This is read by an automated voice. Please report any issues or inconsistencies here.
A fast-moving atmospheric river is heading toward California this week and could pack a punch, with the possibility of periods of heavy rain, and a risk of flooding and debris flows in recently burned areas.
After arriving in Northern California on Wednesday, the storm system is expected to land in Southern California on Thursday.
It could produce the most rain downtown Los Angeles has seen in at least a month, and possibly since February.
The National Weather Service office in Oxnard called the forecast storm "potentially significant," with roadways expected to flood in spots, rockslides possible on canyon roads and a chance of mudflows.
The storm could snarl traffic in Los Angeles and Ventura counties as the bulk of the rain is expected in those counties on Thursday afternoon and evening, bringing ponding of water on roadways. The L.A. metro area could be hard hit during Thursday evening rush hour, with localized freeway flooding in low spots, forecasters warned.
In the L.A. area, there could be 1 to 2 inches of rain on the coast and in the valleys, with 2 to 4 inches possible in the mountains and foothill areas. "We're looking at moderate to locally heavy rain rates with this system," said Rose Schoenfeld, a weather service meteorologist in Oxnard, on Monday afternoon.
"There is a chance of burn area flash flooding and debris flows," Schoenfeld added.
Winds also could be an issue, with peak gusts of 50 mph along the Grapevine section of Interstate 5 and in the Antelope Valley. Winds could hit 21 mph in downtown L.A., 23 mph in Long Beach, 25 mph in Santa Clarita, 30 mph in Redondo Beach and 44 mph in Lancaster.
Most areas will experience around six to eight hours of precipitation, the weather service office in Onxard said. Heavy rain could fall over the course of one to three hours, with rates of up to three-quarters of an inch per hour. There's also a 20% chance of rain rates of as much as 1 inch per hour.
The key threshold for rainfall rates that can trigger significant debris flow is half an inch per hour or greater.
"If models stay consistent, it's likely that a flood watch will be issued for the most vulnerable areas, including the most recent burn scars," the weather service said.
There's a slight chance of showers and thunderstorms in Southern California into Friday, according to Schoenfeld. "With thunderstorms, we could see brief heavy downpour, small hail, gusty winds and lightning."
The trajectory of the storm will bring in winds from the south and southwest, Schoenfeld said, and that means south-facing slopes will see particular risk for enhanced rainfall as the storm moves over those slopes, squeezing out even more moisture from the storm. These areas of enhanced rainfall include the northwestern corner of San Luis Obispo County, the Santa Ynez Mountains north of Santa Barbara, and all south-facing slopes of Ventura County.
The coming storm could make a difference in California's fire season "if we get the kind of storm totals we're anticipating," according to David Gomberg, NWS fire program manager. It will take a couple of weeks following the storm to assess how vegetation responds, he said, but the expected precipitation "definitely should start to make a big difference in the fire season."
Officials generally like to see 3 to 4 inches of widespread rain in the lower elevations to end the fire season. Downtown L.A. already received 1.38 of an inch of rain on Oct. 14. Prior to that, the last calendar day where downtown got more than an inch of rain was on March 13, when 1.03 of an inch fell.
The wettest calendar day so far this year was on Feb. 13, when 2.8 inches of rain fell on downtown L.A.
Temperatures were expected to plunge as a result of this week's storm. Downtown L.A. hit a high of 92 degrees on Monday -- one degree short of the record of 93 for the calendar day -- but was expected to reach only 61 degrees by Friday. Anaheim's high of 88 degrees Monday was expected to fall to a high of just 64 degrees by Friday; in San Diego, Monday's high of 89 degrees at Brown Field Municipal Airport was expected to fall to a high of 61 degrees by Friday.
In San Bernardino County, wind gusts of up to 60 mph could hit the mountains and into the deserts.
In the San Francisco Bay Area, the most intense rain is expected late Wednesday and Thursday, with half an inch to 1.5 inches in the valleys and up to 3 inches in the coastal ranges. San Francisco could get somewhere between 1 inch to 2.5 inches of rain. The region could see gusts of up to 40 mph, and "downed trees and power lines may result in power outages," the weather service office in Monterey said.
Sacramento could get up to 2 inches of rain.
The storm could bring heavy snow to the Sierra Nevada, and meteorologists were already discouraging travel during the peak time of the snowstorm -- between Thursday morning and Friday morning. Donner Peak could get 12 to 18 inches of snow, and the California Highway Patrol is expected to require motorists to use chains to cross California's mightiest mountain range.
Wind gusts in the Sierra could exceed 100 mph along the ridgetops, the weather service office in Reno said.