now feel like a lifetime ago.

But as 2022 came to an end, meteorologists began predicting that storms driven by atmospheric rivers would bring significant rain and snow, at last offering a respite from the drought that had plagued the state since 2020.

In the months that followed, Californians experienced the other side of the pendulum swing between weather extremes — swings that have become wider as climate change has made dry periods hotter and rainy cycles more intense.

The winter and spring last year were so jarring and deadly, that when a hurricane appeared to be on track to hit Southern California in August, officials issued warnings that were dire.

What became Tropical Storm Hilary caused damage less catastrophic than had been anticipated. But now, as another atmospheric river barrels toward California, forecasters and officials are urging residents to take the warnings seriously.

Jill Cowan, Judson Jones and John Keefe contributed reporting.