Wildfires are raging across parts of the desert Southwest and California, where scant rainfall, sweltering temperatures and wind are combining to create ideal conditions for rapid fire spread.

A spate of fires broke out over the weekend between Phoenix and Tucson, while other brush fires were burning near San Luis Obispo in California on Monday, prompting evacuations. Relative humidities have plummeted to as low as 5 percent in parts of Arizona, Nevada, California and other locations in the Southwest, with winds gusting above 40 mph fanning the flames.

Evacuations were underway near the Bush Fire northeast of Phoenix, which had already torched nearly 64,513 acres in the Tonto National Forest of Arizona as of Tuesday morning and was 0 percent contained. The U.S. Forest Service in Phoenix urged the 1,500 residents of Tonto Basin and Punkin Center to evacuate immediately.

“Danger in these areas is imminent and life threatening,” they wrote. “If you choose to ignore this advisement, you must understand emergency services may not be able to assist you further.”

The fire remained at zero percent containment on Tuesday morning.

A flare-up of wildfires across the Southwest

Meanwhile, additional fires continued to burn east of Phoenix, where the Sawtooth Fire was nearing 25,000 acres at 81 percent containment. Some 225 personnel were originally called out to fight the fire sparked by lightning — though it did not threaten any populated areas, and no evacuations were necessary.

The North Rim of the Grand Canyon has been closed since the weekend due to a 30,000-acre inferno dubbed the Mangum Fire. More than 500 firefighters continue to combat the blaze, which was only 3 percent contained late Tuesday morning.

Farther south, evacuations were ordered ahead of the Bighorn Fire burning northeast of Tucson, within view of the city. The blaze, which began with a lightning strike on June 5, was burning along the western reaches of the Santa Catalina Mountains just east of Highway 77. Dry and windy conditions have moved the fire closer to populated areas.

Nearly 15,000 acres had burned as of Monday evening, with the fire about a third contained. Catalina State Park, and a number of popular hiking trails — including Romero Canyon, Pima Canyon, and Finger Rock — have been closed to the public.

How a sluggish monsoon and ‘dry lightning’ contribute to Arizona’s ongoing fire season

June is peak fire season in greater Phoenix and central Arizona. The month only averages 0.02 inches of precipitation — barely enough to measure — and afternoon highs top out around 104 degrees on average. The pendulum swings rapidly in July thanks to the Southwest monsoon; with a mean rainfall total of 1.05 inches, July is historically Phoenix’s wettest month — coming on the heels of the city’s driest.

But that means the fires will likely rage on until the Southwest monsoon actually kicks in.

“We don’t really start to see the moisture or storms forming here until mid-July,” said Andrew Deemer, a meteorologist at the National Weather Service office in Phoenix. “Usually we’ll start to see storms forming over the mountains.”

Unfortunately, those initial storms tend to do more harm than good, since they often produce dry lightning, which is lightning accompanied by little to no rainfall at the surface. That in turn sparks fires, but without the quenching deluges to quell them.

“It’s very apparent and very concerning during the initial part of the monsoon,” Demmer said. “Folks in other parts of the country are used to rain with their storms. But here, maybe a little bit of rain or even no rain with thunderstorms … if the rain’s not falling on top of [where the lightning strikes], you can get fires really easily.”

To make matters even worse, the same ingredients that favor dry thunderstorms — namely a layer of increasingly hot, dry air near the ground — can also result in dry microbursts and erratic, gusty winds. That frequently exacerbates ongoing fires and can also lead to unpredictable fire behavior, posing an enormous challenge to firefighters.

Widespread relief will only come once the monsoon begins to take hold. And until then, it’s a waiting game.

“Is it fire season until then? Unfortunately, kind of,” explained Deemer. “We need moisture to really help with those fuels … they’re so sensitive to the extreme heat we have. They just need a spark. And a lot of times, the best way to fight those is to get the moisture in. Usually, June all the way through mid July is peak Arizona fire season.”

An early start to a long season in California

A number of smaller wildfires were also threatening communities in California, including the Avila Fire near San Luis Obispo. The 400-acre fire was 10 percent contained Tuesday morning, but evacuation orders were canceled. Highway 101 was reopened late Monday.

East-southeast of Sacramento, the Grant Fire continued to burn but was 92 percent contained. A little over 5,000 acres had been consumed by the flames.

Much of California’s Central Valley, the Great Basin of Nevada, northern Arizona and the Four Corners region are under red flag warnings on Tuesday — denoting conditions that would immediately favor the spread of wildfires.

The National Weather Service Storm Prediction Center also outlined an area of critical fire weather conditions from northern Arizona through Utah and into extreme northwest Colorado. Relative humidities of 5 to 20 percent were expected — further drying out any remaining moisture from vegetation. Meanwhile, a low-level corridor of high winds, known as a low-level jet stream, passing overhead late Tuesday will foster an uptick in surface winds in many of these areas.

Some improvement is forecast Wednesday, but it will take a larger-scale pattern shift to make a noticeable difference in ongoing fires across the Southwest.

And in California, fire season is just beginning — with a long summer ahead. Drought is already firmly established across the northern half of the Golden State, with the Climate Prediction Center calling for anomalously hot conditions and near to below-average rainfall over the next three months.

The 2020 fire season is taking place amid the coronavirus pandemic, complicating evacuation orders and preparation decisions.

Cases of covid-19 are on the rise in several states in the West, including Arizona, New Mexico, Oregon and California. These states, along with Utah, also recorded their highest incidence of new cases last week since the start of the pandemic, a spike that likely was not due simply to increases in testing, according to a Washington Post analysis.

Diana Leonard contributed to this report.