Storm Pummels Pasadena and Altadena, Triggers Mudslides and Debris Flows

A destructive storm is making its way out of Southern California Friday, but not before flooding streets, neighborhoods and freeways, causing crashes and congestion, and leading to evacuation warnings and orders in recent burn areas.

Heavy rain pummeled most of the Southland Thursday and intensified late in the day and evening, sparking flooding and mud flows in Altadena and Pasadena.

“We survived the Eaton fire, and now we have flash floods,” lamented one X follower who lives in Kinneloa Mesa.

“We did have moderate mud/debris flow on Park Vista where mandatory evacuations had already occurred, and a few other residents voluntarily left after the slide between two homes.  There will be clean-up this morning on streets especially near foothills of Upper Hastings Ranch but no other major storm-related incidents we’re aware of yet,” Pasadena City spokesperson Lisa Derderian told Pasadena Now early Friday.

Derderian reminded residents that the 10 mandatory and 24 warning evacuations are still in effect until 2 p.m. Friday and can be extended if the need exists.

The storm system began with generally light rain Wednesday, with additional precipitation falling overnight and into Thursday morning across a widespread area. But as the day wore on Thursday, the rainfall intensified, prompting the National Weather Service to issue a series of flash flood warnings for much of eastern, western and central Los Angeles County, including the Eaton Fire burn areas.

According to the NWS, rainfall rates were recorded in some areas at levels as three-quarters of an inch to 1.2 inches per hour — well above the rate that can typically trigger flooding or debris flows.

In issuing flash flood warnings, NWS forecasters said, “There is now a high risk for a life-threatening debris flow.”

As of early evening, however, there were no reports of any injuries from the storm.

The flood warnings covered areas including Pasadena, Altadena, Sierra Madre, Arcadia, and Monrovia.

Evacuation warnings went into effect at 7 a.m. Thursday for recent burn areas. Evacuation warnings were issued in the San Gabriel foothills near the Eaton Fire burn area, affecting thousands of residents. The city of Sierra Madre issued mandatory evacuation orders Thursday morning impacting nearly 1,000 residences.

Los Angeles County Sheriff Robert Luna said individual homes in areas patrolled by his agency were also given mandatory evacuation orders if their properties were considered at risk. Speaking to KNX News Thursday morning, Luna said authorities were taking a “surgical” approach to the mandatory evacuations to specifically target hopes considered at heightened danger.

He said mandatory evacuations as of Thursday morning were issued for nine homes in Altadena.

Pasadena City spokesperson Lisa Derderian said Wednesday night that 10 homes in Pasadena were given mandatory evacuation orders hand-delivered by police officers.

L.A. County public works crews had worked for days in advance to prepare for the storm — emptying debris basins, clearing storm drains and distributing sandbags to residents. In the Eaton Fire burn zone, the county Department of Public Works and the sheriff’s department warned residents living in homes that may be at high risk of damage from mudslides or flooding, and advising them to prepare to evacuate.

The NWS has described the multi-day storm system as likely to be “the biggest precipitation producer so far this season.”

By the time the storm exits the area on Friday, roughly 1 to 3 inches of rain are expected in most coastal and valley areas, with 3 to 6 inches anticipated in the foothills and mountains.

Los Angeles County Public Works Director Mark Pestrella said rainfall rates that exceed a half-inch per hour can lead to mud and debris flows, although factors such as topography, the soil system, geology and the status of the flood control system all contribute to the potential for such occurrences. He insisted, however, that crews have been working around the clock to ensure the county’s 154 debris are clear and have the capacity to capture mud and debris that could descend mountain slopes.

Pestrella said that while the county worked to notify residents in the fire zones whose homes may be at increased danger of experiencing mudslides, he urged residents who think they may be at risk to contact the county at 800-675-4357 to have their property evaluated. The service is available at any county location, not just the recent fire areas.

“This service is going to be provided right up to and to the point that we can no longer get to your properties,” he said.