Forecasters warned Wednesday of a new wrinkle in the heat wave gripping the Southland for more than a week, saying onshore winds will combine with high heat, low humidity and dry vegetation to create a “risk of extreme fire behavior” Thursday. As a result, a fire weather watch issued by the National Weather Service, often a precursor to a red flag warning, will be in effect in Los Angeles County in the San Gabriel Mountains and the Antelope Valley from Thursday morning through Thursday evening.
Until now, the winds have been relatively tame, and only a few unspecific warnings of fire had cropped up in NWS statements since the heat wave began June 13.
“While winds have been relatively light so far this week, onshore flow is expected to strengthen on Thursday,” according to an NWS statement. “With this expected wind increase, continuing hot and dry conditions and drying fuels/vegetation, the risk for extreme fire behavior will be on the rise.”
NWS forecasters said southwest winds would blow at sustained speeds of between 15 and 25 miles per hour in the San Gabriels and the Antelope Valley Thursday, gusting to 35 mph, although winds of 20-30 mph with 45-mph gusts are possible in the foothills in the Antelope Valley.
At the same time, humidity levels will range between 3 and 15 percent — lowest at higher altitudes — and maximum highs will continue to range between 100 and 110 degrees in the Antelope Valley and at lower elevations in the San Gabriels.
“If fire ignition occurs, conditions may be favorable for extreme fire behavior, which would threaten life and property,” an NWS statement said.
In the meantime, a strong ridge of high pressure remained parked over Arizona, maintaining near-record heat and low humidity through at least Thursday in Southern California, the NWS said.
An excessive heat warning will remain in force until 9 Wednesday evening in the San Gabriels, where temperatures of between 100 and 105 are expected, and in the Antelope Valley, where highs are expected to range between 106 and 110 through Friday.
A less serious heat advisory will be in force until 9 Wednesday evening in the Santa Monica Mountains Recreational Area and the Santa Clarita, San Fernando and San Gabriel valleys. The advisory, issued Wednesday, was allowed to expire in Los Angeles proper and beach cities.
“The very high temperatures could create a dangerous situation in which heat-related illnesses are possible,” warned an NWS statement. “Temperatures inside vehicles, even if the windows are partially open, can quickly rise to life-threatening levels.
Never, ever leave people or pets in enclosed vehicles, even for a short period of time.”
People who work outdoors were urged to schedule strenuous activity for early morning or evening, wear lightweight clothing and drink plenty of water.
The California Independent System Operator, which manages the statewide power grid, again issued a Flex Alert for Wednesday. It will will take effect at 2 p.m. and remain in force until 9 p.m., just like on Tuesday.
A Flex Alert is issued when the electrical grid is “under stress,” generally from increased power usage due to hot weather.
As part of the alert, residents were urged to make an extra effort to conserve during late afternoon hours, when air conditioners tend to be used most. The agency predicted peak usage on Wednesday would exceed 46,600 megawatts, and noted that widespread air-condition use can put strain on the electrical system.
According to Cal-ISO’s website, electrical demand late Tuesday afternoon topped 44,000 megawatts. The highest demand ever recorded by the system was 50,270 megawatts, which occurred on July 24, 2006.
The NWS forecast a mix of partly cloudy and sunny skies in Los Angeles County Wednesday and highs of 74 at LAX; 82 in Avalon; 84 in Downtown L.A.; 85 in Long Beach; 90 on Mount Wilson; 92 in Burbank and San Gabriel; 93 in Pasadena; 101 in Saugus; 102 in Woodland Hills; 106 in Palmdale; and 107 in Lancaster. Temperatures will be generally lower in coming days but remain in the 100s in the Antelope Valley through at least Tuesday.
Sunny skies were forecast in Orange County Wednesday, along with highs of 69 in San Clemente; 70 in Laguna Beach; 72 in Newport Beach; 85 in Anaheim; 86 in Irvine; 88 in Fullerton; 89 in Mission Viejo; and 91 in Yorba Linda. Orange County temperatures will stay roughly the same at least through Tuesday.