(CBS/AP)

Southern California communities below wildfire-scorched mountains prepared for the possibility of fast-moving floods laden with mud and rocks as a Pacific storm headed for the West Coast.

Sandbags and concrete barriers called K-rail were placed on streets Monday in suburbs northeast of Los Angeles to try to direct any debris flows away from homes.

"There's really nothing else to do but wait and see what happens," said David Wacker, a 25-year resident of La Crescenta, one of a string of communities along the foot of the steep San Gabriel Mountains.

Tropical Storm Patricia approached the Mexican coast, prompting a tropical storm warning for the southern portion of Mexico's Baja California peninsula. Patricia's maximum sustained winds are near 60 mph with some strengthening possible. By Tuesday morning, the tropical storm was centered about 155 miles south-southeast of the peninsula and was moving north near 6 mph.

Patricia is expected to make its closest approach to the peninsula late Tuesday or early Wednesday, but its impact will likely be felt in Southern California as well.

The rain comes on the heels of the late summer Station fire - the largest fire in Los Angeles County history, which burned through more than 160,000 acres, destroyed 89 homes and killed two firefighters. The vegetation and roots that used to hold the ground in place are now gone, reports CBS News correspondent Ben Tracy.

"This is a really burned mountain side and with the right amount of rain we could have a really bad, bad situation," Lucy Jones, of the U.S. Geological Survey, told CBS News.

Unusually strong for October and packing gusty winds, the storm was expected to move into northern and central parts of the state Monday night and reach southern areas Tuesday, the National Weather Service said.

Forecasters said the system was expected to pull in considerable moisture left over from Typhoon Melor, which made a damaging hit on Japan last week after drenching the Northern Mariana Islands.

Rainfall across Southern California was expected to be heavy and widespread, bringing threats of flash flooding and debris flows in burn areas.

Scientists predict that heavy rainfall over a three-hour period would be enough to cause hillsides in the area to slide, reports Tracy. Overall, there's a 80 percent chance of debris flows - enough to cover a football field with mud and rock 60 feet deep, they said.

"All the ingredients necessary for a big rain event are in place," the NWS said.

Forecasters estimated that 3 inches to 6 inches of rain would fall in Santa Barbara County mountains, where an 8,700-acre fire destroyed 80 homes in May. Flash flood watches were issued there and in 10 other counties up and down the state.

Estimates for Los Angeles County mountains and foothills ranged from 2 inches to 4 inches, with the heaviest period Tuesday night and Wednesday.

Debris flows occur because the ground in recently burned areas has little ability to absorb rain, which instead instantly runs off, carrying ash, mud, boulders and vegetation.

Preparations to prevent storm damage have included clearing debris from flood-control basins designed to catch material flowing out of mountain drainages.

The emergency assessment of the Station Fire area by the USGS assumed scenarios with two common types of storms, one lasting three hours and another lasting 12 hours. It also looked at what might happen in various drainages if the catch basins are empty or if they have become filled.