A powerful Eastern Pacific storm system hugely augmented by a subtropical plume of moisture trundled toward the Southland Tuesday, threatening to drench the region starting Tuesday afternoon with the most rain Southern California has seen this rainy season, then trigger flash flooding both in burn areas and far away from them beginning Wednesday, forecasters said.
“A strong storm system originating from the Pacific Ocean will tap into a long fetch of deep subtropical moisture and bring the potential for a very wet and long duration storm event for Southwest California Tuesday through Thursday night,” according to a National Weather Service statement. “This atmospheric river event will likely bring the highest rainfall totals to some portions of Southwest California, so far this season,” which starts in October.
The rain likely will stop late Thursday or early Friday, according to the National Weather Service.
Total rainfall from this storm is expected to range from 2 to 5 inches in coastal and valley areas and between 5 and 10 inches across the foothills and coastal slopes, it said.
A flash flood watch will be in effect from Wednesday evening through late Thursday night not only in burn areas of L.A. County but also in urban areas. It will be in effect in the San Gabriel and Santa Monica mountains; the San Gabriel, San Fernando and Santa Clarita Valleys; Los Angeles, including the coast, metropolitan L.A., Downtown, and the Hollywood Hills; and both coastal and inland Orange County.
“In addition to the flash flooding and mud and debris flow risk in recent burn areas, there will be other flooding threats in non-burn areas due to the long duration and intensity of this storm,” warned the NWS. “Widespread urban roadway flooding is possible as well as rockslides and mudslides, especially near canyon roadways. As a result, there could be significant travel delays and road closures across the region between Tuesday and Thursday night.”
The NWS said rainfall rates up to six-tenths of an inch per hour are possible late Wednesday evening, with rates possibly increasing to three- quarters of an inch per hour or higher at times Thursday. Isolated rainfall rates as high as one inch per hour cannot be ruled out, it added.
“Rainfall of this intensity can produce dangerous mud and debris flows near recent burn areas,” noted an NWS statement. “Flash flooding is a very dangerous situation.
“Southern California residents, in or below the recently burned areas are urged to take the steps necessary to protect their property. Persons in the watch area should remain alert and follow directions of emergency preparedness officials,” it said.
The NWS forecast mostly cloudy skies in L.A. County for much of the day Tuesday and highs of 57 degrees on Mount Wilson; 65 in Avalon; 66 in Palmdale and Lancaster; 67 in Saugus; 69 in Long Beach, San Gabriel and Burbank; 70 in Downtown L.A.; and 71 in Pasadena and Woodland Hills. Temperatures will be slightly lower the rest of the week until Monday, when they’ll revert to Tuesday’s levels.
Mostly cloudy skies were also forecast in Orange County, along with highs of 64 in Newport Beach, Laguna Beach and San Clemente; 70 in Mission Viejo; 71 in Yorba Linda, Anaheim and Irvine; and 72 in Fullerton. Temperatures will be slightly lower until Monday, when they’ll be a degree or so above Tuesday’s.