Record-threatening, long-duration heat wave to roast Western U.S.

Large parts of the Western United States face a prolonged and extreme heat event that is expected to break scores of records over the next week. California’s Central Valley may be particularly hard hit by this heat wave, which the National Weather Service has warned could threaten monthly records over the holiday weekend. Sacramento could be near 110 degrees on Labor Day.

Abnormally hot weather has already swelled over the Pacific Northwest; Seattle and Portland set record highs of 90 and 100 degrees on Tuesday. This heat will intensify, spread over more territory in the West and become entrenched in the coming days - lasting at least a week.

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Except right along the coast, where cooler ocean water will moderate the heat at times, few areas from California to the Northern Plains will be spared. Temperatures 10 to 15 degrees above normal will be widespread - and more typical of mid-July than early September. Some areas will see temperatures more than 20 degrees above normal by Sunday and Monday, elevating the heat into historic territory - the hottest ever recorded in September.

"[T]here is high confidence in extremely hot inland temperatures that may break monthly records," wrote Daniel Swain, a climate scientist at UCLA, 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."

As the heat wave builds, more than 50 million Americans are already under excessive heat watches and warnings and heat advisories from western Arizona to Washington state.

San Francisco, Los Angeles, San Diego and Sacramento are all included in excessive heat watches and warnings over 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.

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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.

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 languish. 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.

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.

Here's a look at forecasts for the coming days.

Phoenix: 107 degrees Friday, 111 Saturday, 110 Sunday, 107 Labor Day and 107 Tuesday.

Las Vegas: 109 degrees Wednesday, 110 Thursday, 109 Friday, 110 Saturday (would break 108 in 1982), 110 Sunday, 108 Labor Day and 107 Tuesday.

San Diego: 86 degrees Friday, 88 Saturday, 90 Sunday, 91 Labor Day and 87 Tuesday.

Los Angeles downtown: 99 degrees Wednesday, 97 Thursday, 94 Friday, 99 Saturday, 104 Sunday, 101 Labor Day and 97 Tuesday.

San Francisco: 82 degrees Saturday, 82 Sunday, 81 Labor Day and 76 Tuesday.

Sacramento: 105 degrees Thursday, 104 Friday, 109 Saturday (would break 108 in 1955), 111 Sunday (would break 108 in 1988), 113 degrees Labor Day (would break 108 in 1988) and 108 Tuesday.

Redding, Calif.: 106 degrees Friday, 108 Saturday, 110 Sunday, 111 Labor Day and 109 Tuesday.

Medford, Ore.: Between 99 and 102 degrees every day through Monday, 97 Tuesday.

Portland: 92 degrees Thursday, 87 Friday, 83 Saturday, 85 Sunday, 83 Labor Day and 86 Tuesday.

Seattle: 85 degrees Wednesday, 82 Thursday, 85 Friday, 76 Saturday, 78 Sunday, 75 Labor Day and 75 Tuesday.

While the core of the heat is expected near the West Coast, 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 9News.

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.

The most extreme heat will affect the most heat-prone location in the world: Death Valley, Calif. The National Weather Service is predicting a high of 123 degrees on Saturday, 122 on Sunday and 123 on Labor Day. The September record in Death Valley is 123 degrees, so the area has multiple chances of tying or even 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 thousand-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.

While the heat in the Pacific Northwest will be more short-lived, temperatures will remain dangerously hot in California, the Great Basin of Nevada and parts of the Desert Southwest through at least the middle of next week. The heat looks to peak Labor Day or Tuesday, but signs point to another fortification of the heat dome much deeper into next week.

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