Los Angeles, San Diego face serious flood risk as storm threatens California with days of significant rain

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California faces increasing flood threat

The heaviest rain is expected to fall north of Los Angeles around Santa Barbara, California.

LOS ANGELES – Rounds of rain are inundating California, raising the risk of serious flash flooding in Los Angeles and San Diego through at least Thursday.

A strong upper-level low-pressure system pumped in a steady plume of moisture from the southwest, which could turn more southerly.

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On Wednesday, the heaviest rain targeted the Central Coast region, but will threaten areas from Santa Barbara to San Diego on Thursday, the FOX Forecast Center said.

More than 20 million Californians were under a Flood Watch and forecasters were also paying attention to the threats of mudslides.

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The storm system made driving treacherous with rain at lower elevations and frozen precipitation in the mountains.

As much as 3-5 inches of rain is likely, fueled by the potential for rainfall rates up to 1 inch per hour, the FOX Forecast Center said.

The overall rainfall amounts combined with the potential for bursts of thunderstorm-triggered heavier rain, especially over recent wildfire burn-scar areas, has prompted the WPC to put much of Southern California at a Level 3 out of 4 risk of flash flooding.

For many communities, the heavy precipitation is the heaviest event since the remnants of Hurricane Hilary impacted the region in August.

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"Southern California … although we do get handed some active weather in El Niño setups, they're not used to having 8 inches of rain in the mountains outside of Los Angeles," FOX Weather Meteorologist Britta Merwin said. "Although it's a setup that we see plenty in Southern California, it’s an area that can’t handle that type of rain, so it always translates to a flood risk; it’s always feast or famine – you don’t get the in-between."

At times, some instability will develop in the atmosphere, leading to locally higher rainfall rates. This could even result in a few rumbles of thunder, even in Los Angeles.

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By the end of the week, between 5 and 8 inches of rain is expected to have fallen in several areas, with some places experiencing even higher amounts. This could result in significant flooding, including flash and urban flooding.

"We’re working this rain up into terrain," Merwin said. "You've got that orographic lift; you squeeze out all the moisture possible, and you get these gnarly rain totals up in the hills that can approach 7-8 inches."

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Additionally, wind gusts will reach 30-50 mph during the peak of the storm, with isolated gusts to 60 mph, especially over the mountains.

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Rain pushes inland Friday

The storm lingers into Friday before finally pushing inland late Friday near the Mexico border and heading into the Desert Southwest. However, long-range forecasts indicate additional rain is likely in California next week.

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Snow blankets the Sierra Nevada

The same system is also dropping heavy snow over the highest elevations of California's Sierra Nevada Mountains, with lower snow over the highest elevations of the Washington and Oregon Cascades and parts of Idaho and Nevada.

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