A wet storm slowly made its way into the Southland Thursday, raising fears of flash flooding in some areas and concerns of mud and debris flows on slopes denuded by wildfires.
The rain began falling Thursday morning across San Luis Obispo and Santa Barbara counties, then spread to Ventura and Los Angeles counties in the afternoon.
Dense fog loomed over much of the Los Angeles area for the bulk of the day as the rain system advanced. Sometimes-heavy rain was expected to fall overnight and into Friday morning, when the precipitation is expected to clear, according to the National Weather Service.
All city- and county-funded winter homeless shelters in the San Gabriel Valley were notified to stay open around the clock in response to the expected storm. The shelters will be open until 7 a.m. Saturday.
In Los Angeles County, light rain is expected to start falling late this morning or early this afternoon, then intensify and "come through hard," said NWS meteorologist Curt Kaplan said the storm "will be a pretty good rainmaker."
An NWS statement put it this way: "The warm nature of this storm system ahead of the cold front will bring periods of heavy rain. Rainfall rates capable of triggering mud and debris flows within the recent burn areas will be possible. Rain will turn to showers by Friday morning." The storm will be the biggest so far of the rainy season, which runs from October to May, said NWS meteorologist Andrew Rorke.
Rainfall totals will range between a half-inch and 1.5 inches in coastal and valley areas and between 1 and 3 inches in the foothills and along south- and southwest-facing mountain slopes. A flash flood watch will be in effect through Friday morning in the so- called burn areas of L.A. County -- in the San Gabriel and Santa Monica mountains and the San Fernando, Santa Clarita and San Gabriel valleys, as well as in areas of Ventura County.
"Southern California residents in or below the recently burned areas are urged to take the steps necessary to protect their property," according to the NWS. "Persons in the watch area should remain alert and follow the directions of emergency preparedness officials."
"Periods of heavy rain late (this) afternoon and evening through Friday morning could lead to flash flooding and debris flows for the Sand, Fish, Sage, Old, Solimar, Springs and other recent burn areas," the statement added. Not much atmospheric instability is seen in this storm, meaning no immediate forecast of thunderstorms.
Instead, it's expected to be very wet because embedded in it is a subtropical band of moisture from Hawaii, forecasters said. Also threatening the region are strong and potentially damaging winds. A high wind warning, projecting winds blowing or gusting at 58 miles per hour or more, will be in force until 10 a.m. Friday in the San Gabriel Mountains and the Antelope Valley.
The wind in those areas is expected to blow at between 20 and 35 mph and gust at 60 mph before quickly tapering off Friday morning, forecasters said. "Strong winds can make driving difficult, especially for drivers of high profile vehicles and vehicles towing trailers," warned an NWS statement. "Winds this strong may down trees and power lines and cause property damage."
Forecasters said snow levels are forecast to drop rapidly on Friday, falling to near 4,000 feet, but only minimal accumulation is expected over passes such as the Grapevine.