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A man takes shelter from the sun as he waits for the bus in Oakland, Calif., on Monday, Sept. 5, 2022. Heat warnings and advisories have been extended until Thursday.

An ice cream vendor sells cold treats to beachgoers at Crissy Field in San Francisco on Monday.

A woman cools herself off in a water mister at the Scottish Highland Games in Pleasanton on Sunday. The games are occurring just as an intense heat wave grips the California Bay Area.

People get water as they finished the Kilted Mile at the Scottish Highland Games in Pleasanton on Sunday, amid the intense heat wave.

Reese Burbank, 7, tries to stay cool by spraying herself with water at the Scottish Highland Games in Pleasanton on Sunday. 

See the latest California heat wave live updates here. Plus, our meteorologist looks at the forecast: when is the heat wave over?

The worst of California's heat wave has arrived for the final day of Labor Day weekend celebrations, with temperatures reaching 110 degrees or higher in a number of Bay Area cities.The intense heat tested California’s power grid like never before. Two Bay Area cities recorded the highest-ever temperature in the region.

What's behind the sizzling temperatures? It's called a heat dome: an atmospheric lid is trapping hot air over the Western U.S., triggering intense heat. You can read more here about the science of California's worsening heat waves, as explained by the Chronicle weather science team.

Our reporters fanned out today to bring you the latest news from around the Bay Area.

The state power grid canceled its emergency notices on Monday at 9 p.m. as demand dropped and supplies were sufficiently available -- meaning rolling blackouts are extremely unlikely on Monday night. A Flex Alert -- a request for Californians to reduce electricity use -- remains in effect until 10 p.m. The grid will face a fresh crisis on Tuesday when demand is forecast to be at its highest-ever level, driven by a combination of the heat wave and people returning to work (which boosts electricity demand outside the home).

The Brentwood Family Aquatic Complex is closed through Tuesday "due to the excessive heat forecasted on those days," the East Bay city said. The Brentwood Community Center, located at 35 Oak Street, was open as a cooling center Saturday through Tuesday from 11 am to 6 pm for those needing to escape the heat.

Murphy's Irish Pub and Restaurant in Sonoma canceled a music jam on Monday evening as temperatures peaked at 110 degrees, although it was open for drinks and food in air-conditioned rooms. Ray's Deli and Tavern in Petaluma is closing on Tuesday "due to the extreme heat" for the safety of their staff, but hopes to reopen for regular business hours Wednesday. Harvey's Donuts in Sonoma will stay closed until Friday to beat the heat.

In a sign that power supplies continue to be badly strained, the California grid operator extended its stage 2 emergency notice until 9 p.m. on Monday. The prior notice had expired at 8 p.m. Although power demand has peaked, there is still a tremendous extra load from air conditioning and no solar power after dark except what is stored in batteries. If the grid passes through stage 3, rolling blackouts ensue -- a concern for Tuesday as well when state power demand is forecast to be at its highest-ever level.

Even cool coastal San Francisco has not been spared the brutal heat, with the National Weather Service issuing a heat advisory for downtown from Monday evening until Wednesday. The hottest temperatures are expected Tuesday afternoon, with a high of 87, the agency tweeted. 

Livermore joined Fairfield with the unwelcome distinction of having the highest-ever temperature in the Bay Area on Monday: 116 degrees. Livermore's previous record was 115 degrees in 1950, according to the National Weather Service.

Numerous records set today around the Bay Area including many new daily high temperature records, a few September monthly records, and one all-time record. #cawx pic.twitter.com/zsYDQJgpXX

The Franklin Fire at  Cummings Skyway and SR4 is approximately 50 acres, according to Cal Fire, which is providing air support. It is showing a "dangerous rate of spread," the agency said. Wildfires are primed to break out in a high-heat environment, when vegetation is extremely dry. Read more on the Franklin Fire here.

Franklin Incident at Cummings Skyway and SR4. CalFire air support. pic.twitter.com/1K0G6dXpvy

Record-shattering heat across the Bay Area led to "unplanned heat-related outages" for 22,418 customers as of 6 p.m. Monday, according to a PG&E spokesperson. The main issue was transformer outages: Transformers need time to cool down, and typically do so overnight when temperatures drop. But during a heat wave temperatures are still high when it gets dark, and transformers may fail. It's more common in coastal areas rather than inland, where higher temps are typical and transformers are built to handle more heat. These blackouts were not state-ordered rotating outages -- which are due to a statewide power shortage and have not occurred as of Monday at 6:40 p.m. -- nor are they public safety power shutoffs to prevent wildfires. 

From 6:30 p.m. to 8 p.m. the California grid will move to Energy Emergency Alert Level 2 -- the highest level of emergency so far in this heat wave. If the grid moves through Emergency Alert Level 3, it will turn turn to rolling blackouts as demand overwhelms supply. (Correction: An earlier version of this post gave an incorrect end time for the Level 2 alert. It ends at 8 p.m.)

Novato Unified School District will run a compressed schedule at Lu Sutton Elementary School, Novato High School and Hill Education Center, schools that have limited or no air-conditioning, during Marin County's excessive heat warning from Tuesday to Friday. At the high school, each period will be shortened and the school day will finish by 1:15 p.m.

Preliminary high temperatures for the Bay Area’s scorching heat wave are rolling in, and early reports show Livermore possibly soared to 113 to 116 degrees, which would break its daily record high of 108 degrees that was set in 1950. Buchanan Field Airport in Concord tied its all-time record of 112 degrees from 2020, while downtown Oakland reached 100 degrees, breaking its daily record high of 95 degrees in 2008.

The San Francisco Airport tied its daily record high of 93 degrees from 2020. A high of 102 degrees was recorded both at San Jose International Airport and in Mountain View, breaking the previous daily record highs of 99 and 97 degrees in 2009, respectively. Palo Alto reached 98 degrees, breaking the 2008 daily record high of 95 degrees.

The Sonoma County Airport station hit a high of 111 degrees, breaking the 109-degree daily record set two years ago. The Napa County Airport reached 109 degrees, surpassing the 106-degree record in 2020. The National Weather Service Bay Area office will start verifying the records Monday afternoon, but it is unclear when a final report will go out.

Fairfield, in the Travis AFB area, preliminarily recorded a high of 116 degrees -- an all-time record for the Bay Area. The city's temperatures could go higher still on Tuesday, according to Chronicle meteorologist Gerry Diaz. Livermore was another city that saw excruciating temperatures on Monday.

Even as the grid veered toward a power shortage on Monday, officials preemptively declared another emergency on Tuesday from 5 to 9 p.m. Power demand is expected to reach its highest-ever levels on Tuesday, causing the specter of rolling blackouts to linger over California.

Fairfield hit 116 degrees late Monday afternoon which -- assuming the number is finalized by the National Weather Service -- would be the highest temperature ever recorded in the nine-county Bay Area, according to Chronicle meteorologist Gerry Diaz. The weather service will begin to verify records on Monday afternoon. The figure was recorded in the vicinity of Travis Air Force Base.

The California grid operator's request that Californians reduce their electricity use -- a request known as a Flex Alert -- is now in effect. Californians are asked to refrain from using energy-hungry appliances such as the dryer or dishwasher, turn out unneeded lights and keep their air conditioning to 78 degrees or higher (85 for those who are not at home). The goal is to prevent rolling blackouts, which could happen if the grid is overtaxed by electricity loads. Regulators have said Monday will be the most challenging day for the grid so far during this heat wave.

A young girl tries to stay cool in the stands at the Scottish Highland Games in Pleasanton on Sunday. Monday is expected to be even hotter.

Tropical storm Kay is spinning off the southern coast of Baja California Sur, and is forecast to hit Baja as a hurricane by the middle of the week. Its remnants will bring plenty of moisture into Southern California. This moisture will help spawn scattered wet thunderstorms from San Diego to Santa Barbara, along with portions of Kern County and the Mojave, according to Chronicle meteorologist Gerry Diaz. The latest weather models are signaling that some of this remnant moisture will make it into NorCal next weekend as well. If enough moves in, Northern California could see wet storms as well. But if only sprinkles of remnant moisture move in, the chance for these storms producing dry lightning in the heat-strained forests of Northern California next weekend cannot be ruled out.

Up to 5,000 homes and businesses were without power in Napa County, as were more than 5,000 in an area west of Vacaville, according to a PG&E outage map. The utility could not immediately be reached for an explanation, but extreme heat is known to strain all infrastructure, including power equipment. The outages are not part of statewide rolling blackouts, which are a possibility -- though not an inevitability -- for the late afternoon and evening Monday. 

Cal/OSHA has told companies in the construction, agriculture, tree-trimming, landscaping, car wash and warehouse fields that it plans to conduct “targeted inspections” to ensure workers safety during the heat wave. By law, employers must supply fresh water, access to shade and regular breaks — and closely monitor employees for signs of heat illness at extreme temperatures.

The nearly unprecedented strain on the power grid will peak on Monday and Tuesday, with grid officials looking at all possible ways to avert rolling blackouts. The best thing Californians can do, they say, is slash electricity use from 4 p.m. to 10 p.m. on Monday night (and similar hours this week), to offset the immense power demand from air conditioners. Read more here about how the public can help avert blackouts.

The heat wave in the Sacramento Valley is reaching its apex today and tomorrow, meaning it's about to experience some of the hottest temperatures seen in the continental United States this year. Weather models are projecting today's highs reaching 115-119 degrees, according to Chronicle meteorologist Gerry Diaz. Sacramento, Modesto and other cities are forecast to surpass their all-time records the next couple of days. Little overnight heat relief is expected, with nighttime temperatures staying in the 70s, while some of the peaks along the Sierra and Diablo Ranges will likely stay in the 80s overnight. Temperatures will finally drop below 110 degrees in most of the valley by Thursday, but 100-105 degree weather will still be rampant from Thursday to Saturday.

"There is a 10-minute delay on the Antioch Line in the Antioch and SFO directions due to warm weather conditions," BART said in its alert feed. The service had also slowed down trains on Sunday for the same reason.

Chronicle meteorogist Gerry Diaz reports here on which Bay Area cities are likely to hit 110 on Labor Day.

A person jogs under noon rays at Lake Merritt in Oakland on Monday. Heat warnings and advisories have been extended until Thursday.

"If you are not already at Stinson Beach. ...  we'd suggest not going," the Marin County Sheriff tweeted Sunday. There are "hours-long delays getting into the area, and the parking lots and street side parking is all full." On Sunday, the sheriff's office said it had issued more than 145 citations in Stinson Beach. Visitors are requested not to block driveways or park in yards, and to be respectful of residents.

If you are not already at Stinson Beach... We'd suggest not going. Expect hours long delays getting into the area and the parking lots and street side parking is all full. pic.twitter.com/1XMRE5qxPt

On the desk of Gov. Gavin Newsom is AB2238, a bill that would direct the state’s Environmental Protection Agency to create a ranking system for heat waves by 2025. Hurricanes, earthquakes and even atmospheric rivers get ranked by their severity — so why not heat waves? It’s unclear how the current heat wave would rank, but officials say its extreme temperatures and long duration make it particularly dangerous. Read more here about the heat wave ranking bill, which was passed unanimously with bipartisan support.

Along the Russian River near Healdsburg -- a city expected to see temperatures of 110 degrees on Monday -- Myra Perez of Sonoma joked it might be the hottest she’s ever been. Her children, 6 and 3, played in the water, apparently immune to the heat. “It’s too hot to barbecue,” Perez said. “We’re going to In-N-Out. It's the type of choice many Californians are facing on what is normally an outdoors-oriented holiday.

The Golden Gate National Recreation Area tweeteed that parking lots in Stinson Beach, Muir Beach and Rodeo Beach in Marin County are full, as are most of its lots in San Francisco. "We welcome our visitors escaping the heat, and remind them to be patient, get shade, and don't take chances with cold ocean currents or coastal bluffs," the park service tweeted.

As temperatures rise, children play in the water at Crissy Field in San Francisco on Monday.

Heat in San Francisco is considered extreme when it surpasses 85 degrees - and shortly before noon, the downtown reading was already at 89 degrees. The city will remain cooler than most parts of the sweltering region and state, however.

Amid scorching temperatures across California, the state's electrical grid operator is forecasting the highest-ever demand for power in state history. Californians burned through 50,270 megawatts of power on July 24, 2006, but California Independent System Operator President and CEO Elliot Mainzer said Monday at a news conference that the expected demand has surpassed that historic high for Tuesday. He called on Californians to conserve their power during the key hours of 4 p.m. to 10 p.m. this week and hoped that those reductions and importing power from other states could get the state through Monday and Tuesday without rolling blackouts.

In a sign of the intense stress on power supplies caused by the heat wave, the California grid operator has issued an Energy Emergency Alert 1 notice for Monday, effective from 5 p.m. through 9 p.m. This is the first level-1 emergency alert for the grid so far this heatwave, and it is designed to lower power demand from large customers and create more supply. If the grid passes through emergency stages 2 and 3 -- something grid officials think could happen Monday -- then it is time for rolling blackouts because demand, sent sky-high by air conditioning use, is exceeding supply. Read more here about the nearly unprecedented strain on California's power grid.

Officials are warning against any activity that could start a fire, as the extreme temperatures have further dried out vegetation and made it highly vulnerable to a spark. Fire danger is rated high through Friday in most of California. 

What feels like an endless heatwave is peaking on this Labor Day, with widespread 105-110° F temperatures coating the North Bay, East Bay and Sacramento Valley. Even San Francisco is getting in on some of the heat wave as parts of the city climb to the 80s today. This historic heat wave isn't done with California yet. With the heat wave expected to drag out, all excessive heat warnings and advisories for the Bay Area and the Sacramento Valley have been extended through 8 p.m.. Thursday.

Fans take advantage of the shade while watching San Francisco Giants play the Philadelphia Phillies  at Oracle Park in San Francisco on Sunday.

Officials rescued a hiker who had experienced heat exhaustion at Montara Mountain in San Mateo County on Sunday, according to a tweet from Cal Fire's CZU branch. Read here about officials' warnings about outdoor exertion this weekend, following a string of high-profile heat-related deaths over the past year.

Coastside Fire Engine 44, @CAStateParks & @CHP rescued a hiker suffering from heat exhaustion on Montara Mountain Sunday. Standby for more details…. 🚁 pic.twitter.com/GNii2RztLS

During that time, electric grid officials are asking Californians to refrain from using energy-intensive appliances such as dryers or dishwashers and turn off unnecessary lights. They also ask people to keep their air conditioning at 78 degrees or higher during that time (pre-cooling your home before 4 p.m. is a good idea). This is the latest in a string of Flex Alerts during the heat wave.

A biker rides by Lake Merritt during the heat wave on Labor Day weekend in Oakland.

Power grid conditions are expected to deteriorate late Monday afternoon as air conditioners work overtime and electricity demand soars. Grid officials anticipate emergency conditions and will ask consumers and businesses to save power from 4 to 10 p.m. in a bid to avert rolling blackouts. Read more here about why grid officials are so worried.

Heat waves are typically defined as unusually hot weather lasting more than two days. But what’s considered unusual can vary across California’s distinct geographical regions. For example, a high of 95 degrees Fahrenheit may describe a normal summer day in the Central Valley. But these temperatures could be unbearable in San Francisco, where residents typically don’t have air conditioning. Weather experts consider these differences when providing warnings about extreme heat. Go here to read more about the science of heat waves.

Kate Galbraith is The Chronicle's Climate Editor, overseeing energy and environment coverage. Previously she edited COVID and business coverage. Kate has also worked for the Texas Tribune, CALmatters, The New York Times and the Economist. A native of Washington, DC, Kate began her writing career at Let's Go and Lonely Planet travel guides. She is co-author of The Great Texas Wind Rush, a book about how the oil and gas state came to lead the nation in wind power.

Gerry Díaz is The San Francisco Chronicle's first ever Newsroom Meteorologist.

He previously served as a meteorologist for the National Weather Service Bay Area, working from the agency's Monterey office. In that position he led an initiative aimed at increasing outreach to Spanish speaking communities during extreme weather events.

Most recently he worked as a meteorology specialist for utility Southern California Edison.

Díaz enjoys hiking through California's national parks and shooting panoramic photography, including of the Central Coast.

Matthias Gafni is an enterprise reporter for The San Francisco Chronicle. He investigates stories in the East Bay and beyond. For almost two decades, Gafni worked for the Bay Area News Group - San Jose Mercury News, East Bay Times and Vallejo Times-Herald - covering corruption, child sexual abuse, criminal justice, aviation and more. In 2017, Gafni won a Pulitzer Prize for breaking news for his work on the Ghost Ship fire. In 2018, he was named SPJ Reporter of the Year in Northern California. The following year, he was a finalist for a Pulitzer Prize for breaking news for his work covering the Camp Fire. In 2020, he won a Polk Award for military reporting for his coverage of the Capt. Crozier saga involving a COVID-19 outbreak aboard a U.S. aircraft carrier. He was born and raised in the Bay Area and graduated from UC Davis. He lives with his wife and three kids in the East Bay.

Julie Johnson covers the changing climate, sea level rise and strategies to stem California's wildfire crisis. Before joining The Chronicle, she spent 11 years as a staff writer at the Santa Rosa Press Democrat, where she had a leading role on the breaking news team awarded the 2018 Pulitzer for coverage of the 2017 Wine Country fires. Julie has covered murderous pot deals, police corruption and marijuana's rocky path from a black-market trade to a legitimate industry.

Kellie Hwang is the engagement reporter for The San Francisco Chronicle. Before returning to the Bay Area, she held roles as transportation reporter and trending news reporter at the IndyStar in Indianapolis. Previously, Kellie covered dining news and trends, visual arts, events and nightlife for the Arizona Republic, and freelanced for the former Contra Costa Times. Kellie also serves as co-director of the Asian American Journalists Association Features Forum. She is a University of Washington graduate.

Mallory Moench is a San Francisco City Hall reporter. She joined The San Francisco Chronicle in 2019 to report on business and has also written about wildfires, transportation and the coronavirus pandemic.

She previously covered immigration and local news for the Albany Times Union and the Alabama state legislature for the Associated Press. Before that, she freelanced with a focus on the Yemeni diaspora while studying at the City University of New York Graduate School of Journalism.