Major atmospheric river storm slams Southern California

Rain from a major atmospheric river storm moved into Southern California early Thursday, causing minor flooding during L.A.’s morning commute and concerns about greater destruction.

The moisture-heavy system continues to forge a trail of wet, windy weather across the state, dumping significant rain and snow across Northern California on Wednesday — including some historic rainfall amounts — and causing roadway flooding and power outages.

While showers continue across the state, the storms’ strength has turned toward the Southland, with the heaviest rainfall expected through noon.

Early Thursday, the National Weather Service issued a flood advisory for much of Los Angeles County and eastern Ventura County through 8 a.m., warning of likely flooding in low-lying and poor-drainage areas.

As of 5:30 a.m., weather officials said the rainfall in L.A. County had “caused widespread urban, roadway and small stream flooding.”

Rainfall totals had already reached an average of 1 to 2.5 inches across much of L.A. County, with some places already over 3 inches — and more rain is still to come.

A cold front also moved in with the atmospheric river, bringing dangerous winds across the region, with some gusts already recorded above 60 mph. Officials warned of downed trees and possible power outages.

A winter storm warning is in effect for the mountains of Los Angeles, Riverside and San Bernardino counties through late Thursday, with the highest elevations expected to get up to 18 inches of snow. A dusting to a few inches of powder is possible as low as 4,500 feet, and forecasters warn that “travel may be difficult to impossible” in the mountains, including along the 5 Freeway corridor.

Heavy snowfall was also forecast in the southern Sierra, with as much as 4 feet possible, depending on elevation. Forecasters warned that in the foothills, where the precipitation doesn’t freeze, flooding and mudslides could be likely.

High surf was expected to bring waves up to 12 feet along the beaches of Ventura, Los Angeles, Orange and San Diego counties, and rip currents will make any ocean activities dangerous, the weather service warned.

This system is the first of back-to-back, major atmospheric river storms, which state officials worry is the start of an extended weather pattern that could cause problems.

This first front is aligning with forecasters’ projections of a fast-moving storm, which officials had hoped would help minimize damage from the otherwise powerful system. But another atmospheric river barreling across the Pacific toward Southern California is looking much slower, forecasters warn, which could further challenge the state’s infrastructure and ability to handle extreme weather.

South and east of Los Angeles, as well as parts of San Bernardino, Riverside, Orange and San Diego counties will remain under a flood watch through late Thursday, with excessive rainfall likely to cause ponding on roadways, flooding in low-lying areas and along some waterways.

Ryan Kittell, a meteorologist with the National Weather Service in Oxnard, said some areas could see up to 1 inch of rain per hour Thursday morning, a rate that is sure to cause flooding.

San Diego, which was still reeling from last week’s historic rains that caused devastating flash floods, was bracing for almost guaranteed flooding again near Fashion Valley along the San Diego River, where the latest river forecasts showed flood stage would be surpassed by evening.

Winds could reach up to 40 mph across much of Southern California and as high as 65 mph in the mountains — causing possible delays at Los Angeles International Airport, downed trees and power outages.

And there won’t be much of a break until the next system moves in.

The latest predictions show that Sunday will likely bring the start of a second, moisture-heavy system, expected to move at a slower clip and drop even more rainfall on an already soaked Southland.

Rainfall amounts could reach 3 to 5 inches, with up to 10 inches possible in the mountains and foothills — totals that could bring “damaging flooding,” Kittell said.

“The potential is growing for a very significant storm,” he said.