A Once-in-a-4,433-Year Heat Wave Is Hitting the Western U.S.

It is difficult to describe how bonkers this week’s heat wave is in the southwestern United States. Alan Gerard, a typically even-keeled meteorologist with 35 years of forecasting experience under his belt, attempted to do so in a recent edition of his newsletter, Balanced Weather. He settled on: “jaw-dropping,” “insane,” “truly historic,” “literally flabbergasting,” “incredible,” and “anomalous [even for] the middle of summer.”

Jeff Berardelli, the chief meteorologist at WFLA, the NBC affiliate in Tampa, tried to contextualize it on social media, noting that based on historical patterns, Phoenix could expect a March day as hot as it was on Thursday — 105 degrees Fahrenheit — only once every 4,433 years.

Phoenix is just one part of the story. The heat wave — which began ramping up on Tuesday, peaks Friday, and won’t subside until early next week — has set or tied March record highs in at least 480 locations so far, stretching from New Mexico to Southern Oregon. California has already broken the record for the hottest winter day ever recorded anywhere in the U.S.: 109 degrees on Thursday at a station in the eastern Coachella Valley. “The extent and magnitude of this particular heat wave is without comparison to anything that we’ve seen in March,” John Abatzoglou, a professor of Climatology at the University of California, Merced, who specializes in climate impacts in the West, told me.

That’s partially because this heat wave would be “virtually impossible for the time of year in a world without human-induced climate change,” per a report released Friday by scientists from World Weather Attribution. Heat waves have one of the clearest climate signals of any extreme weather event because a hotter planet means a hotter baseline. “Across almost the entire western U.S., temperatures [this week] were made at least five times more likely due to climate change,” Zachary Labe, a climate scientist at Climate Central, which maps the effects on daily temperatures, told me.

The March 2026 heat wave is likely to become a reference point in the same vein as the 2021 heat dome in the Pacific Northwest, subject to study, research, and scrutiny by climatologists, public health experts, hydrologists, and emergency managers in the months and years to come. The consequences of the current heat wave will also outlast the record temperatures. When it is this hot — and, more importantly, when it is this hot this soon — the effects compound, touching everything from hydropower capacity to the coming wildfire season.

To make matters worse, “this week is exacerbating conditions that were already bad,” Labe said.

Let’s take a look.

Hydropower

About half of the total utility-scale hydroelectricity in the U.S. is generated in the three West Coast states, but it is part of the energy mix in almost every state experiencing the heat wave this week, including also Arizona, Colorado, Idaho, Montana, Nevada, New Mexico, Oregon, Utah, and Wyoming. In these states, high-elevation snowpack acts as a kind of battery; the slow release of melting winter snow from the mountains over the spring and summer helps keep generation reliable. In the case of a major heat wave, however, the water can run off too fast or evaporate, limiting supply in the summer-peaking months.

Though reservoirs in California are mostly in good shape at this point, with the rivers flowing high this early, there will be less water available when the squeeze comes in July and August. “This year, at least for precipitation, it’s been really quite good [in California],” Abatzoglou, the UC Merced professor, told me. The issue is, it’s also “just been way too damn warm.”

Every major river basin in the western U.S. experienced its first- or second-warmest winter this year, setting a grim stage for the high temperatures that have settled over the region this week. What little snowpack there already was — 97% of the snowpack-monitoring weather stations in Colorado are in snow drought — is now being hammered by the kind of heat the region doesn’t often experience before late spring or early summer. Daniel Swain, a climate scientist with the University of California Agriculture and Natural Resources, has warned that, as a result, we might see “June snowpack levels by April 1” in some parts of the West.

Of particular concern is what this will mean for Lake Powell, the reservoir created by the Glen Canyon Dam on the Colorado River, which supplies electricity to more than 5 million customers across seven states. The reservoir is a mere 40 feet away from the minimum volume required to turn the turbines in the dam, Bloomberg reports.

The early-season heat-driven runoff will further diminish the river’s already reduced flow in the coming months. Snow accumulation in the basin above Lake Powell was only at 67% of the 30-year median for March. Earlier this week, the U.S. Colorado Basin River Forecast Center downgraded its two-week-old forecast for inflows to Lake Powell during the critical April through July period from 2.3 million acre-feet of water to less than 1.8 million acre-feet. That would represent just 27% of the river’s 30-year historical average inflow; to understand how much the unseasonable heat has affected that, projections were more than 3.6 million acre-feet of water at the start of the year.

Though it is the bottom left corner of the country that has drawn the most attention for its triple-digit temperatures this week, records are also toppling in southeastern Oregon and southwestern Idaho, where highs are 20 to 25 degrees Fahrenheit above normal. Kurt Miller, the executive director of the Northwest Public Power Association, a nonprofit that represents public utilities in the region, told me his modeling shows that two consecutive 80-degree days in Boise are enough to trigger runoff starting “in earnest” in the Pacific Northwest, where dams meet about 60% of the region’s electricity demand.

The good news is that the high temperatures in Idaho are forecast to be “peaky” rather than prolonged, as they will be in the southwest, meaning the Pacific Northwest isn’t likely to string together the series of hot days necessary to trigger a catastrophic melt-off scenario. Additionally, while snowpack in the Northwest has been dismal this year, hydropower in the region is largely determined by upstream conditions in British Columbia and Montana, which have been closer to seasonal norms and, more importantly, are out of range of this week’s heat event.

Wildfires

There is another obvious downside to the early snowmelt in the West: Fire season will likely start sooner. “This is basically hitting fast-forward,” Abatzoglou, the University of California, Merced professor, told me. “It’s pushing us much faster toward the crispy season.”

Especially given the already historically low snowpack (in the northern Sierras, snow is at just 38% of its normal levels), the heat current wave could move up the start of fire season by weeks as high elevations melt out and soil and vegetation begin to dry. Usually, such conditions aren’t seen before the late spring or early summer.

While major wildfires have mostly spared high-elevation landscapes in recent years, “I expect that will not be the case this year,” Swain, the scientist at UC ANR, said in a video posted earlier this week. He further predicted that “we’ll see an especially severe and early start to fire season in the four corners — Arizona, New Mexico, Utah, and Colorado,” as well as a potential “severe peak” later in the forests of Northern California, Oregon, and the Rockies. (If there’s a saving grace, though, it’s that the lack of precipitation in the West has also curbed the fuel loads by keeping vegetation growth to a minimum.)

Adding to the alarm is the fact that “this year’s snow accumulation pattern most closely resembles 2015, followed by 2005,” as the National Interagency Fire Center wrote at the start of the month. Both were historically bad fire seasons: In 2005, a then-record 8.7 million acres burned, and in 2015, the U.S. broke more than 10 million acres burned for the first time.

Some research also indicates that longer fire seasons can lead to more severe wildfires, which, in addition to posing greater risks to people and property, take a deeper toll on state and federal firefighting resources and personnel. If the season starts sooner, wildland firefighters are more likely to be exhausted by the time the severe fire days of “dirty August” and “Snaptember” finally come around.

Still, any estimates of the direct impact of this week’s heat wave on the upcoming fire season should come with a hefty margin for error. Wildfires are influenced by a number of factors, both climate-related and not, ranging from historic forest management practices to the timing of the “green up” of local fuel loads to, yes, when and where the snow melts off. But an early dry fuel bed also means prescribed burning efforts can begin sooner, the NIFC notes in its March forecast (although that said, the dry fuel bed may also eventually “curtail burning late spring into early summer if timely moisture intrusions do not materialize”).

Other bad things

It could still take months for the immediate-term impacts of this week’s heat wave to come into focus. Excess mortality takes weeks to calculate and sometimes years to pin down precisely; researchers didn’t conclude their formal study of the 159 deaths resulting from the 2021 heat wave in Washington state until 2023.

What we do know is that early-season severe heat is especially dangerous for human health because people aren’t yet acclimatized to it. In fact, between 50% and 70% of outdoor heat-related fatalities occur in the first few days of a heat wave, according to the Occupational Safety and Health Administration. That’s because it can take between four days and two weeks to adapt physiologically to handling heat stress, including the slow process of building up adequate blood plasma to cope with the increased cardiovascular and cooling demands on the body.

And lest we forget, this heat has arrived staggeringly early, in some places breaking the daily temperature record by as much as 11 degrees. It took until August last year for Los Angeles to experience a similarly “strong” heat wave, senior AccuWeather meteorologist Heather Zehr said in a press release.

Drownings increase during heat waves as people seek out water to cool off, but there is a reason to be especially concerned about water this week. That’s because many people will head to rivers, and that water will likely be cold and high due to rapid snowmelt in the mountains, Abatzoglou told me. In addition to its extreme heat warnings this week, the National Weather Service has also been posting PSAs reminding Americans that “cold water can kill.”

There could be impacts on agriculture, too. It is fruit- and nut-blossom season in California’s Central Valley, which produces about three-quarters of those products consumed in the United States. It’s the Valley’s Mediterranean-esque climate, in part, that makes the region so productive; this time of year, daily highs are more typically in the mid-60s, perfect for the trees. Now, however, they’re cresting 90 degrees, threatening the Valley’s $21 billion in annual exports and more than 200,000 agricultural jobs. Further south near San Diego, there are also mounting concerns for the avocado industry, as the plants have particularly heat-sensitive blooms.

Though the heat wave is expected to begin breaking next week, a definitive end to the dry, unseasonably warm troubles in the West is nowhere in sight. April is the crucial transition month in the West, when states can sometimes stabilize after winters with poor snowpack or low precipitation through late-season storms. But “unfortunately, taking a look at some of the longer-range outlooks, it’s more likely than not there will be warmer than normal temperatures continuing across the West into April,” Labe said. “So just all around bad news.”