Tuesday, December 23, 2025

Atmospheric River Brings Flood, Mudslide Risks to Altadena, Pasadena Through Christmas

A long-duration atmospheric river is expected to bring heavy rain, strong winds and an elevated risk of flooding and debris flows to Altadena, Pasadena and the San Gabriel foothills from late Tuesday through nightfall on Christmas Day, according to the National Weather Service office in Los Angeles/Oxnard.

Forecasters have issued a Flood Watch for Altadena and Pasadena from Tuesday afternoon through late Thursday, warning that the most dangerous conditions are likely from late Tuesday night into Wednesday morning, when rainfall rates are expected to peak.

The storm system is classified as an atmospheric river, with National Weather Service meteorologists and partner agencies indicating the event aligns with an AR3-category system for parts of Southern California.

Heaviest rain expected overnight into Wednesday

According to the National Weather Service’s Area Forecast Discussion and quantitative precipitation outlooks, rain will begin increasing across the San Gabriel Valley late Tuesday afternoon, intensifying Tuesday night as the first major surge of the storm moves through the region.

For Altadena, which sits directly below the San Gabriel Mountains, forecasters project one to two inches of rain Tuesday night, followed by an additional three to four inches during the day Wednesday.

Another three-quarters of an inch to one inch is possible on Wednesday night.

Pasadena is forecast to receive slightly lower totals, with one to two inches Tuesday night and two to three inches Wednesday, followed by up to three-quarters of an inch Wednesday night.

National Weather Service forecasters expect peak rainfall rates in Los Angeles County between about 2 a.m. and 5 a.m. Wednesday.

In foothill and mountain areas, hourly rainfall rates are projected to range from roughly 0.55 to 0.85 inches per hour, with broader regional guidance indicating peak rates between 0.60 and 1.25 inches per hour on favored slopes.

A second surge of the storm is expected on Christmas Day, bringing another round of moderate to heavy rain.

While the National Weather Service has not yet published specific rainfall totals for Thursday for Altadena and Pasadena in its point forecast products, forecasters warned that additional rainfall could increase flooding and debris-flow risks due to already saturated ground.

Flooding and debris-flow concerns in foothill areas

The NWS warned that the prolonged and intense rainfall could lead to widespread urban flooding, rockslides and mudslides, particularly in foothill communities such as Altadena and upper Pasadena. Meteorologists noted that debris flows are possible near recent burn scars in the Angeles National Forest and may also occur outside burn areas because of the storm’s duration and intensity.

A Flood Watch for Altadena and Pasadena is in effect from 4 p.m. Tuesday until 10 p.m. Thursday.

The Los Angeles County Office of Emergency Management and law enforcement agencies are advising residents in designated at-risk zones to evacuate before the rain begins.

Evacuation warnings and orders for vulnerable homes identified by LA County Public Works will take effect at 11 a.m. Tuesday.

“GET SET for possible evacuations if you are near San Gabriel Foothills in Altadena. Gather loved ones, pets and supplies. Those who need more time evacuating should consider leaving now,” the alert said.

Sheriff’s deputies are conducting door-to-door outreach at those residences, and the Sheriff’s Homeless Outreach Services Team is contacting unhoused individuals living near local waterways to warn them about the storm and offer services.

Winds add to power and tree risks

In addition to heavy rain, strong winds are expected across Southern California as the storm moves through.

The National Weather Service has issued a Wind Advisory for the Altadena and Pasadena area from midday Tuesday through Thursday afternoon.

Forecast discussions indicate widespread wind gusts of 35 to 55 mph are possible, with stronger gusts expected in foothill and mountain locations.

The National Weather Service warned that saturated soils combined with gusty winds could result in downed trees and power lines.

Snow levels remain high for now

Snow levels are expected to remain relatively high during the storm, generally between 7,500 and 8,000 feet through Christmas Eve, lowering to around 7,000 feet on Christmas Day, according to the National Weather Service.

Forecasters also indicated that a colder system later in the week could lower snow levels further after Christmas.

Evacuations and road impacts

Los Angeles County has issued evacuation warnings for some burn-scar areas elsewhere in the county ahead of the storm, directing residents to check the Genasys Protect evacuation map for address-specific information.

No storm-related closures have been reported in Altadena or Pasadena as of 4 a.m. Tuesday