The West is facing a new and serious fire threat as temperatures heat up again and moisture from the Southwest monsoon surges northward, triggering thunderstorms that unleash lightning but little rain. The dry lightning presents a heightened risk of new wildfires igniting between Sunday and Tuesday across a broad swath of the West, stretching from the California coast into northern Montana.

The fire threat is emerging as a heat wave builds over the northern Rockies and southern Canada, where scores of blazes are already active. The tinderbox conditions from the hot and dry weather, combined with possible dry lightning strikes, could result in a dangerous proliferation in fire activity.

This is a “multiday, multi-region classic monsoon-burst ignition event,” said Brent Wachter, a fire meteorologist in Redding, Calif., with the Predictive Services arm of the National Interagency Fire Center (NIFC).

Already, 70 large blazes are burning across a dozen states, according to the NIFC. On Thursday, it raised the national preparedness level to 5, the highest, based on the amount of wildfire activity and need for firefighting resources.

Recent precipitation related to the Southwest monsoon has eased fire conditions in Arizona, New Mexico and in southern Utah and Nevada, but the fire risk remains high in California, the Pacific Northwest and the Northern Rockies.

While monsoonal thunderstorms are common in summer in the West, especially in the mountains, the level of flammability across the landscape right now is far above normal and breaking records in many areas. Relentless heat, on top of severe drought, has made fires much more likely to ignite and spread rapidly.

Lightning on Sunday through Tuesday will overlap with areas that have seen repeated, record-breaking heat waves in June and July.

The Northern Rockies is in the bull’s eye of this weekend’s heat wave, with excessive-heat warnings and heat advisories in effect for the eastern two-thirds of Montana and the southern half of Idaho, where temperatures may be as many as 20 to 25 degrees above average. It could see a significant lightning event over extremely dry vegetation. The threat may be highest on Monday, when, the National Weather Service says, the fire danger is “critical” and fire weather watches are in effect.

Coleen Haskell, a Predictive Services fire meteorologist in Missoula, Mont., said the pattern will feature “high-based” thunderstorms, with cloud bases 8,000 to 10,000 feet above ground level. This setup usually means any rain produced will evaporate before it reaches the ground, resulting in strong outflow winds.

“During active southwest monsoon years, we typically get high-based thunderstorms with a mix of wet and dry storms,” Haskell wrote in an email. “Some of those ignitions can hold over several days after the lightning strikes and then fires become more active during the next increase in wind.”

Last week, lightning brought more than 100 new fire starts to the area, most of which remained small, though some grew larger when wind aligned with dry vegetation, she wrote.

An “extremely large area of varying density smoke” has spread across much of the northern United States and southern Canada as a result of these fires and those in the Pacific Northwest and Canada, according to the National Oceanic and Atmospheric Administration.

‘Impactful’ event in California

Meanwhile, red flag warnings and fire weather watches, for high fire danger, are in effect for much of central and northern California through Monday.

The thunderstorm forecast has drawn comparisons to last summer’s lightning siege in Central and Northern California, which began Aug. 16 and resulted in more than 1.5 million acres burned by the end of that month.

While Wachter doesn’t expect a repeat of August 2020 this week, he does expect the event to be problematic.

“That one was epic — this one is going to be impactful,” he said. “There are going to be ignitions, and some of them could become large.”

Statewide, vegetation is actually drier than it was during last year’s lightning siege and breaking records for July, except along the immediate coastline, where flammability has been moderated by fog and cooler temperatures.

Lightning fires may not become apparent immediately, so the magnitude of the event may take days or even weeks to be fully realized.

The Tamarack Fire, south of Lake Tahoe near the California-Nevada border, was ignited by lightning on July 4. It remained under 50 acres until it exploded in size during windy conditions on Friday, growing to 21,000 acres in less than two days and forcing the evacuation of the nearby town of Markleeville.

Erratic winds could also lead to rapid fire spread soon after ignition.

“The potential for large fire will be high over Central California west of the Sierra Sunday through Monday morning due to the expected thunderstorm activity over extremely dry fuels,” Predictive Services meteorologists in Southern California wrote in a forecast outlook. “Downdraft winds from the thunderstorms may cause rapid rates of spread and long-range spotting on lightning sparked ignitions.”

Jason Samenow contributed to this report.