After days of unseasonably hot temperatures and powerful Santa Ana winds, Southern California is bracing for a dramatic weather shift with cooler temperatures, widespread rain, thunderstorms and the potential for flooding and debris flows this week.
The approaching storm could bring up to 3 inches of rain to the mountains and foothills, with the heaviest precipitation expected Wednesday night into Thursday morning, according to the National Weather Service.
Forecasters said the greatest danger includes localized flooding, debris flows near recent wildfire burn scars, hazardous driving conditions and coastal erosion. The approaching rain also comes as crews work to stabilize a damaged hillside beneath Pacific Coast Highway near Laguna Beach.
Carol Ciliberti, a meteorologist with the National Weather Service in Los Angeles/Oxnard, said much of the region can expect between half an inch and 1.5 inches of rain, with higher totals in the mountains and foothills.
"There's going to be quite a bit from southern Santa Barbara County across Ventura County and most of LA County," Ciliberti told Patch.
Ciliberti said just over an inch of rain is expected at Los Angeles International Airport, about 1 inch in Oxnard and 1.3 inches in Santa Barbara. South-facing mountains and foothills could receive 1.5 to 3 inches in the Los Angeles/Oxnard forecast area.
Ciliberti said there is a 30 percent to 40 percent chance of thunderstorms, which could bring dangerous lightning, heavy downpours, small hail and even waterspouts that could move onshore as weak tornadoes.
"With these types of storms, a lot of times what gets us is shorter term, 15- to 30-minute heavy showers that can deliver a lot of rain in that period of time," she said.
Recent wildfire burn areas are particularly vulnerable.
"There's a minor risk of flood and debris flows in our recent burn areas," Ciliberti said.
She added that there is a 10 percent chance of heavier rainfall rates that could produce more significant flooding or debris flows near recent burn scars.
Farther south, Sam Zuber, a meteorologist with the National Weather Service in San Diego, said rain could begin in Orange and San Bernardino counties as early as Wednesday morning before spreading across the region by the afternoon and evening.
The heaviest rain is expected Wednesday night into early Thursday, particularly in Orange County and the Inland Empire.
About an inch of rain is expected in the mountains, with approximately three-quarters of an inch in coastal Orange County and the Inland Empire. Parts of northern San Diego County could also receive three-quarters of an inch.
"As you get further south, it kind of starts to taper off, but it's not out of the question that even downtown San Diego sees about a half an inch in this 48- to 72-hour timeframe," Zuber told Patch.
There is also a slight chance of thunderstorms Wednesday night into Thursday morning across Orange, Riverside, San Bernardino and San Diego counties.
The approaching rain comes as crews work to stabilize a damaged hillside beneath Pacific Coast Highway near Laguna Beach.
A large crack appeared in the bluff supporting the roadway near Crystal Cove State Park, prompting Caltrans to close one southbound lane between Irvine Cove Drive and Reef Point Drive.
Crews have been covering the hillside with plastic tarps ahead of the rain. Laguna Beach City Manager Dave Kiff said Caltrans also plans to place large boulders at the base of the slope and develop a longer-term stabilization plan. The extent to which the approaching storm could affect the damaged hillside remains uncertain.
Along the coastline, elevated surf and higher tides could also contribute to erosion and flooding.
Ciliberti said the approaching full moon will bring higher tides, which, combined with elevated surf, could cause minor to locally moderate coastal flooding and coastal erosion.
Meanwhile, the timing of the heaviest rainfall could create problems for Southern California commuters, particularly Wednesday afternoon and Thursday morning.
"Unfortunately, that could coincide with some of the earlier people heading to the office Thursday morning on the roads," Zuber said.
She warned that heavy rain could reduce visibility and urged drivers to slow down, leave additional space between vehicles and allow extra time to reach their destinations.
Ciliberti also encouraged residents to prepare for possible localized flooding, especially if their properties have experienced drainage problems.
"Watch out for localized flooding," she said. "Slow down because the roads could be slick. There could be lower visibility."
The storm marks a sharp reversal from last week's heat and Santa Ana winds, which brought warm temperatures and heightened fire danger across the region.
Temperatures will continue falling through midweek, with many coastal and valley communities dropping into the 60s and 70s by Wednesday and Thursday.
Despite the dramatic shift, forecasters said the changing conditions are typical for fall in Southern California.
"It's a pattern change. It is fall," Ciliberti said.
Zuber agreed, noting that October often brings alternating periods of wet weather and dry Santa Ana wind conditions.
"This is not abnormal for this time of year at all," she said. "We start to see our winter storms coming down now."
Unlike colder winter storms, the approaching system is expected to be relatively warm, with no snow forecast at higher elevations.
"It's just going to be a nice wetting rain," Zuber said.
The storm is expected to move out of the region by Friday, with drier weather and sunshine returning. Temperatures are expected to rebound over the weekend, when another round of Santa Ana winds could develop, according to the weather service.
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