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Spokane, Washington  Est. May 19, 1883

Major California storm could unleash life-threatening flooding, forecasters warn

Giant sequoia trees stand as snow falls in the Grant Grove of giant sequoia trees during an atmospheric river storm on Thursday in Kings Canyon National Park, Calif.  (Mario Tama)
By Dan Stillman and Diana Leonard Washington Post

Forecasters at the National Weather Service are warning of a “significant threat of widespread and very dangerous flash flooding” Saturday night through Monday for much of densely populated Southern California. The cause is a particularly intense atmospheric river that will aim a fire hose of moisture at the coast and run up against the region’s mountainous terrain.

The potential for flooding is highest from around Santa Barbara to the south. Strong winds, mountain snow and dangerously high surf are also a concern, with storm impacts expected to reach as far north as the Bay Area.

The Weather Service office serving the Los Angeles region is cautioning that the expected torrents could trigger rock and mudslides in the mountains, and flooding of small streams, rivers and roadways throughout the region. Burn scars left behind by wildfires will be particularly prone to flooding and debris flows.

“People need to start preparing now for a major flooding event,” the Weather Service wrote in a discussion Friday.

The potential for flooding comes just days after another atmospheric river dumped several inches of rain across portions of the state, including record daily amounts in southwest California.

The storm is still a few days away, so precipitation estimates are still coming into focus; some recent model projections backed off forecasts that were considered extreme. But many foothill and mountain areas could see 6 to 12 inches or more of rain. Santa Barbara, a coastal city with steep mountains behind it, could see 10 to 15 inches of rain in the higher terrain. Such amounts have caused “major” problems in the past, the Weather Service said.

“Between the tremendous amount of moisture that this storm has with it, good southerly flow and slow movement – those are all the ingredients for some of our biggest storms,” said David Bruno, a meteorologist with the Weather Service in Oxnard, California. “But I want to stop short of saying that this is going to be something we’ve never seen before.”

Downtown Los Angeles is most likely to see 3 to 5 inches of rain, which wouldn’t be unprecedented for a two-day period. “However, if we start talking 6 or 7 inches of rain in two days, then we’re close to the top few events ever,” Bruno said.

The Center for Western Weather and Water Extremes, based in La Jolla, California, predicts the atmospheric river will reach category 3 on its 1-to-5 scale, with 5 being the most extreme. The scale defines a category 3 atmospheric river as “strong” and producing a “balance of beneficial and hazardous impacts,” including the threat of flooding but also helping to refill reservoirs. A category 3 is considered by meteorologists to be on the higher end of atmospheric rivers that hit Southern California.

Downpours will be fueled by unusually warm waters off the California coast, intensified by the El Niño climate pattern. Sea surface temperatures are much as 3 to 4 degrees Fahrenheit above normal.

“This warm ocean increases amount of moisture in lower levels of atmosphere (& thus in storms) due to increased evaporation, and may also be contributing to apparent propensity for extreme hourly t-storm downpours this year,” Daniel Swain, a climate scientist at the University of California Los Angeles, wrote on X, formerly Twitter.

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The forecast for Southern California

Rain chances increase late Saturday afternoon and evening in Southern California, with the heaviest rain and highest risk for widespread flooding of roads, streams and rivers anticipated late Saturday night through Sunday night in Santa Barbara, Ventura and San Luis Obispo counties, and Sunday night into Monday in Los Angeles County. Strong and damaging winds from the southeast could cause power outages, while travel delays and road closures are likely because of heavy mountain snow and possibly mudslides.

Particularly heavy rain is likely on and below the south side of mountain ranges such as the Santa Ynez and Topatoga ranges. Rainfall rates of a half-inch per hour could last for many hours with up to one inch per hour possible on and below south-facing slopes. There’s also a slight chance for thunderstorms in San Luis Obispo and Santa Barbara counties.

The exact timing of the heaviest rain in Los Angeles County, how long it will last and ultimately how much will fall are uncertain because models indicate the storm’s plume of moisture could temporarily stall over Santa Barbara and Ventura counties Sunday into Sunday night, and there’s some question as to how slow or fast it will advance through Los Angeles county once it starts moving again.

The Weather Service is predicting an average of 3 to 6 inches in most coastal and valley areas through Monday, 6 to 12 inches in the foothills and mountains, and as much as 15 inches across the mountains of southern Santa Barbara and western Ventura counties. Southern Santa Barbara County through western Los Angeles County has the best chance of seeing rainfall totals reaching the upper end of the forecast ranges, especially if the moisture plume slows or stalls there.

The rainfall will come after calendar rainfall records were set Thursday at several locations in southwest California, including 2.93 inches at Santa Barbara, 2.49 inches at Los Angeles and 2.45 inches at Long Beach.

Farther south, Orange and southwestern San Bernardino counties are expecting 3 to 4.5 inches in coastal areas and 5 to 8 inches in the mountains, with locally greater amounts on the coastal slopes. For San Diego and western Riverside counties, which experienced serious flooding about 10 days ago, additional rainfall of 2 to 3 inches is possible near the coast and 3 to 5 inches in the mountains and 0.5 to 1 inch in the lower deserts.

The storm will also bring strong winds, moderate coastal flooding and dangerous surf and rip currents. Winds along the coast could reach or surpass 50 mph Sunday and Monday. Surf heights are forecast to 8 to 12 feet south of Point Conception, the Weather Service said. In the mountains, winds could top 60 mph.

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The forecast for the Bay Area and Central Coast

Flood watches are up for the Bay Area and Central Coast late Saturday afternoon into Monday morning after many locations recorded 4 to 5 inches of rain Wednesday into early Thursday. The Weather Service is warning of flooding on creeks, streams and rivers, and in urban and low-lying areas.

Rain should increase Saturday evening along the Big Sur coast before spreading north and east through Saturday night. The heaviest rain arrives early Sunday with highest amounts through Monday expected along the Central Coast at Big Sur and the Santa Lucias, where rainfall could reach 5 to 7 inches. Lesser amounts around 2 to 3 inches are forecast around San Francisco and Oakland.

A high wind warning has been issued for Sunday for the San Francisco Peninsula Coast, Santa Cruz Mountains, Eastern Santa Clara Hills, East Bay Hills and Northern Monterey Bay because of the potential for gusts up to 60 mph that could blow down trees and power lines. In some areas, especially southern zones, the winds could be stronger than the previous storm.

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Heavy mountain snow could fend off drought threat

While most storms this winter have targeted coastal California, the upcoming atmospheric river could also deliver 2 to 4 feet of snow to the higher elevations of the Sierra Nevada – a welcome change given that precipitation has been lagging behind in this key region for the California’s water supply. Statewide snowpack is only about half of where it should be this time of year, though it has improved this week.

Because of the deficits, the latest federal Drought Monitor placed a swath of the Sierra Nevada in the “abnormally dry” category, which can be a precursor to drought.

However, California’s reservoirs are still above average and flush with water because of last year’s record-breaking snowy winter. In fact, some reservoirs are releasing water for flood protection ahead of this week’s storms.

“Even though this winter has provided below average precipitation overall, it can only take one storm to produce severe flooding in the right circumstances,” the California Department of Water Resources said in a statement this week. “Communities in San Diego, the Bay Area and along the North Coast have already experienced flooding impacts this winter, highlighting the need to always be prepared for flood risk.”

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Jason Samenow contributed to this report


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Video: Floodwaters submerged residential areas of Los Angeles County on Feb. 1 as an atmospheric river brought heavy rain to the area.© 2024 , The Washington Post

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