Life-threatening flooding looms for Southern California as dangerous atmospheric river

LOS ANGELES – California is in a brief respite from extreme weather after being impacted with the latest round of record rain and heavy snow Thursday from a Pineapple Express atmospheric river event. However, another potent, even stronger storm is already approaching from across the Pacific Ocean, with the potential for much heavier rainfall totals. 

The National Weather Service (NWS) in Los Angeles is already using strong language in its forecasts, indicating a significant threat for widespread and dangerous flash flooding Sunday into Monday.

“This is shaping out to be a life-threatening flooding situation, especially during the Sunday through Monday time period,” the NWS said in their Thursday night forecast discussion.

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‘Extreme’ rainfall totals possible across Southern California

An area of low pressure will aim another atmospheric river toward the California coast on Sunday and bring a slug of moisture straight from the tropics, the FOX Forecast Center said. 

Unlike the last system, this storm won’t be in a hurry to leave. That will keep the atmospheric river of moisture pointed at California for 2 to 3 days. 

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“The last atmospheric river event that we went through, there was movement,” FOX Weather Meteorologist Britta Merwin said. “This time there’s going to be less movement, which means extreme rain totals.”

While all of California will get in on the rain, it is becoming clear that areas of most concern will be along the southern coastal ranges down through the Los Angeles Basin, the FOX Forecast Center notes. The heaviest rain arrives Sunday through Monday with peak rain Sunday night.

The Los Angeles metro may see 5 inches of rainfall or more Sunday into the middle of the week, with higher totals approaching 8-9 inches in the mountains.

“This is the type of rain that they cannot handle,” Merwin said. “This is a guaranteed flood setup. There’s no way around it. We know it’s going to be bad, and there’s going to be huge impacts.”

This storm comes on the heels of Thursday’s soaking Pineapple Express storm, and the soil in Southern California is already super-saturated. Thursday’s record rainfall of 2.45 inches in Long Beach flooded highways, including the busy I-710. Los Angeles also set a daily record with 2.49 inches of rain.

“With the ground already saturated from (Thursday’s) rain, onset of dangerous flash flooding will be much quicker with this next event,” NWS Los Angeles forecasters said. “And everyone, especially those near or in south-facing mountains, needs to start preparing now for possible evacuations during or even before the storm hits.”

A Level 3 out of 4 risk for flash flooding across coastal central and Southern California has been issued for Sunday, spreading south into the Los Angeles area on Monday.

“It shows you the confidence that we know this is going to be a bigger storm,” Merwin said. “It has a lot more moisture to it. There’s a much larger flood threat for Southern California.”

Evacuation warnings were issued for parts of low-lying Santa Barbara County on Friday and could be turned into evacuation orders, if flooding becomes problematic.

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Several feet of snow likely in Sierra Nevada

The plume of moisture will generate a significant winter storm for the Sierra Nevada, with 4-6 feet of snow likely above 5,000-6,000 feet, the FOX Forecast Center said. Disruptions to daily life, including difficult to impossible travel conditions, are expected. 

In the mountains of Southern California, snow levels are expected be around 7,000 feet, but above that level, 2-4 feet of snow is possible.

Damaging winds possible around San Francisco, central Coast

In addition to the heavy rain threat, damaging winds are in the forecast for the central California and southern California coasts toward Santa Barbara, and even into parts of the San Francisco Bay Area. 

High Wind Warnings are already posted for gusts along the coast and coastal mountains reaching 50-60 mph. Wind Advisories warn of possible wind gusts to 50 mph in the San Francisco Bay Area.  

Gusts topping 40-plus mph will likely extend well inland into the Central Valley. These winds will have the potential to bring down many trees and cause numerous power outages. 

At the coast, these winds will also whip up the Pacific into a frenzy with 10-20 foot waves likely to slam into beaches up and down the state.

Moisture streaming in behind the storm will keep periods of rain in the forecast through much of next week, though there will be longer dry periods between the showers to wring out.