Flash Flooding Alert through Friday: Rain, Gusty Winds Across San Gabriel Valley

Steady rain generated by a storm out of the Gulf of Alaska drenched the Southland early Friday, raising fears of flash flooding and mud flows down fire-stripped slopes, as treacherous travel conditions developed in the mountains.

Periods of heavy rain were feared to trigger flash flooding and debris flows down slopes stripped of vegetation in the Sand, Fish, Sage, Old, Solimar, Springs and other recent fires.

During the night, the National Weather Service issued a flash flood warning for areas at the foothills of the San Gabriel Mountains below the sites of the Fish and Reservoir wildfires. It expired at 6:30 a.m. without major incidents involving homes, but some water and mud did slide into Duarte -- located near the site of the Fish wildfire in June -- prompting school officials to keep Valley View elementary school closed Friday.

Later, the NWS cancelled a flash flood watch in some areas but left it in place until noon in others. The watch was cancelled in the Santa Monica Mountains and the Santa Clarita and San Fernando valleys but allowed to remain in force until noon in the San Gabriel Mountains and San Gabriel Valley.

Prompting the decision to leave the watch in force in some places was rainfall in some areas at rates of between a quarter-inch and a third of an inch per hour, according to the NWS.

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 and intensified as the day wore on. The bulk of the precipitation materialized overnight -- before the storm clears out, possibly by mid morning.

In the San Gabriel Mountains, a winter weather advisory will be in effect from 2 this afternoon until 10 a.m. Saturday. The NWS said the snow level would remain above 8,000 feet through early this morning in advance of a cold front, but then drop to as low as 2,000 feet tonight.

"This -- in combination with strong winds, icy roadways and dangerous wind chill readings -- will bring the potential for treacherous driving conditions through Saturday morning across the north-facing slopes of the local mountains, including Interstate 5 near The Grapevine," warned an NWS statement early this morning.
The storm will be the biggest so far of the rainy season, which runs from October to May, according to the NWS. 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, although 4 inches is possible in places.

Not much atmospheric instability is seen in this storm, meaning no forecast of thunderstorms. Instead, it was a very wet storm because embedded in it was a subtropical band of moisture from Hawaii, forecasters said.
Also threatening the region are strong and potentially damaging winds.

A wind advisory denoting an expectation of 35-mile-per-hour winds or gusts will be in effect until 9 tonight along the coast, in metropolitan Los Angeles, including downtown L.A., and the Hollywood Hills, and until noon Friday in the Antelope Valley and the San Gabriel Mountains. In the Santa Monica Mountain Recreational Area, a wind advisory will be in force from 6 tonight until 6 Saturday morning. Expected in the areas subject to a wind advisory are winds of between 20 and 30 mph gusting to 45 mph.

"Gusty winds will make driving difficult, especially for drivers of high-profile vehicles," said an NWS statement. "The combination of the winds and the overnight ring (early today) could lead to downed trees and power lines."

The NWS forecast highs in Los Angeles County of 54 degrees Fahrenheit on Mount Wilson; 55 in Palmdale; 56 in Lancaster; 57 in Saugus; 58 in San Gabriel; 59 in Pasadena and Burbank; 60 at LAX and in downtown L.A., Woodland Hills and Avalon; and 61 in Long Beach. Saturday's temperatures will be a few degrees lower in many communities but the sun will be shining and will do so over the next several days.

In Orange County, the weather service forecast highs of 62 in Laguna Beach, Yorba Linda, Fullerton and Anaheim; and 63 in Newport Beach, San Clemente, Fullerton, Anaheim, Irvine and Mission Viejo. As in L.A. County, Saturday's temperatures will be a little lower under sunny skies.