A wet and potentially damaging storm was bearing down on the Southland Wednesday, bringing what forecasters are calling an “atmospheric river” of moisture that will persist into Friday.
Although some showers are possible across the Los Angeles area Wednesday, the brunt of the storm is not expected to hit until Thursday. Rain was already falling Wednesday over San Luis Obispo County, where the system remained largely stalled.
“The front sags southward into Ventura and Los Angeles counties by Thursday afternoon into Friday,” according to the National Weather Service. “Heaviest rainfall across the southern counties will occur Thursday night into Friday morning. Several inches of rain are expected with flooding concerns including recent burn areas. Several feet of high elevation mountain snow is also expected, along with moderate to strong winds.”
The National Weather Service initially issued a winter storm watch for the Los Angeles County mountains, excluding the Santa Monica range, from Thursday afternoon through Friday afternoon, but the agency later upgraded it to a winter storm warning. The warning will be in effect from 3 p.m. Thursday to 3 p.m. Friday, with 1 to 3 feet of snow potentially accumulating at elevations above 6,000 feet.
A flash flood watch will be in effect Thursday afternoon through late Thursday night for the mountains and the Antelope, Santa Clarita and San Gabriel valleys, mainly affecting areas near the Lake, Bobcat and Ranch 2 burn areas. Forecasters said the areas could seen rain rates of 0.75 inches per hour, potentially leading to “significant mud and debris flows.”
Winds in mountain areas will blow at 20 to 30 mph, with gusts ranging up to 50 mph. A wind advisory will be in effect for Los Angeles County mountains until 3 p.m. Thursday. Forecasters said the strongest winds are likely in the San Gabriel Peaks and the Grapevine area, making “driving difficult, especially for high-profile vehicles.”
A flood watch will be in effect for most of Orange County from Thursday afternoon through Friday afternoon. Forecasters said the heaviest rain there is anticipated to begin before midnight Thursday night, continuing through Friday morning, including a chance of thunderstorms and rain rates topping a half-inch per hour.
According to the NWS, the low-pressure system was gathering strength off the Oregon coast, and will likely remain mostly stationary through Thursday.
“This will create a steady and nearly persistent stream of moist southwest flow, known as an atmospheric river, focused over San Luis Obispo and Santa Barbara counties … lasting through at least Thursday with two to three days of nearly constant rain,” according to the weather service.
Forecasters said Los Angeles County could see 1.5 to 3.5 inches of rain, with 2 to 5 inches possible in the foothills and mountains, although the largest rainfall totals are likely to occur in Ventura County.
Whenever persistent rain is in the forecast, fears are raised about possible mudslides.
“With the potential of this event, the area may approach critical thresholds for mud and debris flows in and around the recent burn areas,” according to the NWS.
Los Angeles County health officials have issued a cold weather alert through Thursday for the mountains and Antelope Valley, and through Tuesday for the Santa Monica Mountains and the Santa Clarita Valley.