A windstorm that forecasters described as potentially destructive and life-threatening will develop across Southern California Tuesday morning, dramatically raising the risk of wildfires for the next several days.
“Residents across Southern California are urged to remain vigilant and monitor the latest forecasts,” the National Weather Service advised. “There is high confidence in strong offshore winds with the potential to be the strongest wind event of the season, especially for the Southland valleys. Trees, large tree branches, and power lines may be downed by the wind … Residents living the near the foothills and mountains are advised to review the ‘Ready, Set, Go!’ fire plan. Most residents should be in the ‘Set’ stage and being prepared to go in the event of the fire evacuation.”
Red flag warnings of critical fire danger will take effect at 4 a.m. Tuesday and remain in place until 6 p.m. Thursday for the Malibu coast, the Santa Monica Mountains Recreational Area, the San Gabriel Valley, the San Fernando Valley, Calabasas, the Santa Clarita Valley, the San Gabriel Mountains, and the 5 and 14 freeway corridors.
At 4 p.m. Tuesday, the warning will extend to include Los Angeles County beaches, the Palos Verdes Hills, Catalina Island and the inland Los Angeles County coast, stretching into downtown Los Angeles.
The red flag warning will be in place from 10 a.m. Tuesday to 6 p.m. Wednesday in the Antelope Valley and Antelope Valley foothills.
“Confidence is high for a life threatening, destructive, widespread windstorm Tuesday afternoon through Wednesday morning, especially focused on the LA County mountains and foothills, San Gabriel Valley, San Fernando Valley, Hollywood/Beverly Hills, and Simi Valley,” according to a statement from the National Weather Service. “… With widespread damaging winds likely from Tuesday afternoon into Wednesday across portions of Los Angeles and Ventura counties, there will be a high risk for downed trees and power lines, as well as power outages.
“There is also the potential for strong north to northeast winds for the LA county coast, especially Hollywood Hills, Beverly Hills, and Sepulveda Pass, extending into Palos Verdes and Catalina Island late Tuesday afternoon into Wednesday, resulting in the red flag warning for these areas. There is a high probability of strong mountain wave wind conditions during the peak of the event Tuesday afternoon into Wednesday morning, which will likely bring strong/damaging winds to areas that typically do not see strong winds, including portions of the San Gabriel Valley and San Fernando Valley, LA Basin and Simi Valley.”
According to the NWS, Santa Ana winds could gust at speeds of 60 to 80 mph — with some ridgetops seeing gusts topping 90 mph — in western LA County valleys, the Santa Monica Mountains and into Malibu.
In the Inland Empire, a High Wind Watch is in effect from 4 p.m. Tuesday to 4 a.m. Thursday, and a Fire Weather Watch is in effect from 4 a.m. Wednesday to 6 p.m. Thursday. The Weather Service said the times for the two advisories may be adjusted, depending on the climatic activity.
The NWS said that “gusts of 35 to 45 mph are possible into the Inland Empire, below the Cajon Pass and … near the foothills of the Santa Ana Mountains.”
“Wind-prone locations have the potential to see gusts exceed 75 mph during the peak of the winds,” the NWS said about the Inland Empire. “Winds look to very slowly weaken from their peak, with gusts 40 to 60 mph still possible into late Wednesday afternoon.”
Forecasters warned particularly of the possibility of “breaking mountain waves” at high elevations across Southern California.
“These occur when very strong winds aloft intercept mountain tops at nearly perpendicular angles, which will happen with these events,” according to the NWS. “These short lived and very difficult to predict events can cause considerable local damage wherever they occur. The most likely areas for this phenomena are the eastern San Fernando Valley and the northern San Gabriel Valley.”
Humidity levels are also expected to drop, most notably on Wednesday, creating dry conditions that could amplify the fire danger.
Forecasters said that despite the Santa Ana winds, temperatures across the region could actually fall by 5 to 10 degrees on Tuesday due to the amount of cold air accompanying the system. But the temperatures will bounce back on Wednesday, recovering those 5 to 10 degrees.
In the Riverside and San Bernardino metropolitan areas, daytime highs will reach the mid-60s Tuesday to Thursday, with lows in the low 40s.
San Fernando Valley temperatures should hit the high 60s on Tuesday and the high 70s on Wednesday and Thursday. Lows will be in the mid- to upper 40s.
San Gabriel Valley highs will be in the mid-60s Tuesday and low 70s Wednesday and Thursday with lows in the mid-40s.
In Orange County, temperatures will be in the low 70s Tuesday through Thursday with lows in the mid-40s to low 50s.
Long Beach highs will be in the mid-60s on Tuesday and the low 70s Wednesday and Thursday with lows in the low 50s.
South Bay temperatures will be in the high 60s on Tuesday, and low 70s Wednesday and Thursday, with lows in the low 50s.
There is a remote chance of light rain and sprinkles in the Inland Empire, possibly even snow flurries above 5,000 feet, as the cold front and trough of low pressure make an express run through the region. However, meteorologists said the moisture content will evaporate quickly as relative humidity plunges to 10% across the Inland region by Wednesday morning.
“Any fires that develop will likely spread rapidly,” the NWS said. “Outdoor burning is not recommended.”
Due to the elevated fire danger, Caltrans will close Topanga Canyon Boulevard between Mulholland Drive and Pacific Coast Highway from 10 a.m. Tuesday through 6 p.m. Friday. Only residents and local business traffic will be permitted into the canyon during the closure.
As is standard during high-fire-danger conditions. Southern California Edison customers in some areas could have their power turned off under the utility’s Public Safety Power Shutoffs program. The program is designed to de-energize power lines that could potentially be damaged and spark a wildfire during red flag conditions. According to SCE’s website, more than 70,000 customers in Los Angeles County and more than 9,400 in Orange County are under consideration for power shutoffs during the upcoming wind event.
Updated information about power cuts is available at https://www.sce.com/outage-center/outage-information/psps.