A week of heavy rains has caused flooding and overnight landslides as another storm moves into Southern California.

Most of downtown Laguna Beach was closed by police after streets were inundated with floodwaters, and Laguna Canyon Road was shut down.

[Updated at 6:15 a.m.: Laguna Beach police reported rockslides in canyon areas and urged residents to remain in their homes for now and avoid downtown, portions of which were under several feet of water.]

Numerous Orange County roads were partially or fully closed, including Ortega Highway, the toll road California 241 and Pacific Coast Highway in Newport Beach.]

In Silverado Canyon, the Orange County Fire Authority was responding to reports of rock and boulder slides. Silverado has experience major flooding and rock slides in previous heavy downpours.

A transition road to the 10-71 interchange in Pomona was closed after being hit by mud and rockslides.

A mudslide has closed Metrolink tracks between San Juan Capistrano and Oceanside.

The incidents came as Los Angeles braced for another powerful storm. 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 was "definitely going to be the main event." Rainfall rates were expected to be as high as three-quarters of an inch to 1 1/2 inches per 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.

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-- Louis Sahagun, Richard Winton and Rong-Gong Lin II