Crews hoping for wet weather to help combat lightning-sparked wildfires in California faced a new threat Wednesday: too much rain.

Flash flood warnings were posted for much of the eastern Sierra Nevada on Wednesday as the National Weather Service predicted a 40 percent chance of rain over a cluster of wildfires in Plumas National Forest.

Fire officials worried that crews battling a 22,000-acre blaze about 125 miles northeast of Sacramento could face mudslides on burned-over slopes.

That blaze was only about 20 percent contained late Tuesday, after afternoon lightning sparked some new spot fires on the parched terrain, fire information officer Jill Poulsen said. Residents of the nearby communities of Janesville and Milford were urged to prepare to evacuate, though no order had been issued, Poulsen said.

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Elsewhere in the Sierra, crews in the Inyo National Forest were busy trying to control new spot fires and gained little on a 35,000-acre wildfire, roughly 55 square miles, which was about 80 percent contained Tuesday, said U.S. Forest Service spokeswoman Nancy Upham.

"We were hit with these thunderstorms, so there were a lot of winds in the area," Upham said. "It did not jump the containment lines at all, but clearly we were not able to extend them any further."

The Inyo fire, sparked by lightning on Friday, has destroyed six homes in an area north of Mount Whitney. Eleven firefighters have been injured.

They were among large wildfires were active in 12 Western states, according to the National Interagency Coordination Center.

Five of the top priority wildfires are in Utah, where crews are fighting the biggest fire in state history, which had spread across 514 square miles by early Wednesday, authorities said. The fire about 120 miles south of Salt Lake City was 30 percent contained Wednesday.

Utah fire teams also were on alert for a storm front expected to arrived Wednesday that could deliver more high wind and dry lightning.

Nevada's largest blaze — near the Idaho line — had grown to about 124 square miles Wednesday and was 40 percent contained, Elko Interagency Dispatch Center Manager Bill Roach said. Heavy rain fell in western Nevada, dousing a handful of lightning fires around Reno, authorities said.

The weather was cool and cloudy in the southwest corner of South Dakota, where crews expected to make significant progress on a wildfire near Hot Springs that killed a homeowner and destroyed 30 houses. The blaze had covered more than 15 square miles and was 40 percent contained Wednesday after a night of temperatures in the 40s. Residents who were evacuated Saturday had been allowed to return to their homes.

In the Southwest, a fire that threatened buildings at the Kitt Peak National Observatory near Tucson was 50 percent contained Wednesday at 6,110 acres, about 9.5 square miles, and was unlikely to threaten any buildings, authorities said. Flames were about two miles from the astronomy complex, which remained closed to visitors. Workers had returned to the facility and were preparing to resume observations on Friday, said observatory spokesman Doug Isbell.

Another round of record heat was forecast in the Northwest, adding to the problems faced by crews battling fires in Washington. High temperatures ranging up to 107 degrees were forecast for Thursday and Friday in south-central Washington, and Wednesday's forecast high in Seattle, on the usually cooler side of the state, was 97.

Along with the heat, brisk wind and new thunderstorms threatened eastern Washington. "With the very dry fuels, even if some storms have rain in them, they still could start fires," said Jeremy Wolf, a National Weather Service meteorologist in Spokane.

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