Vast swaths of Los Angeles County are under a flash flood watch Wednesday as monsoonal moisture lingers in the area until at least Wednesday evening, creating the chance of rain and thunderstorms and the threat of mud and debris flows down slopes stripped bare in wildfires, forecasters said.
A flash flood watch issued by the National Weather Service will be in force through Wednesday evening in the San Gabriel Mountains and the Antelope, Santa Clarita and San Gabriel valleys. The NWS said the threat of thunderstorms and high humidity could persist through Friday.
“The surge of monsoonal moisture that pushed into the region (Tuesday) will linger through at least Wednesday evening, keeping a chance of showers and thunderstorms across much of the area,” according to an NWS statement.
“Due to abundant moisture throughout the atmosphere, there will be the potential for intense downpours with any thunderstorms. This will bring a risk of flash flooding.”
“Flash flooding and debris flows will be a particular threat in and below the recently burned areas” of Los Angeles County, warned the NWS, although the only burn areas it named specifically Wednesday morning are the sites of the Whittier and Alamo fires in Santa Barbara county.
Inland temperatures, meanwhile, are expected to be in the 90s Wednesday, and will approach 100 in some valley areas.
The National Weather Service forecasts a 20 percent chance of rain Wednesday in Los Angeles and Orange counties.
Some quick flashes of rain hit the Antelope, San Gabriel and San Fernando valleys Tuesday night, but a flash flood advisory issued by the NWS expired without incident.
Strong winds, rain and street flooding swept across Riverside and San Bernardino counties Tuesday afternoon, and The U.S. Open of Surfing in Huntington Beach sent thousands of fans home early due to fears of lightning strikes.
The NWS issued a heat advisory as a spike in humidity made temperatures feel higher than they really were.
“This is a dangerous situation, with an increased threat of life- threatening heat-related illness,” the NWS warned.
The NWS attributed the high heat to an upper-level high-pressure system over Northern California and Nevada.
Forecasters noted that thunderstorms bring with them the threat of lightning strikes.
“Anyone planning outdoor activities in the mountains and deserts this week should be prepared for the possibility of monsoonal thunderstorms and the associated flash flood risk if storms do develop,” the NWS said.
Along the Southland coast, high surf and strong rip currents are expected to persist through Thursday. Forecasters urge swimmers caught in a rip current not to panic but to try to relax and float, remaining parallel to shore until able to break free. “If unable to escape, face the shore and call or wave for help,” the NWS said.
But as of early Wednesday morning, no beach hazards statement or high surf advisory was in effect.
The weather service’s temperature forecast page reflected an expectation of thunderstorms throughout L.A. County Wednesday, along with highs of 81 degrees in Avalon; 82 at LAX; 88 on Mount Wilson; 89 in Long Beach; 91 in Downtown L.A.; 93 in Burbank; 94 in San Gabriel; 95 in Pasadena; 97 in Saugus; and 99 in Woodland Hills, Palmdale and Lancaster.
Partly cloudy skies was the salient element forecast in Orange County Wednesday, along with highs of 74 in Laguna Beach; 76 in San Clemente; 78 in Newport Beach; 90 in Irvine and Mission Viejo; 91 in Anaheim; 92 in Fullerton; and 94 in Yorba Linda.
The high temperatures will persist for several more days, according to an NWS forecast.
