A powerful low-pressure system is forecast to bring heavy rain, gusty winds and possible thunderstorms to the Los Angeles area beginning Thursday night and continuing through the weekend, with mountain areas potentially receiving up to 4 inches of rainfall, according to the National Weather Service.
The storm is expected to reach Los Angeles County Thursday night, with most areas seeing 1 to 2 inches of rain through Friday. Mountain and foothill neighborhoods, including the Santa Monica Mountains and surrounding canyons, could receive 2 to 4 inches, with some models suggesting totals as high as 5 inches in specific locations.
"Periods of moderate to heavy rain are expected Thursday through Saturday, and possibly into early next week, as a storm system moves into the region," forecasters wrote in a weather discussion Wednesday morning. "Quite a bit of uncertainty in the progression of the storm exists, and widespread flooding is not out of the question."
The National Weather Service issued a Slight Risk of excessive rainfall — level 2 of 4 — for the area on Friday. Rain rates could reach a quarter to half inch per hour, with isolated areas potentially seeing three-quarters of an inch per hour.
Forecasters warn of a 10 to 20 percent chance of thunderstorms, which could produce heavy downpours, small hail and gusty winds. There is also a very small but non-zero chance of a weak, brief tornado.
The storm system is part of a highly amplified upper-level trough developing a strong surface cyclone off the coast of northwest California and southwest Oregon. High wind warnings are in effect for coastal regions from San Francisco Bay north toward Cape Mendocino, where sustained winds of 25 to 35 mph may be accompanied by 60 mph gusts.
As the cold front and moisture stream slide south Thursday and Thursday night, conditions will deteriorate across Southern California Friday. The Northern Sierra Nevada Range is expected to receive 4 to 8 inches of snow through Friday, with 6 to 12 inches above 9,000 feet and up to 18 inches in the highest peaks of the central to southern Sierra Nevada Range.
Marine conditions will also become hazardous, with Small Craft Advisories likely and a 40 to 70 percent chance of gale-force winds for waters west through northwest of Point Conception from late Wednesday night through Thursday evening. A large west-northwest swell will bring high surf to coastal areas Thursday through Saturday, with waves reaching 12 to 16 feet along the Central Coast and 8 to 11 feet in Ventura County.
Residents should prepare for potential travel delays, localized street flooding and increased flows in creeks and streams. Burn areas in the region may face flash flooding or debris flows. Drivers should watch for blown palm fronds, rockslides, mudslides on canyon roads and ponding water on local streets.
The National Weather Service advises residents to monitor forecasts, clear gutters, check windshield wipers and avoid outdoor activities Thursday through Saturday. Residents should not drive through flooded roads and should go indoors if thunder is heard.
Rain chances are expected to continue into the weekend across Los Angeles County, with some models suggesting showers could persist through Sunday morning.