Heavy rain is on its way, and that could mean dangerous mud flows in the Southern California foothills

San Dimas Public Works Supervisor Terry Gregory clears a clogged drain in January as heavy rains caused flooding and mud flows in San Dimas, Glendora and Azusa.
(Robert Gauthier / Los Angeles Times)

Powerful winds and rain will sweep across Southern California on Thursday, bringing the potential for dangerous mud flows in the foothills of Los Angeles and San Bernardino counties, the National Weather Service said.

Starting early Thursday in San Luis Obispo and Santa Barbara counties, the storm will spread southeast across Los Angeles County and the San Gabriel Mountains, where it’s expected to drop 1 to 3 inches of rain by Friday morning, meteorologist Joe Sirard said.

The National Weather Service issued a flash flood watch for the Santa Monica and San Gabriel mountains and foothill communities in the San Fernando, Santa Clarita and San Gabriel valleys.

Mud flows are possible when it rains more than two-tenths of an inch in 15 minutes, and the incoming storm is expected to drop rain at a significantly higher clip than that, Sirard said.


Downtown and the coast will get at least a half-inch of rain, he said.

It will also be windy, Sirard added. In the Antelope Valley, gusts may reach 60 mph Thursday night, while much of the rest of the Southland will see winds of 15 to 25 mph, according to Sirard.

Snow levels could drop to 4,000 feet along Glendora Ridge Road in the San Gabriel Mountains. Glendora Ridge Road from Glendora Mountain Road to Mt. Baldy Road will be closed at 4 p.m. Thursday. The stretch of road will reopen Monday after the snow and debris have cleared.

The storm is expected to last all day Thursday and dissipate by Friday afternoon.


Snow showers and ice could affect Interstate 5 near the Grapevine on Friday night, according to the weather service.

It could be one of the biggest storms of the early rainy season, but weather officials said its long-term impact on Southern California’s drought won’t be known for some time.


For breaking California news, follow @JosephSerna on Twitter.


L.A.'s warehouse crackdown will target owners, not tenants, city attorney says

The 102 million dead trees in California’s forests are turning tree cutters into millionaires


Off-duty L.A. sheriff’s deputy opens fire on bear that charged at his three children


6:08 a.m. Dec. 15 : This article was updated with details about snow levels and road closures.

This article was originally published at 4:35 p.m. Dec. 14