What to Know
A storm fueled by an atmospheric river pulling Tropical moisture into California is expected to pummel SoCal Thursday
Widespread rainfall is falling Thursday morning, marking the final round of rain during a week of storms
Several hillside areas were under evacuation orders ahead of the storm
A fourth consecutive day of rain started out with downpours in some parts of Southern California as a Pacific storm fueled by a conveyor belt of moisture bears down on the West Coast.
Flash flood warnings are in effect for parts of Riverside and San Bernardino counties. The warnings will be in place until about 1:30 p.m.
In Ventura County, a flood warning was issued late for residents near Foster Park and the Ventura River. The warning is set to expire at 2 p.m.
A week of storms has left communities near the Woolsey, Holy and Thomas fire burn areas and others around the state concerned about the potential for mudslides that could impact thousands of people living in foothill and canyon areas. Immense areas of the state were razed by wind-whipped flames, leaving hillsides bare of vegetation that could stabilize soil and prevent mudslides.
"It's been raining over and over and over, and it's adding to what's already on the ground," said NBC4 forecaster Shanna Mendiola.
In Los Angeles County, a flash flood watch will be in force until 1 p.m. along the coast, in the Santa Monica Mountains Recreational Area, the San Gabriel Mountains, the San Fernando and Santa Clarita valleys, beach cities, Long Beach, Beverly Hills and Hollywood. In all, it covers the Woolsey, Hill, South, and Stone burn scars in Los Angeles and eastern Ventura Counties.
A flash flood watch means that conditions may develop that lead to flash flooding.
Several roads in burn areas were closed, including a stretch of Topanga Canyon Boulevard in Malibu.
In the Hollywood Hills, unstable soil began sliding at a construction site in th 1800 block fo Stanley Avenue. No injuries were reported, but the nearby area was evacuated.
Rain is expected to decrease Thursday evening. Dry conditions are in the weekend forecast.
The heaviest rains are due to a phenomenon called an atmospheric river. The robust plume of subtropical moisture ushers tropical air up from the Pacific like a conveyor belt. The long, narrow band of water vapor in the sky makes land, then glides up from sea level and condenses, turning to rain.
"It kind of wrings out like a sponge once it heads up those hills," said Mendiola.
In a typical year atmospheric rivers account for about 30 to 50 percent of the West Coast’s annual rainfall. They've contributed to historic rain seasons and flooding, including the 2017 winter storms and the Great Flood of 1862, which devastated parts of the state.