The first of two back-to-back storms will slide into the Southland tonight and be followed by an even wetter and more powerful weather system, which likely will dump up to 4 inches of rain in the foothills and give rise to fierce winds while threatening to trigger mud slides in areas denuded by wildfires.
"These two storms will bring significant rainfall, high elevation snow and gusty winds," warned a National Weather Service advisory.
Today's rains -- part of what the NWS calls "a dramatic change in the weather" that will result in "significant rainfall" through Saturday -- will first hit Southern California across San Luis Obispo and Santa Barbara counties by this afternoon, spread south and east into Ventura and Los Angeles counties this evening and depart Thursday evening, according to the advisory.
That storm is expected to produce between a half-inch and an inch of rain, though up to 1.5 inches could drench"upslope areas," it said.
The snow level will remain high -- above 7,500 feet during daytime hours, dipping to around 6,500 feet tonight and Thursday -- and gusty south-to- southwest winds will buffet mountain areas, especially "over higher terrain," the advisory said.
A wind advisory will be in effect in Los Angeles County in the San Gabriel Mountains and the Antelope Valley from noon today until 9 p.m. Thursday, the NWS said, forecasting south-to-southwest winds of between 20 and 30 miles per hour and gusts of between 45 and 55 mph this afternoon.
The winds will be largely confined to the foothills this afternoon but spread overnight, forecasters said.
"The second storm will impact the area from late Thursday night through late Saturday night and will likely be the strongest storm the area has experienced in about three years," according to the NWS advisory.
Moderate to heavy rain will first strike San Luis Obispo and Santa Barbara counties late Thursday, then expand into Ventura and L.A. counties early Friday morning and persist into Friday evening, it said, adding that widespread showers likely will follow Friday night and last through Saturday, giving way to scattered showers Saturday night.
"This weather system will also be accompanied by an unstable air mass, resulting in a slight chance of thunderstorms over most of the area Friday through Saturday," according to the NWS, which said that such thunderstorms could produce "brief intense rainfall and small hail."
Rainfall amounts resulting from Friday's storms "could be very impressive," according to the NWS advisory. It cited a potential for between 1.5 and 2.5 of inches of rain in coastal and valley areas and between 2 and 4 inches in the foothills and mountains, with up to 6 inches possible "along south-facing slopes."
The snow level generally will be above 7,000 feet Friday but drop to between 5,000 and 5,500 feet by Saturday, NWS forecasters said. "Significant snowfall" is expected, along with strong, gusty south-to-southwest winds in the mountains, creating winter storm conditions, according to the NWS.
NWS forecasters said they are particularly concerned about the possibility of flash flooding and mud and debris flows around the sites of the 1,952-acre Colby Fire in the hills above Glendora and Azusa in January, the 250- acre Madre Fire in the Angeles National Forest, also in January, the 125-acre Madison Fire in the Monrovia area in April , the 22-242-acre Powerhouse Fire in the Angeles National Forest in June and the 28,000-acre Springs Fire in Ventura County in May.
"Given the potential for heavy and intense rainfall, residents located near these burn areas should be alert to the potential of mud and debris flows Friday through Saturday," according to the NWS advisory.
The Los Angeles Board of Public Works, which oversees the Bureau of Sanitation, urged residents to prepare for the storms by taking these measures:
- Sweep driveways
- Remove debris to prevent it from entering storm drains
- Report downed trees or limbs
- Avoid watering outdoor plants
- clean up after pets
To report storm-related emergencies, Los Angeles residents should call 800 996-2489 or 311.
Despite the wind and the rain, temperatures will not be particularly low. The NWS forecast highs today of 64 in Newport Beach; and 67 in Anaheim.
--City News Service