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 Cañada 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 Cañada Flintridge, foothill suburbs under threat of flood waters draining from the fire-scarred slopes of the San Gabriel Mountains.
I will not be parking my car under a tree that's for sure.
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.