Heat wave in Western U.S. could kick off sweltering summer

A jet and a city skyline are silhouetted by the sun.

A significant early-season heat wave headed for western North America is threatening to deliver stifling temperatures that could break records, prime the landscape for wildfires and kick off a sizzling summer.

A powerful high-pressure ridge, or heat dome, will bring unusually hot temperatures to the Golden State by the middle of this week before spreading into the Pacific Northwest and Southwestern Canada, according to Daniel Swain, a climate scientist with UCLA. Temperatures could remain well above normal across much of the region for as long as 10 to 14 days.

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The hottest parts of California won’t be the inland desert regions that typically experience high temperatures, but rather portions of Northern California and the Sierra Nevada foothills, Swain said during a briefing Friday. The Sacramento Valley could see widespread triple-digit temperatures, including more than 110 degrees in the central and northern valley by Wednesday and Thursday — as much as 20 degrees above normal.

“This will be another event where a fair chunk of California’s population — the near coastal dwellers in the Bay Area and in Southern California — probably won’t see extreme heat,” Swain said. “But you might not have to go that far inland to see dramatically hotter temperatures, and if you go far enough inland, you might even see record-breaking heat.”

Far northern California is likely to see its first 100-degree day of the year by Wednesday, according to the National Weather Service. The Central Valley is under an excessive heat watch, with a chance of soaring to 110 degrees in the San Joaquin Valley.

The event bears some similarity to a record-shattering heat dome that smothered the Pacific Northwest for 27 days in June and July 2021, though it will be of a lesser magnitude, Swain said. That event spawned numerous studies and research papers, and was associated with hundreds of deaths, mass die-offs of marine life, crop loss and infrastructure damage.

The 2021 heat dome also intensified the 2021 fire season — a pattern that could repeat this year.

Though California has enjoyed two relatively tame fire seasons thanks to back-to-back wet winters, it has seen an explosion of new vegetation that can be cured by the incoming heat, creating fuel for future flames. The state has already seen several small fires, including a 1,300 acre blaze in Santa Barbara County.

“There’s no real indication of extreme fire risk with this pattern, but it will set the stage for faster drying of vegetation in June following a couple of wet winters, and could potentially accelerate wildfire conditions later in the season,” Swain said.

The incoming heat could also be the death knell for the remainder of the state’s snowpack, which has dwindled to about 44% of normal for this time of year after peaking in April. The melt won’t trigger flooding, but could cause rivers to run high and cold — a potential hazard for swimmers seeking relief from high temperatures, according to the weather service.

While the heat wave’s worst effects may dissipate by mid-June, it could also mark the start of a long, hot summer, forecasters said.

The latest seasonal temperature outlook from the National Oceanic and Atmospheric Administration indicates June, July and August will bring above-normal temperatures to nearly all of the United States, with the highest likelihood of hotter-than-usual conditions in the four corners region of Arizona, New Mexico, Colorado and Utah.

A seasonal outlook indicates a high likelihood of warmer-than-normal temperatures in June, July and August.

In California, odds favor above-normal temperatures across the state, with a stronger likelihood farther inland.

Last year, Arizona made headlines when Phoenix suffered a record 31-day streak of temperatures of 110 degrees or hotter. In Death Valley, the mercury climbed to 128 degrees — just shy of an all-time world record.

Dan Collins, a meteorologist with NOAA’s Climate Prediction Center who authored the latest seasonal outlook, said the simmering summer will be heavily influenced by changes in the El Niño-La Niña cycle, as well as climate change.

“These seasonal outlooks are largely driven by those two factors,” he said.

El Niño, a climate pattern in the tropical Pacific, has been a factor in record-hot temperatures across the world since its arrival last year. April marked the 11th straight month of record-setting global temperatures, and 2023 was the planet’s hottest year on record.

Though El Niño has weakened, it has still influenced boiling tropical ocean temperatures that are driving atmospheric circulation patterns, including the incoming heat wave, Collins said.

But part of the reason the West is expected to see an anomalously hot season is climate change, which has driven an overall warming trend in the region over the last several decades.

“El Niño may have been one contributor to the global temperatures being warmer, but of course, there was a long-term temperature trend as well,” Collins said.

The incoming heat wave also won’t be the first notable heat event this year. Several cities in Florida have broken daily records in recent weeks, including 97 degrees in Tampa and Orlando, and 96 degrees in Miami and West Palm Beach. The heat indexes — measurements that include temperature and humidity — were several degrees higher.

Heat has also stifled portions of the Caribbean, Central America and Mexico, where temperatures in Oaxaca soared to a record 118.4 degrees on May 26. Portions of Mexico are also undergoing extreme drought, with officials warning that Mexico City will soon run out of water.

Extreme heat is one of the deadliest effects of climate change, and the heat in Mexico has already been associated with more than 60 deaths this year. Phoenix’s heat stretch last year killed at least 645 people, while the Pacific Northwest heat wave in 2021 is believed to have contributed to at least 600 deaths — including many in households without air conditioning.

Collins said the incoming heat wave has the potential to not only deliver record daytime temperatures, but record nighttime temperatures as well.

“It’s not really just summer arriving on schedule,” he said. “It’s an extreme event that’s predicted.”

Swain, of UCLA, said there are still some uncertainties about how the heat wave will play out for the Western U.S., in part because it could linger into mid-June. But what is clear is that it’s going to be notably hot for millions of people.

“These are pretty darn hot temperatures,” he said. “These are some big-time numbers.”