LOS ANGELES (CNS) - A winter storm arrived in the Southland with a bang today, as a large thunderstorm parked over the Santa Monica Mountains for much of the afternoon and prompted a flood warning near Thousand Oaks.

Radar showed light-to-moderate rainfall falling over western L.A. County and the Santa Rose Valley of Ventura County, as a strong thunder cell developed and did not move.

The heaviest rain was in Ventura County, just west of the L.A. County line, the NWS said. A “heavy downpour” was spotted just north of the Ventura (101) Freeway near Camarillo, and heavy rain was falling on state Route 118 west of Moorpark.

The flood warning was for the Santa Rosa Valley, northwest of Thousand Oaks.

That’s just north of the Springs Fire burn area, where rain triggered landslides in December and January.

State Route 1, the Pacific Coast Highway, remained closed after a predawn mudslide brought fire debris onto the roadway, northwest of Mugu Rock.

That’s beyond the stretch of roadway near Malibu that was closed for 77 days to repair mudslides and related coastal wave erosion.

The slow-moving storm likely will bring local flooding across portions of West Los Angeles and the San Fernando Valley from Woodland Hills to areas including Van Nuys and North Hollywood, NWS officials said in a special weather statement at 2:33 p.m.

Heavy rain, hail up to one-half inch and lightning also were possible in those regions, according to the weather service

Rain also fell in Marina Del Rey and surrounding coastal communities, as it was in portions of Orange County, including Anaheim, where Disneyland patrons and employees were forced to use umbrellas and don rain gear.

The storm — fueled by a cold low-pressure system — began its sweep into Southern California Saturday, bringing blustery winds from coastal regions to inland mountains ranges.

“A moist and unstable air mass in place from a cold upper low pressure system will bring showers and a slight chance of thunderstorms to the area through Monday,” the NWS said in a statement released at 2:09 p.m. “A weaker storm system will follow on its heels later Monday, allowing for lingering showers into early Tuesday.”

Dry and warmer weather was expected late Tuesday through Friday, the NWS said.

The weather service said approaching precipitation expected tonight through Monday could cause hazardous conditions on Interstate 5 over the Grapevine, and perhaps in the foothills near the Antelope Valley (14) Freeway.

Showers — and snow above 6,000 feet in elevation — were expected to increase in the mountains, with the snow level dropping to between 3,500 and 4,000 feet today and tonight. Both the 5 and 15 freeways top out just above 4,100 feet.

Thunderstorms, thunder snow and hail storms are possible, NWS officials said.

Chain requirements were in effect overnight for Highway 243 near Idyllwild, and for the highways to and through the Big Bear and Lake Arrowhead resort areas.