A powerful winter storm began bearing down on the Los Angeles area Tuesday, prompting forecasters to warn of dangerous flooding, damaging winds and the potential for mudslides through Christmas Day.
The National Weather Service issued a flood watch for all of Southwest California as the storm system threatened to bring up to 11 inches of rain to foothill and mountain areas by Thursday evening. Forecasters described the event as a "very dangerous holiday storm" and urged residents to take protective action before conditions deteriorate.
"This is a major storm to affect Southern California," meteorologists wrote in their forecast discussion Tuesday morning. "Anyone traveling on Christmas Eve or Christmas Day will need to exercise extreme caution."
The storm began with light rain Tuesday afternoon, with the heaviest precipitation expected Tuesday night through Wednesday. A second wave of rain was forecast for Christmas Day.
Los Angeles County officials also sent out warnings to residents who live near burned areas saying landslides or other problems are possible.
Coastal and valley areas south of Point Conception can expect 3 to 6 inches of rain, while foothills and mountains could see 5 to 11 inches through Thursday, according to the weather service. Areas north of Point Conception will receive slightly less, with 2 to 4 inches expected along the coast and valleys and 4 to 7 inches in higher elevations.
Peak rainfall rates south of Point Conception were expected to reach 0.60 to 1.25 inches per hour, with a 20% to 30% chance of isolated rates as high as 1.50 inches per hour across south-facing slopes. The intense rainfall rates, combined with the prolonged duration of heavy rain, prompted forecasters to designate south-facing mountains and foothills as high-risk areas for widespread flooding.
"Widespread and significant urban flooding will be likely along with rock/mudslides and debris flows," forecasters warned, noting the threat extends beyond recent burn areas. Streams, rivers and creeks were expected to see rapid flows, increasing the risk of swift-water rescues.
Strong winds will compound the flooding threat. High wind warnings were issued from noon Tuesday through 3 p.m. Thursday for Santa Barbara and San Luis Obispo counties, as well as mountains and deserts in Ventura and Los Angeles counties. Gusts of 60 to 80 mph were forecast for those areas, while widespread gusts of 35 to 55 mph were expected across the region.
The combination of saturated soil and powerful winds raised concerns about widespread downed trees and power lines, particularly in areas under high wind warnings.
The Storm Prediction Center placed the region under a marginal risk for severe thunderstorms Tuesday night and Wednesday. Any thunderstorms that develop could bring brief but intense rainfall and damaging winds. There was also a chance of weak, short-lived tornadoes or waterspouts, especially Thursday when colder air aloft creates additional instability.
The storm system features an atmospheric river with precipitable water values peaking around 1.5 inches and integrated vapor transport values between 750 and 1,000 kilograms per meter-second. A strong upper-level southwesterly jet will interact with this very moist tropical air mass to produce the prolonged heavy rainfall.
Snow levels will remain above 7,500 to 8,000 feet through Christmas Eve before dropping to around 7,000 feet Christmas Day, bringing accumulating snowfall to mountain resorts.
Marine conditions were expected to become exceptionally dangerous Tuesday afternoon through Thursday, with gale to storm-force winds north of Point Conception. The weather service strongly advised vessels, especially small craft, to remain in safe harbor. South-facing harbors were considered particularly vulnerable to the combination of south swell and strong winds.
A high surf advisory was in effect through Saturday morning, with large, choppy and dangerous surf expected to produce hazardous rip currents. Minor to locally moderate coastal flooding was possible for south-facing shores through Thursday.
Following the main storm system, cooler and showery conditions were forecast Friday and Saturday, with snow levels dropping to 4,500 to 5,000 feet. An additional half-inch to one inch of rain was expected in mountains and foothills Friday, with lighter amounts Saturday.
The forecast office issued flood watches, high wind warnings, wind advisories and high surf advisories covering most of Southwest California through the holiday period.