CHICO — Firefighters battling California’s largest wildfire of the year are preparing for treacherous conditions entering the weekend when expected thunderstorms may unleash fire-starting lightning and erratic winds that could erode progress made over the past week. Dry, hot conditions posed similar threats across the fire-stricken West.

Weather, fuels and terrain will pose challenges for the 6,000 firefighters battling the Park Fire, which has spread over 614 square miles (1,590 square kilometers) since allegedly being started by arson in a wilderness park in the Sierra Nevada foothills east of the Sacramento Valley city of Chico.

The fire’s push northward has brought it toward the rugged lava rock landscape surrounding Lassen Volcanic National Park, which has been closed due to the threat.

“Lava rocks make for hard and slow work for hand crews,” Cal Fire said in situation report. “Crews are being flown into access areas that have been hard to reach because of long drive times and steep, rugged terrain.”

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Thunderstorms Bring New Risks

After days of benign weather, increasing winds and a surge of monsoonal moisture were expected to increase fire activity and bring a chance of thunderstorms Friday night into Saturday, said Ryan Walbrun, incident meteorologist with the National Weather Service.

“The concern with thunderstorms is any gusty outflow winds that would push the fire itself or create some new fire ignitions within the vicinity of the Park Fire,” Walbrun said.

Collapse of thunderstorm clouds can blow wind in any and all directions, said Jonathan Pangburn, a fire behavior analyst with Cal Fire.

“Even if there’s not lightning per se, it is very much a safety-watch-out environment for our firefighters out there,” Pangburn said.

Walbrun said there was little prospect of beneficial rains from the storms and the forecast for next week calls for continued warming and drying.

“As we look forward in time, we’re really just entering the peak of fire season in California,” he said.

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Multiple Fires Across the West

The Park Fire, which has destroyed at least 480 structures and damaged 47, is one of almost 100 large fires burning across the western U.S.

A wildfire on the edge of metro Denver crept within a quarter-mile of evacuated homes, but authorities said Thursday they were hopeful that hundreds of threatened residences could be saved despite sweltering temperatures and firefighters suffering heat exhaustion.

The Quarry Fire southwest of the Denver suburb of Littleton encroached on several large subdivisions. Neighborhoods with nearly 600 homes were ordered to evacuate after the fire, of unknown origin, spread quickly Tuesday afternoon and overnight when relatively few firefighters were yet on the scene.

Jim and Meg Lutes watched from an overlook near their house northeast of the fire as smoke plumed up from the ridges. Their community west of Littleton was not yet under evacuation orders, but the couple had been ready to start packing a day earlier when flames could be seen blanketing the mountains.

“It can come over that hill pretty quick if the wind changes,” said Jim Lutes, 64, pointing to a nearby ridge.

Five firefighters were injured Wednesday, including four who had heat exhaustion, said Mark Techmeyer, a spokesperson with the Jefferson County Sheriff’s Office.

The fire was in steep terrain that made it difficult to access but had been held to about a half-square mile (1.4 square kilometers) with no houses yet destroyed, authorities said.

Miles to the north near the city of Lyons, Colorado, officials lifted some evacuations and reported making progress on the Stone Canyon Fire. It has killed one person and destroyed five houses. The cause was under investigation.

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The fire was among several threatening heavily populated areas of the Colorado foothills, including one in which a person was killed earlier this week.

New, large fires were reported in Idaho, southeastern Montana and north Texas.

Scientists say extreme wildfires are becoming more common and destructive in the U.S. West and others parts of the world as climate change warms the planet and droughts become more severe.