Heat, wildfire and now the monsoon: What to know about this triple threat

The early arrival of monsoon activity in central Arizona on Friday could set the outdoor stage for an unsettled weekend, the chances of storms colliding with a persistent heat wave and dangerous wildfire conditions from the low deserts to the high country.

Weather forecasters were predicting a rise in humidity and an increasing chance of thunderstorms through the weekend as moisture from a tropical cyclone streamed northward. The increase in storm chances — 30% to 40% in Phoenix by Saturday and Sunday — will likely push temperatures closer to normal for late June.

The temperature reached 115 degrees at Sky Harbor International Airport on Thursday, 9 degrees above the average for the date, and a record-tying 117 degrees on Friday. The heat is the result of a strong high-pressure system that moved into Arizona.

Amid the heat and the threat of wind and lightning, wildfires of different sizes were burning across the state. The Garner Fire north of Oracle Junction burned about 9 acres before forward progress was stopped, while much larger blazes like the lightning-caused Foote fire near Alpine burned over 2,000 acres in eastern Arizona with 0% containment.

The confluence of heat, humidity and monsoon activity is not altogether abnormal, but experts still encourage caution.

"This isn't a very dangerous setup, but the heat always has its risks," said Ryan Worley, meteorologist for the National Weather Service. "It's not an overly dangerous situation, but it's something people should be aware of and keep an eye on the forecast."

Is Arizona's monsoon early?

Although monsoon season officially began on June 15, monsoon activity typically doesn't ramp up until early July. Phoenix has a 30% to 40% chance of showers and storms on Saturday and Sunday.

"This is really early," said Michael Crimmins, a climatologist at the University of Arizona. "We don't normally see precipitation this early in the season, so that'll catch people a little bit by surprise. It's almost a month earlier than it was last year."

Monsoon season begins when a subtropical ridge shifts north from Mexico during the spring and early summer, bringing hot temperatures and moisture over the southwest. The string of high temperatures likely helped jump-start storm development.

"There really isn't a heat dome over the Southwest," Crimmins said. "Heat dome is a colloquial term that's used more broadly to talk about large high-pressure systems that have high heat associated with it."

A large high-pressure system responsible for heat waves across the eastern U.S. has moved toward the Southwest, bringing the heat with it, and heat is one of the elements that typically jump-starts monsoon activity.

"We're definitely not underneath it, but we're under the influence of it," he said.

To the west, there is a low-pressure system over California that brought cooler temperatures this week. Arizona is between these two big weather features, contributing to increased heat and humidity. A former tropical cyclone, Alberto, produced its own plume of moisture from the Southwest.

Chances for storms vary across the state, but the southeast will likely see some activity that could reach into the high country and northeast portion of the state, according to Crimmins.

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Invasive species make fires burn hotter and faster

The heat also fuels wildfires in Arizona, already aggravated by invasive plant species that took root in the desert environment, creating grasslands that become tinder readied by the hot Arizona sun.

A study published in May cited invasive plants that rapidly overtake the desert in wet winter months, filling in the gaps of the native environment and creating fuel that allows fires to spread farther and burn more intensely.

These plants were mainly grasslike weeds, such as red brome, which produces blades and stems that burn quickly when dry.

The invasive plants, while quick to spread, are not acclimated to the Arizona climate and die in the hot summer months, setting the path for wildfires to spread more easily.

These invasive plant species allowed wildfires to travel down and up mountains, something that typically didn't happen before. They also connected desert environments with woodlands, where fires typically occurred and were isolated, now making nearly all the state ready to burn.

The study used the 2020 Bush Fire and 2021 Telegraph Fire, both of which started in desert lowlands with the help of invasive plants, expanding to forested environments and burning nearly 200,000 acres of land in the Tonto National Forest.

As of Thursday afternoon, 33 wildfires had burned more than 46,000 acres in Arizona since April, according to The Arizona Republic's daily wildfire monitor.

Three wildfires, the Wildcat Fire, Wolf Fire and Spring Fire, were responsible for nearly 30,000 acres burned.

In 2023, more than 1,600 wildfires burned across the state, scorching around 180,000 acres of earth. More than 70% of those fires were human-caused.

Add heat for the monsoon

Thunderstorm chances in Phoenix were not expected to bring enough rain to quash active wildfires but could trigger heavier winds and stronger lightning, which could spread existing fires and spark new ones.

But the onset of the monsoon could be good news for the longer-term wildfire outlook.

"The monsoon is so important for ending the fire season here in Arizona," Crimmins said. "The humidity has had such a dramatic increase across the state, and I think that's really going to help keep fire starts down and help any fires underway right now."

If monsoon conditions continue over the next few weeks, the shift in weather could effectively end fire season. But with any storm activity, there is always a chance of lightning strikes that could cause fire across the desert and forest lands.

Although temperatures may dip slightly over the next few days, Worley says "there's no end in sight" to temperatures over 105 degrees in Phoenix.

Similarly, unsettled weather was forecast for Flagstaff, where storm chances are expected to rise to 70% by Sunday with temperatures in the mid-80s, and Tucson, with high storm probabilities through the weekend and temperatures around 103 to 105.

This article originally appeared on Arizona Republic: Arizona weekend weather: Heat, wildfire, monsoon set to collide