Where did the Phoenix heat wave come from and when will it end?

• A historic heat wave is bringing record-breaking triple-digit temperatures to Phoenix through the weekend.
• Experts say that while no single event can be attributed to climate change, the increasing frequency of extreme weather is a clear trend.
• The heat wave is causing an early snowmelt, which threatens the region’s water supply and could lead to an earlier fire season.

Call it spring hellfire, the Heat Miser’s meddling or the harbinger of summer — a historic heat wave is descending on Arizona, bringing three-digit temperatures to Phoenix. But this week’s record-breaking heat is not the stuff of myth or legend, experts warn: It’s on trend for a warming planet.

On March 18, in the hangover glow of Saint Patrick’s Day, temperatures are expected to reach 102 degrees — the earliest-ever triple-digit day in the city’s recorded history. It will supplant the previous earliest 100-degree-day of March 26, 1988. Temperatures will continue to climb through the week, with each day forecast to set its own record. March 20 and 21 will likely tie at 106 degrees.

The heat is the result of a high-pressure system spinning across the West, causing “an expansive dome of unusually hot temperatures,” NOAA’s National Weather Service Weather Prediction Center said in a Facebook post on March 16. 

“ As we go from winter into spring and into summer, we get low-pressure systems, which can give us storms and cooler weather, and then we can get high-pressure systems, which give us dry conditions and hotter temperatures,” said Erinanne Saffell, Arizona’s state climatologist. 

But even with the weather system, the temperatures are unprecedented. 

“We don’t usually see high-pressure systems this strong in the middle of March,” said Katherine Berislavich, a meteorologist at the weather service’s Phoenix office. The city tends to hover around the mid- to upper-70s this time of year, making this week’s temperatures 20-25 degrees above normal. 

The heat spell comes on the tail end of Phoenix’s warmest winter ever — February signed off with a stretch of its own record-setting 90-degree days. Over the past year, Arizona and the Southwest have seen above-average temperatures every month, Saffell said. A warmer year has put environmental experts on high alert. As spring turns into summer, they are keeping their eyes on the upcoming fire season and the state’s imperiled water supply. 

Are we ready? Arizona in for heat shock as temps about to top 100 earlier than ever

Can we link the heat to climate change?

This meteorological spring, which began March 1, has so far seen more high-pressure ridges and fewer or less-deep low-pressure troughs to cool us off, Berislavich said. While the intensity and timing of this week’s heat wave might be unusual, she cautioned that no singular weather event can be attributed to climate change. 

“But seeing these extreme weather events increase in frequency over time, yes, we can make the case that climate change has to do with this overall shift,” Gabriel Lojero, a meteorologist at the Phoenix NWS office, told The Arizona Republic earlier this week. 

Since 1970, the average spring temperature has increased by 3.9 degrees in Maricopa County, according to Climate Central, a news and research non-profit. That might not sound like a lot, said Shel Winkley, a Climate Central meteorologist. But he likened such temperature shifts to a fever. 

“When you’re talking about this long range, it’s kind of like if your body temperature rose three degrees — you would feel unwell,” he said. 

While we can’t know that climate change is causing a specific extreme weather event, researchers are increasingly able to study how climate change can make such events more severe or more likely to occur. It’s called attribution science, and it examines the links between global warming and human interference. 

Climate Central tracks the number of days where temperature has likely been affected by climate change using its Climate Shift Index. As of March 10 this year that number is 38 in Phoenix, but it will soon go up. The index shows that the heat wave rolling across the West right now is made at least five times more likely by climate change. 

Globally, temperatures are increasing. The Copernicus Climate Change Service announced in January that 2025 was the third-hottest year on record — it also had the highest carbon dioxide concentration in the atmosphere than any year in the last 14 million, according to scientists from the National Oceanic and Atmospheric Administration’s Mauna Loa Observatory. 

 ”We have gained a greater ability to more rapidly attribute certain events and developments to changing climate,” said Max Boykoff, a professor of environmental studies at the University of Colorado Boulder and lead project investigator for its Media and Climate Change Observatory where a recent study showed that in 2025 media coverage of climate change went down by 14% —  and from 2021, by 38%. 

There are a few hypotheses Boykoff shared for the decline in coverage: mass media consolidations laying off climate journalists, increasing external pressure, crowding out of climate stories by “flooding the zone” with other news, and climate fatigue — readers being less inclined toward stories about global warming that they’ve heard over and over again. 

“ Even in the face of increasing CO2 concentrations in the atmosphere and temperature records being broken globally and heat waves from climate change coming through, as coverage is diminishing, it runs the risk of being perceived as a diminished set of challenges, as diminished priorities,” Boycoff said. 

Rising temperatures: How climate change and La Niña turned up the winter heat in Phoenix

What does the heat wave mean for the environment?

The West’s warmer-than-average winter means that snowpack at its highest elevations, in places like Arizona, Colorado and Utah, is suffering. Experts are calling it a snow drought. 

“That snow pack is what we count on for the water supply,” Saffell said. Usually, it melts slowly through March and April, filling rivers, streams and aquifers. But a heat spell like this week’s could thaw what little snow is still clinging to mountain peaks. 

“We have had snowpack as late as April, but with it being this hot right now, we’re not expecting to have any of that snow come back anytime soon,” she said. 

Decreased snowpack this winter, the worst since 1981 in Colorado and Utah, could lead to critical declines at reservoirs that depend on the Colorado River. Already, the river is facing the lowest levels of runoff in a quarter century. Inflow at Lake Powell is expected to reach just 36% of the 1991-2020 average. 

As the season heats up, warmer and windier conditions mean that Arizona’s fire season could start earlier, too, Winkley said. 

“The moment we don’t have any snow left, that’s when fire season starts,” said Noah Baker, the Flagstaff Fire Department’s wildfire, climate, and community health specialist.

 “I have never had to prepare for fire season in the way that I typically do before mid-April,” he said. This year he started those preparations on March 10. 

Staying cool — and safe

Triple-digit temperatures can be life threatening. 

“Staying inside, in the AC, is the best,” Saffell said. “It takes a while for your body to get used to hotter temperatures, and we’re going to be getting hotter temperatures quickly.”

Those without access to air conditioning, with chronic health conditions and the elderly are especially vulnerable to the heat. Physicians at Valleywise Health Medical Center urge residents to check on friends, family and neighbors who might be at risk. 

“One of our concerns with early spikes in outdoor temperatures is that our out-of-town visitors are not acclimatized to the heat,” said Dr. Paul Pugsley, an emergency medicine physician at Valleywise, in a recent news release. “Their bodies aren’t physiologically ready to ramp up and respond, and they might not recognize the signs of early heat illness.”

Symptoms of heat exhaustion include headache, nausea, dizziness, thirst, sweating and an elevated body temperature. To stay cool, experts suggest keeping water on hand, limiting alcohol and drinks with high levels of caffeine or sugar and wearing light colored clothing. 

Saffell advised that the best time to be outside exercising during a heat wave is around sunrise. 

“I think it’s helpful for folks to understand that the temperatures are still going to be hot after sunset,” she said. “We’re going to get our hotter temperatures around three or four in the afternoon, and then we’ll have sunset but it’ll still be warm.”

To read more about the heat: Sign up for AZ Climate, The Republic’s weekly environment newsletter.

Looking at the Climate Prediction Center’s forecast, she said the rest of March looked warm and dry for the most part. But will the intense heat persist?

“We’ll just have to wait and see.”

Sarah Henry covers environmental issues for The Arizona Republic and azcentral. Send tips or questions to sarah.henry@arizonarepublic.com.

Environmental coverage on azcentral.com and in The Arizona Republic is supported by a grant from the Nina Mason Pulliam Charitable Trust.

Follow The Republic environmental reporting team at environment.azcentral.com and @azcenvironment on Facebook and Instagram.