Potentially catastrophic rain is heading toward Los Angeles — a region already scarred by recent extreme weather and climate events, making it more vulnerable to the influx of water.
Millions of people across California are on alert for flooding over the Christmas holiday, according to the National Weather Service.
But Southern California will be under the bullseye of a prolonged, strong atmospheric river that will bring periods of heavy rain beginning Tuesday night and lasting into Thursday evening.
Significant and widespread flooding caused by excessive rainfall is likely, with potential debris flow impacts across recent burn scars.
The first round of rain — beginning at about 9 p.m. Tuesday — will be the strongest. Heavy rain is expected first over the Santa Barbara area before moving to the Los Angeles area around midnight.
The rain over Los Angeles could continue until as late as 6 p.m. Wednesday, equating to 18 hours of moderate to heavy rain over the region on Christmas Eve.
Rainfall rates may exceed 1 inch per hour at times, especially at higher elevations.
Widespread rainfall totals over the 18-hour period are expected to reach between 3 inches and 7 inches, but some localized spots could see 9 inches or more.
The month of December typically brings around 2 to 3 inches of rain to the region, on average, records show.
The National Weather Service’s Weather Prediction Center issued a rare “High Risk” (level 4 of 4) for excessive rainfall and flash flooding on Wednesday across portions of Southern California. This includes portions of Santa Barbara, Ventura, Los Angeles, and San Bernardino counties.
The “High Risk” includes Los Angeles, Burbank, Altadena, Glendale, San Bernardino, Santa Clarita, and Thousand Oaks.
Being under a “High Risk” is a rare occurrence. This risk is only issued about 4% of days, accounting for one-third of all flood-related fatalities and 80% of all flood-related damages, according to the National Weather Service.
The Weather Prediction Center has issued a level 4 of 4 risk, a major risk, for excessive rainfall and flooding for Ojai, Thousand Oaks, Simi Valley, Santa Clarita, San Fernando, Glendale, West Hollywood, El Monte, San Bernardino and the Angeles National Forest.
Los Angeles and Ventura counties were devastated by the Palisades and Eaton fires in January.
Evacuation warnings are in effect from Tuesday to Thursday at 11 p.m. for the Palisades, Sunset and Hurst burn scar areas.
In this Jan. 7, 2025, file photo, firefighters battle the Eaton Fire in strong winds as many homes burn, in Pasadena, Calif.
David McNew/Getty Images, FILE
Evacuation orders are in place for a select number of vulnerable properties. Los Angeles police were going door-to-door, alerting residents at vulnerable addresses, ABC Los Angeles station KABC reported.
Recent wildfire burn scar areas are especially prone to dangerous, flash flooding and could also trigger debris flows and mudslides. The threshold for triggering flash flooding decreases with burned soil. Lower rainfall totals could still trigger flash flooding and it can unfold very quickly.
“With as much total rain as what we’re expecting, widespread, significant flooding will likely occur in urban and poor drainage areas, especially in and around the higher terrain,” National Weather Service at Oxnard Meteorologist-In-Charge Ariel Cohen said during a press conference Tuesday afternoon. “There will almost certainly be numerous rock slides and mudslides, along with areas of severe flooding in urban locations and along freeways, which is why being out on the roads will be exceptionally dangerous.”
Rainfall that would normally be absorbed into the ground, instead remains as runoff and can trigger dangerous and significant flash flooding quickly, especially downstream and downhill from a wildfire burn scar area.
The National Weather Service highlights that in some instances, burned soil can be as water repellant as pavement, A general rule of thumb is that half an inch of rainfall in less than an hour is sufficient to cause flash flooding in a burn area. The susceptibility to flash flood within the burned area is greatest during the first two years following the fire.
In recent years, parts of California have been experiencing a shift from major drought stages to an extended period of above-average precipitation, which allowed for abundant vegetation growth.
In this Feb. 13, 2025, file photo, heavy rains created a mud filled debris flow sending cars down Tanoble Drive onto Mendocino Street in the Eaton fire burn area, in Altadena, Calif.
Gina Ferazzi/Los Angeles Times via Getty Images, FILE
Hydroclimate whiplash — the rapid shift between wet and dry conditions — likely contributed to the severity of the wildfires burning in Southern California, experts say.
Flood impacts include significant and widespread urban roadway flooding, a high risk of major mudslides and rockslides and rapid rises to creeks, streams and rivers, which will likely lead to swiftwater rescues.
Residents should expect major travel delays, cancellations and road closures.
Much of California also faces strong winds beginning Tuesday night. The winds will be strongest in the higher elevations and mountains.
While the dangerous flash flooding threat is centered over southern California, the northern half of the state will face the brunt of the strong winds in the coming days.
High wind warnings have been issued for cities such as Sacramento, Redding, Santa Barbara, and Santa Clarita. Wind gusts of 60-70 mph are possible in these areas, bringing the threat of power outages and wind damage. Isolated gusts over 70 mph are possible in some of the mountains.
Wind advisories are in effect for many of the major cities along the coast, including Los Angeles, San Francisco, and San Diego with winds gusts of 40-50+ mph possible.
ABC News’ Dan Peck contributed to this report.