Extended California heat wave brings extreme health, fire risk; power shutoffs likely (2024)

Authorities are warning of extreme health and wildfire risks across California this week, as the longest heat wave of the year is set to kick off Tuesday, bringing triple-digit temperatures, with little overnight cooling, to the vast majority of the state.

“This is really just a long-duration heat event that will provide little to no overnight [temperature] relief,” said Antoinette Serrato, a National Weather Service meteorologist in Hanford, Calif. The heat wave is expected to bring dangerous temperatures through the Fourth of July holiday and into early next week in many areas, particularly across Northern California, the Central Valley and southwestern deserts, she said.

Much of Los Angeles County’s inland valleys and mountains, including the San Fernando Valley and San Gabriel Mountains, are included in an excessive heat watch from Wednesday through Sunday, when high temperatures from 95 to 110 degrees are expected. However, Southern California’s coastline should be largely spared from the worst of the heat.


“This will be a statewide heat event and it may last quite a long time — beginning July 1 and perhaps not meaningfully receding until July 8 [or] 9 (or even later in some spots),” UCLA climate scientist Daniel Swain wrote in a blog post Sunday. “Across inland areas, as well as at higher elevations, it’s entirely possible that temperatures may be continuously above average for the next 2+ weeks–straight, into mid-July.”

He said the duration of the heat event, caused by a type of high-pressure system moving in from the Pacific known as a heat dome, and the lack of overnight cooling across such a widespread area “will amplify both human heat stress and wildfire risk.”

On Tuesday, a combination of excessive heat is expected to overlap with a red flag warning — an alert for extreme fire weather — in parts of the northern San Francisco Bay Area and Sacramento Valley, raising concerns about potential fire starts. Low humidity, winds up to 30 mph and hot temperatures could create the perfect storm for wildfires, officials warned.

Pacific Gas & Electric Co., the state’s largest utility, announced that power cuts were likely for Tuesday and Wednesday in some Northern California counties, “due to high winds and dry conditions,” part of its safety shutoff program that aims to reduce the chance for accidental fire ignitions. The counties that could be affected on Tuesday are Colusa, Glenn, Lake, Napa, Shasta, Solano, Tehama and Yolo, according to the utility’s website. Colusa, Glenn, Shasta and Tehama could see continued shutoffs Wednesday.

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“Right now it has the potential to affect 12,000 customers in eight counties and two tribal areas,” said Denny Boyles, a PG&E spokesperson. He said the shutoffs, if they occur, could begin very early Tuesday and last through Wednesday, or whenever weather conditions improve.


The California Governor’s Office of Emergency Services also announced Monday that it had pre-positioned fire engines and personnel across the state.

Good morning. Heat and fire weather dangers will build through the week! Excessive Heat Warnings, Heat Advisories, and Red Flag Warnings will be in effect for parts of the Bay Area and Central Coast throughout the week. See https://t.co/IELcH0TY3U for safety tips! #CAwx pic.twitter.com/9YSWgDfuQf

— NWS Bay Area 🌉 (@NWSBayArea) July 1, 2024

“A combination of strong wind, low relative humidity, and hot temperatures can contribute to extreme fire behavior,” the National Weather Service wrote in its red flag warning for the Sacramento Valley and North Bay interior mountains, lasting from Monday night through Wednesday.

And given the timing of the heat and red flag warnings around the Fourth of July holiday, Swain said, there’s an added layer of risk for wildfires because of the expected uptick in fireworks use.

Even before the worst of the heat, firefighters were battling two major fires in Fresno County, including the Basin fire, which continued to grow Monday. By Monday afternoon, it had exceeding 12,000 acres, almost double its size from the day before, without any containment, according to the California Department of Forestry and Fire Protection. Firefighters were also still responding to the Fresno June Lightning Complex blaze, which had remained at about 10,600 acres the last few days. It was about 70% contained as of late Monday.

But weather officials say the greatest concern this week comes from the extreme, extended heat, which is expected to bring major health risks.

“As we start moving into the weekend, there are a lot of areas ... that are going to see extreme heat risk,” Serrato said, referencing the weather service’s new heat risk tool. “Overnight low temperatures will be around 70 degrees for a lot of areas, maybe even higher.”


From northern Humboldt County to inland San Diego County — and practically everywhere in between — the National Weather Service has issued an excessive heat warning beginning Tuesday and lasting at least until Friday, and in some areas Monday.


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“This level of rare, long-duration extreme heat with little to no overnight relief affects everyone, especially those without effective cooling and/or adequate hydration,” the heat warning said for the Central Valley, where high temperatures are forecast to reach or exceed 107 degrees from Tuesday through July 8, the following Tuesday.

Highs up to 121 degrees are forecast for the Coachella Valley and San Diego County deserts. In the Antelope Valley, temperatures will reach 105 to 115 degrees.

In Death Valley, highs could peak at 125 later this week, while “low temperatures may not fall below 90 degrees for several days,” the weather service warned.


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In the state’s farthest northwestern corner, officials are forecasting potentially record-breaking temperatures, with highs above 110 expected in northern Humboldt County from Wednesday into the weekend. Trinity and Lake counties could have areas up to 115 degrees, the weather service warned.

According to the excessive heat warning, much of the inland Bay Area could see highs up to 110 degrees. San Francisco, meanwhile, was placed Monday afternoon under a less-intense heat advisory, where highs could reach the mid-90s.


The Bay Area Air Quality Management District also issued a “Spare the Air” alert for the area Tuesday, with air quality expected to be unhealthy for sensitive groups due to high ozone concentrations, or smog, which tend to rise during warm weather. The agency asked people to reduce air pollution when possible Tuesday.

The National Weather Service’s detailed forecast extends only a week, but long-range predictions show a high chance for above-average temperatures to continue through at least July 10, according to the Climate Prediction Center.

Swain said this could become what he called a “double peaked” heat wave, with the most extreme heat first hitting around Wednesday before a second peak arriving next weekend, probably more focused inland.

This kind of extended, extreme heat will affect everyone, including vulnerable populations such as babies, the elderly and pregnant people, said Serrato, who urged residents to take precautions and seek out cooling centers set up across the state if necessary.

“Avoid going outside during the hottest hours of the day and make sure to stay hydrated,” Serrato said. “Try to stay cool and stay safe.”

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Extended California heat wave brings extreme health, fire risk; power shutoffs likely (2024)
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