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.
At around 3:30 a.m., the NWS issued a flash flood warning scheduled to remain in force until 6:30 a.m. for areas at the foothills of the San Gabriel Mountains below the sites of the Fish and Reservoir wildfires. It said rain was falling at rates of up to a half-inch per hour in those areas.
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 was expected overnight before the storm clears out, possibly by mid morning.
National Weather Service meteorologist Curt Kaplan said the storm “will be a pretty good rainmaker,” and an NWS statement added:
“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.”
An urban and small stream flood advisory was issued by the NWS before 2 a.m. and due to expire at 7:45 a.m. Explaining why it was issued, the weather service said it is expecting rainfall rates of a quarter to a third of an inch of rain per hour at times overnight, with as much as a half inch per hour along coastal slopes.
“This will cause urban and small stream flooding early this morning … along with minor mud and debris flows in the recent burn areas. In addition, there could be rapid increase in water levels across the creeks, washes, and rivers, as well as rockslides across canyon roadways,” according to an NWS statement.
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.
The threat of flash flooding targeted 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, the NWS said.
Periods of heavy rain could lead to flash flooding and debris flows down slopes stripped of vegetation in the Sand, Fish, Sage, Old, Solimar, Springs and other recent fires, according to the NWS.
Not much atmospheric instability is seen in this storm, meaning no 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, but perhaps not as fierce as initially anticipated. A high wind warning, projecting winds blowing or gusting at 58 miles per hour or more, was scheduled to be in force until 10 a.m. in the San Gabriel Mountains and the Antelope Valley. But before dawn, it was changed to a wind advisory, denoting winds or gusts of 35 mph, and scheduled to expire at noon.
Forecast in the advisory areas were winds of between 20 and 30 mph with 50-mph gusts.
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 Vieho. As in L.A. County, Saturday’s temperatures will be a little lower under sunny skies.
