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Record-setting, long-duration heat wave is roasting Western U.S.

Los Angeles and Sacramento are included in excessive heat warnings through the Labor Day weekend

Forecast highs on Labor Day from the National Weather Service. (PivotalWeather)

An intense, long-duration heat event has begun in the Western United States that may become one of the most extreme September heat waves ever observed in the region. It is already breaking records, prompting appeals for energy conservation, fueling fires and leading to heat-related illnesses.

On Wednesday, record highs for Aug. 31 were set from California to Montana. In Southern California, Burbank and Anaheim established records for the entire month of August as the mercury soared to 112 and 106. That followed record highs of 90 and 100 degrees in Seattle and Portland on Tuesday.

Wednesday’s excessive heat prompted California ISO, the state’s grid operator, to issue a “Flex Alert” calling for voluntary electricity conservation; the alert was then extended into Thursday.

“Additional Flex Alerts could be issued through the Labor Day weekend as triple-digit temperatures are forecast across much of California and the West,” the operator cautioned.

California’s Central Valley is predicted to be hit particularly hard in the coming days, with highs soaring to 105 to 115 degrees. Temperatures in Sacramento, which just saw its hottest August on record, could hit 110 degrees on Labor Day.

Temperatures this high “increase the chance of heat-related illness for the general population, especially those who are sensitive to the heat, have outdoor plans, or do not have air conditioned homes,” wrote the National Weather Service office in Sacramento.

The heat wave, expected to last about a week, is forecast to peak between Sunday and Tuesday as it becomes entrenched and spreads over more territory.

Few areas from California to the Northern Plains will be spared, except right along the coast, where ocean-cooled air will moderate the heat.

Temperatures 10 to 15 degrees above normal — more typical of mid-July than early September — will be widespread. Some areas will see temperatures more than 20 degrees above normal between Sunday and Tuesday, elevating the heat into historic territory.

“There is high confidence in extremely hot inland temperatures that may break monthly records,” Daniel Swain, a climate scientist at the University of California at Los Angeles, wrote on his blog WeatherWest.com. “There is lower confidence in how close to the coast the heat may ultimately reach, but there’s a good chance that near-coastal areas will get quite hot and even a chance that the beaches will as well.”

Let us know: How has your hometown's summer weather changed?

As the heat wave builds, about 50 million Americans are already under excessive heat watches and warnings and heat advisories from Arizona to Idaho.

Los Angeles, San Diego and Sacramento are all included in excessive heat warnings through the Labor Day weekend, with temperatures forecast to spike to hazardous levels on one of the most popular days for outdoor recreation.

In Death Valley, Calif., famous for holding the world record for highest temperature, the mercury could flirt with 126 degrees this weekend, which would match the highest ever observed globally during September. It set an Aug. 31 record high of 123 on Wednesday.

The Weather Service warned that “extreme heat will significantly increase the potential for heat related illnesses,” describing the conditions as “dangerously hot” and offering “little overnight relief.”

“The prolonged nature of this heatwave, as well as limited overnight relief (especially in urban areas and topographical thermal belts) will be a major challenge,” Swain wrote.

Forecasters also fear that the heat will further desiccate the drought-stricken landscape, elevating the risk of fires. Predictive Services of the National Interagency Fire Center has warned of a “high risk” for fires across Northern California by the weekend; much of the rest of the Western United States is under a moderate risk.

On Wednesday, the Route Fire erupted in Los Angeles County, burning over 5,100 acres among multiple blazes in the region. Six firefighters were hospitalized due to heat-related illnesses battling the blaze, according to the Los Angeles Times.

‘Very dangerous’ heat may reignite fire season in Western U.S.

The heat is the result of a sprawling and unusually intense heat dome, or ridge of high pressure, that is becoming established over the West and is forecast to linger. In addition to bringing hot, dry, sinking air, the high pressure acts as a force field, diverting any inclement weather systems to the north and allowing copious sunshine to pour down unimpeded onto the already parched soil, which in turn radiates more heat.

Expected temperatures

Close to 38 million people, the vast majority of them in California and Arizona, are predicted to experience highs hitting the century mark in the coming week. Some of the hottest weather will settle into California’s densely populated San Joaquin Valley, where highs could approach 115 degrees.

Between Thursday and Wednesday, the Weather Service predicts more than 100 record highs will be threatened or broken in the West.

Here’s a look at forecast highs over the coming days:

Phoenix: 106 degrees Thursday, 105 Friday, 108 Saturday, 109 Sunday, 108 Labor Day and 107 Tuesday.

Las Vegas: 110 degrees Thursday, 109 Friday, 110 Saturday, 110 Sunday, 108 Labor Day and 108 Tuesday.

Los Angeles: 98 degrees Thursday, 92 Friday, 101 Saturday, 103 Sunday, 100 Labor Day and 93 Tuesday.

Sacramento: 105 degrees Thursday, 102 Friday, 104 Saturday, 107 Sunday, 110 Labor Day and 111 Tuesday.

Redding, Calif.: 108 Thursday, 106 degrees Friday, 105 Saturday, 107 Sunday, 110 Labor Day and 110 Tuesday.

In San Francisco, temperatures near the coast may be more tolerable — mostly in the 80s — during this heat wave. However, the Weather Service has still placed coastal areas under an excessive heat watch from Saturday to Tuesday.

“The forecast is a bit trickier along the coast where the marine influence may battle the hotter inland temperatures at times,” the Weather Service office serving the Bay Area wrote.

But just inland, there is “high confidence” temperatures will quickly escalate to dangerous levels, the Weather Service wrote.

While the core of the heat is expected in interior California, the Intermountain West and Northern Plains will also be unusually warm.

Denver is forecast to see highs at or above 90 degrees on each of the first eight days of September. “That’s never happened in Denver’s 150-year climatological history,” tweeted Chris Bianchi, a broadcast meteorologist for Denver television affiliate 9 News.

Record-challenging temperatures are also forecast around Salt Lake City, which has already seen a record number of days at or above 100 degrees this year. It set an Aug. 31 record high of 102 on Wednesday.

Death Valley record

The most extreme heat will affect the most heat-prone location in the world: Death Valley, Calif. The National Weather Service is predicting highs of 124 or 125 degrees between Thursday and Tuesday. The September record in Death Valley is 123 degrees, so the area has multiple chances of breaking that. There’s an outside chance it climbs as high as 126, which would tie the world record for September.

Death Valley National Park still has many roadways closed from severe flooding after 1.46 inches of rain fell in just a few hours on Aug. 5. That proved the second-wettest day on record, and the rapidity of the rainfall qualified it as a 1,000-year rain event there. A number of roads were washed out and vehicles and structures damaged. While California Highway 190 has reopened, several other highways remain closed.

Climate connection

While the heat in the Pacific Northwest will be more short-lived, temperatures will remain dangerously hot in California, the Southwest and Intermountain West until at least the middle of next week. Significant relief should finally arrive by next Friday and into next weekend.

Human-caused climate change is known to intensify excessive heat events and make them more frequent and prolonged.

More on heat waves

Our warming climate: It’s not just you ⁠— summers in the U.S. are getting hotter, and experts say heat waves will likely become even more frequent and intense. Take a look at what extreme heat does to the human body.

How to stay safe: It’s better to prepare for extreme heat before you’re in it. Here’s our guide to bracing for a heat wave, tips for staying cool even if you don’t have air conditioning, and what to know about animal safety during extreme heat. Traveling during a heat wave isn’t ideal, but here’s what to do if you are.

Understanding the science: Sprawling zones of high pressure called heat domes fuel heat waves. Here’s how they work. You can also read more about the link between weather disasters and climate change, and how leaders in the U.S. and Europe are responding to heat.

Tell the Post: What questions do you have about extreme heat, wildfires, droughts or other climate-related topics?

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