CLEARING OPERATION(Reuters)–A bulldozer clears mud near homes in La Canada Flintridge, Los Angeles. Authorities ordered the evacuation of 750 homes in the foothills around LA, fearing that series of powerful storms would trigger flash floods and mudslides

LA CANADA FLINTRIDGE, California (AP) – Hours of heavy rain fell Thursday on saturated Southern California as the fourth Pacific storm in a week came ashore, triggering dire warnings by authorities that huge mud flows were likely in foothill communities and residents of endangered homes should obey evacuation orders.

A teen was rescued from a rushing river but a companion remained missing in Orange County, and the fate of a motorist spotted under tornado-toppled power poles in the state’s remote southeast corner was unknown.

Travel snarls mounted as major highways were closed by snow and tornado damage, and strong winds grounded flights at several airports. Another tornado left a trail of damage in a community northwest of Los Angeles.

The siege of storms has led to several deaths statewide and street flooding in urban areas, and has turned the region’s often-dry river and creek channels into raging torrents.

Muddy water gushed down hills but there were no immediate major incidents, and officials appeared concerned the lack of massive debris flows from wildfire burn areas was misleading for residents.

Los Angeles County Fire Department Chief Deputy John Tripp bluntly warned at the outset of the storm that significant debris flows were likely and probably would block potential rescue attempts.

“For those people that are still in the homes and are in those areas of threat, it’s very likely we will not be able to reach you,” he said.

By nightfall, the storm’s main rainfall was passing but forecasts warned of volatile conditions through the night. Los Angeles Mayor Antonio Villaraigosa said the city’s evacuation orders remained in effect.

“While the worst of the last few storms is behind us, there still is a significant threat from thundershowers that are forming off the coast and have the potential to bring lightning, hail, waterspouts and small tornados,” he said.

In the upper reaches of suburban La Canada Flintridge, where mountainsides rise sharply from the backyards of homes, authorities put pink ribbons on the mailboxes of residents who stayed behind so they would know where to search in the event of a catastrophe.

One person who stayed was Delos Tucker, a retired geologist who has lived in the community since the homes were built in 1962.

“I’m just gambling it’s not going to happen,” he said. “Let’s hope I’m right.”

As an overnight lull gave way to more rain at midmorning, public works crews shoveled mud from yards, driveways and gutters along Ocean View Boulevard in suburban La Canada Flintridge. The neighborhood was otherwise all but deserted, with newspaper and mail deliveries cut off.

The county’s extensive flood-control system was working, but many of the basins designed to catch debris-laden runoff from fire-scarred mountains were full and evacuations remained necessary, said Gail Farber, the Los Angeles County Public Works director.

The basins are located on streams and other water courses emerging from the mountains to intercept surges of mud, boulders and other debris while allowing water to flow into open channels and underground storm drains that empty into the ocean.

The new storm system shut down Interstate 5 in the snowy Tehachapi Mountains north of Los Angeles for the second day in a row. And the California Highway Patrol closed part of Interstate 80 in the Sierra Nevada after about a dozen cars and trucks crashed in a heavy snowstorm.

In Orange County, firefighters pulled a 14-year-old boy from the swollen Santa Ana River, but a 17-kilometers search failed to find a companion the rescued boy said was also in the water. Orange Fire Department Capt. Ed Engler the search was called off by evening after efforts to spot the youth from bridges and helicopters.

A tornado rampaging across desert highways near the Arizona state line toppled high-voltage power poles that trapped a motorist in a vehicle on State Route 78, said Terri Kasinga, spokeswoman for the California Department of Transportation. The person’s status could not be determined because the lines remained electrified.

The tornado also toppled three big-rigs on Interstate 10 and more power lines on US 95. The I-10 reopened late Thursday afternoon, but the other highways were likely to remain blocked for two days.

A fierce wind struck two neighborhoods in Ventura, and witnesses described a tornado, police Sgt. Jack Richards said. Trees were toppled, cars were damaged and a shed was torn apart in a three-kilometer span through two neighborhoods. No one was hurt.

National Weather Service meteorologist Eric Boldt visited the scene and confirmed it was a small tornado.

Southwest Airlines canceled hundreds of flights in Southern California and Arizona due to strong winds and heavy rains.

Southern California Edison said more than 30,000 customers were without power, and repair crews were having trouble reaching equipment in desert and mountain areas because of snow. Another 4,100 outages were reported in Los Angeles.

Acting Gov. Jerry Brown declared a state of emergency in Los Angeles, Orange, Riverside, San Francisco and Siskiyou counties because of the statewide storm impacts.