Los Angeles On Alert For Burn Scar Flooding

A potent atmospheric river will continue to wring out heavy rain in parts of California and the Southwest, leading to flash flooding and the potential for debris flows in areas recently burned by wildfires, including the Los Angeles Basin.

Happening Now

This latest storm arrived in Northern California late Wednesday night into Thursday morning. Its rain is now extending from Southern California to the Sierra foothills to southern Nevada, including Las Vegas.

Early this morning, flooding and rocks were reported on a stretch of U.S. 101 near Gaviota in Santa Barbara County, California, where almost 2.5 inches of rain had already fallen from the storm.

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Some wind gusts over 30 mph have been clocked this morning in the high country from Santa Barbara County to the San Gabriels, Tehachapis and southern Sierra.

There were 22 reports of flooding in Northern California Thursday, particularly around the Bay Area and Sacramento metro areas. Rainfall totals of 2 to 5 inches have occurred in the foothills of the northern Sierra, as well as the Santa Cruz and Santa Lucia mountains south of the Bay Area.

Winds gusted as high as 80 mph in Marin County just north of San Francisco near Mt. Tamalpais on Thursday. Trees and power lines were downed in some areas, including in far northwest California, where multiple downed trees required a closure of the Pacific Coast Highway north of Point Arena.

Alerts

The National Weather Service has issued flood watches for virtually all of Southern California, including the entire L.A. Basin and San Diego. Those watches extend into the Mojave Desert and also into the southern Sierra foothills.

A winter storm warning is in effect in the southern Sierra, and a winter weather advisory is posted for the northern Sierra, where significant accumulation is forecast generally for those locations generally above 8,000 feet in elevation.

Forecast Timing

- Friday: Areas of locally heavy rain will persist over all of Southern California into the Mojave Desert and southern Sierra foothills. Only occasional, light showers are expected elsewhere in the state.

- Saturday: This is the day of greatest flood threat in Southern California, where heavy rain is expected, especially early in the day, possibly accompanied by thunderstorms. Later in the day, that plume of heavier rain could move into the Mojave Desert as far east as southern Nevada and western Arizona. Some of that rain and high mountain snow will also dampen the San Joaquin Valley and southern Sierra.

- Sunday: Some showers will linger over much of California and the Southwest. Another Pacific frontal system will move into Northern California Sunday night with more soaking rain.

- Monday: The band of soaking rain will move through Southern California during the day, then into Nevada and Arizona Monday night. Scattered showers and thunderstorms are possible in the rest of California.

- Tuesday: Some showers may linger in Southern California while the steadier rain and threat of a few thundershowers is in the Desert Southwest from Arizona to southern Utah and southern Nevada.

Impacts

As the map below shows, we expect much of California, parts of Nevada and Arizona to pick up at least another inch of rainfall through Monday night.

The heaviest additional rain will be in Southern California, particularly in the hills and mountains where another 3 to 8 inches of rain could fall.

This poses a danger of flooded roads prompting closures, rockslides and damaging debris flows, particularly in areas recently burned by wildfires.

We suggest avoiding unnecessary travel in Southern California Saturday and Saturday night, during the peak of the storm. If you live near a burn area or an area susceptible to landslides, be ready to evacuate immediately if an order is issued by local officials.

There could also be local flash flooding of normally dry washes and arroyos in the desert, including the Las Vegas and Phoenix metro areas. Never attempt to drive through flood water. Turn around and find an alternate route.

Expect snow-covered roads in the Sierra, especially the southern Sierra, above 8,000 feet in elevation.

Why So Wet?

The reason for this is that an area of low pressure aloft will move very slowly near the California coast before finally pivoting inland Sunday.

When that happens this time of year, it can tap a deep plume of moisture known as an atmospheric river. They're typically responsible for the lion's share of California's precipitation in their wet season, from later in the fall through spring.

While this atmospheric river is of moderate strength, when they move slowly, they can wring out heavy rainfall, especially as the warm, moist air piles into hills and mountainsides, enhancing lift.

Jonathan Erdman is a senior meteorologist at weather.com and has been covering national and international weather since 1996. Extreme and bizarre weather are his favorite topics. Reach out to him on Bluesky, X (formerly Twitter) and Facebook.