LOS ANGELES, CA — The National Weather Service issued a flash flood warning in the southern part of Los Angeles County Thursday where intense downpours flood multiple freeways including parts of the Santa Monica (10), the Long Beach (710), the Harbor (110) and the Ronald Reagan (118) freeways.

At 8:31 a.m., Doppler radar and automated rain gauges recorded heavy rain falling across the South Los Angeles area, according to the National Weather Service.

“Rain rates between 1.0 and 1.5 inches per hour have already been observed — heaviest in the Palos Verdes area,” the NWS warned. “Roadway flooding has been reported, including on the 710 and 110 freeways. AVOID travel in the warning area for at least the next 2 hours if possible.”

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The weather service is warning of flash flooding of small creeks and streams, highways, streets and underpasses as well as other poor drainage and low-lying areas. Areas expected to experience flooding include parts of South Los Angeles, Long Beach, Palos Verdes, Torrance, Port of Los Angeles, Carson, and Harbor City.

The first of two storms expected to batter the Southland began bearing down on the region Thursday, with significant rainfall expected to fall for several hours starting early Thursday morning — ahead of more dramatic downpours anticipated early next week.

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According to the National Weather Service, the Southland will likely see the brunt of the rainfall just in time for the Thursday morning rush hour, with the strongest downpours expected to occur during a one- to three-hour period “when the primary frontal band moves through.”

Forecasters predicted rain rates of about a half-inch per hour, with some localized areas receiving 0.8 inches per hour. Coastal and valley areas are expected to receive 1 to 2 inches of rain during the Thursday storm, with foothills and mountains potentially seeing 3 to 5 inches.

“While the rain will be quite heavy at times, due to the shorter duration of the event impacts are expected to be relatively minor with mainly typical roadway ponding of water and slick driving conditions and flood advisories should be sufficient to handle this event,” according to the NWS. “However, with a 10-20 percent chance of thunderstorms, rain rates in very localized areas could be high enough to require a flash flood warning.”

As of 4 a.m. Thursday, there was 0.94 inches of rain reported at Leo Carrillo, 0.74 inches in Agoura Hills, 0.37 inches in Calabasas, 0.28 inches in Newhall and 0.43 inches in Hungry Valley.

Flooding has been reported at the McClure Tunnel in Santa Monica and on parts of Pacific Coast Highway. Parts of Pacific Coast Highway in Huntington Beach between Seapoint Street and Warner Avenue are closed in both directions because of flooding. The westbound Balboa Boulevard on-ramp on the Ronald Reagan (118) Freeway was closed because of flooding. Rocks have been reported blocking parts of Malibu Canyon Road.

Slippery conditions are causing cars to spin out on freeways in Los Angeles and Orange County and resulting in road closures. California Highway Patrol is reporting at least 50 spinouts and crashes on freeways and roads in the central Los Angeles area.

Long Beach City Health Officer Dr. Anissa Davis issued a rain advisory for recreational beaches and bays in Long Beach. Unhealthy conditions from increased runoff from storm drains and rivers prompted the advisory. Davis is advising to avoid recreational swimming areas for three days. The advisory will be extended if rain continues.

A flood watch will be in effect for most of Orange County from Thursday morning through Friday morning. A high surf advisory will also be in effect for Los Angeles and Orange County beaches from 2 a.m. Thursday through 6 a.m. Saturday, with waves of up to 9 feet possible in the O.C., and potentially reaching up to 12 feet at west-facing shorelines in L.A.

After the main brunt of the storm passes through, the rain should largely taper off by Thursday afternoon, although scattered showers are likely to continue into Thursday evening or possibly Friday morning in northern areas of Los Angeles County.

A winter storm warning will be in effect from 5 a.m. to 10 p.m. Thursday in the eastern San Gabriel Mountains, including Mount Wilson, Mount Baldy, Wrightwood and the Angeles Crest Highway. As much as 18 inches of snow could fall above 7,000 feet in the area, with 6 inches possible at 6,000 feet and 3 inches at elevations as low as 4,500 feet. The snow will be accompanied by winds gusting at up to 55 mph, according to the NWS.

A previously issued winter storm warning was downgraded to a storm advisory in the western San Gabriel Mountains and the Golden State (5) and Antelope Valley (14) freeway corridors. In those areas, forecaster said light to moderate snow is anticipated above 6,000 feet, with a “dusting” of up to 2 inches at elevations as low as 4,500 feet. Winds in those areas will also gust up to 55 mph.

Even before the rain starts falling, the arriving storm system will bring strong winds to much of the region. Areas north of Los Angeles County are expected to see winds of 30 to 40 mph, with gusts up to 50 mph.

Thursday’s downpour, however, will only be a preview of what’s to come beginning as early as Sunday, when the area will be hit by what forecasters are calling “the largest storm of the season.”

The storm is “forecast to spin down the coast and lift an atmospheric river into and across Southern California,” according to the NWS.

“While the exact timing, rates and amounts are still uncertain, it is very likely that this will be a serious two- to three-day storm system,” forecasters said.

“Over the (Sunday to Tuesday) period the early estimates call for widespread rain amounts of 2 to 4 inches for lower elevations and likely twice those amounts in the south facing mountains,” according to the NWS. “There are more than a few (forecasts) that call for much higher amounts than those. Citizens and authorities need to be aware of the likelihood of significant hydrologic issues, including: rock and mud slides in the mountains and flooding of small streams and rivers as well as some chances for flooding of the larger rivers and streams. Significant and hazardous snowfall amounts will be possible above 6,000 feet through Monday night. Lower elevation snow down to around 4,000 feet is possible by later Tuesday into Wednesday.”

City News Service, Inc., Patch Staffer Paige Austin contributed to this report.

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