Sandbags and concrete barriers called K-rail were placed on streets 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.
The U.S. Geological Survey recently warned of potentially massive debris flows from the area burned by the late summer Station Fire. Two firefighters were killed and 89 homes were destroyed as it spread over more than 250 square miles of Angeles National Forest, becoming the biggest fire in Los Angeles County history.
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.
"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.