Storm is a trial by water

Authorities and residents were bracing for flooding, thunderstorms, hail, tidal surges and even small tornadoes Wednesday as the worst of a seven-day series of storms was expected to sweep into Southern California.

Wednesday’s storm was projected to be the most intense of the week, the result of a powerful, cold storm from the Gulf of Alaska colliding with a river of subtropical moisture from the western Pacific Ocean.

“When you get the very cold air mixing in with the very warm air, it can be quite volatile,” said Bill Patzert, a climatologist at the Jet Propulsion Laboratory in La Canada Flintridge. Forecasters said the system could produce lightning and possibly waterspouts offshore and small tornadoes on land.

Patzert said Wednesday is “definitely going to be the main event.”

Rainfall rates were expected to be as high as 0.75 to 1.5 inches an hour, which could cause flooding not only in foothills and mountains but also in low-lying areas, said Stuart Seto, a specialist with the National Weather Service in Oxnard.

“The ground is already permeated. There’s already a lot of moisture,” he said. “With the thunderstorms, the rain rates come faster.... We’re going to see a lot more runoff.”

The weather warnings caused Los Angeles County officials to order the evacuation of 232 residences in La Crescenta and La Canada Flintridge, foothill suburbs under threat of flood waters draining from the fire-scarred slopes of the San Gabriel Mountains.

Los Angeles County Supervisor Michael D. Antonovich pleaded with residents to heed the order: “If you fail to comply, it could result in death,” he said.

Debris flows are capable of sending water, tree trunks and car-sized boulders powering down streets at up to 35 mph. “People need to be aware of how dangerous debris flows can be,” said Sue Cannon of the U.S. Geological Survey.

Sierra Madre officials told some residents to be prepared to evacuate immediately.

Gov. Arnold Schwarzenegger has declared a state of emergency in Kern, Orange, Riverside, San Bernardino, San Luis Obispo and Tulare counties because of the storms.

Wednesday’s rainfall also could prove a problem in areas such as Long Beach, San Pedro and Sunset Beach, which can be overwhelmed if flood-control channels cannot handle bursts of intense rainfall.

The rain will be accompanied by winds of 15 to 25 mph on the coast and in the valleys, and gusts of up to 65 mph in the mountains. That could cause trees with waterlogged roots to fall, crashing onto cars and homes.

“I wouldn’t want to be parked under one,” Seto said.

Officials also warned of damaging tidal surges. The high tide Wednesday morning could reach 6 feet.

Rising floodwaters caused problems throughout the Southland.

In the San Bernardino National Forest, a 29-year-old woman had to be rescued late Monday after her pickup truck was swept away while crossing rain-swollen Lytle Creek.

The woman crossed the creek, north of San Bernardino, shortly before 5 p.m. in her Ford pickup when the high water washed away the road and powerful currents pushed the truck down the creek for a quarter of a mile.

As the water filled her cab to the dashboard, “she did the right thing: She stayed in her cab and dialed 911,” said San Bernardino County Fire Department spokeswoman Tracey Martinez.

When the truck lodged on a small island, a swift-water rescue squad fired a harpoon-like gun and propelled a coiled line to the island. A rescuer traversed the rope across the swollen creek, and eventually the woman was pulled to safety after a harrowing, four-hour recovery effort.

“She’s very lucky she had cellphone coverage in that area,” Martinez said.

Officials pleaded with the public to never cross flooded areas and to avoid flood-control channels and downed power lines.

“This is not a playground. This is no time to do extreme sports,” said Jack Wise of the Los Angeles Fire Department.

“Even 6 inches of water in the flood control channels is enough to wash you down the river.... There’s nobody to hear you scream.”

In Orange County’s Trabuco Canyon, rescuers plucked four hikers to safety after they were marooned in their vehicle all night. In Riverside, a man trapped in 1 to 2 feet of floodwaters was rescued from his vehicle.

Floodwaters rose to knee-high levels in some places in the San Diego area, inundating low-lying streets and forcing the evacuation of several dozen homes and businesses.

Three suspected illegal immigrants were scooped up from the swollen Tijuana River, shivering and desperate for rescue.

In Los Angeles County, much of the San Gabriel Mountains have seen more than a foot of rain, with Tanbark Flats above La Verne recording 18.31 inches since Thursday. Downtown L.A. has seen 6.13 inches in the last six days -- 41% of the city’s average annual rainfall.

If anyone was celebrating the colder storm Tuesday, it was the ski resorts in the San Gabriel and San Bernardino mountains. Bear Mountain saw 6 to 9 inches of snow Tuesday after several days of sloppy rain, with more on the way.

With 7 to 10 inches of snow at Mountain High, the ski resort in Wrightwood was preparing to reopen Wednesday. “The ski resorts, they’ll probably get a foot of snow,” said Seto of the weather service. “So they’ll be happy, especially for Christmastime.”

By Thursday, partly cloudy skies should return and stick around through Friday. A weak storm may arrive on Christmas Day, but it is expected to drop less than half an inch of rain.

ron.lin@latimes.com

robert.lopez@latimes.com

Times staff writers Robert Faturechi, Mary Forgione, Tony Perry and Nardine Saad contributed to this report.