GRASS VALLEY, Calif. — Dark clouds rumbled over the Sierra Nevada mountain range early Sunday morning, sparking 25 lightning strikes and nine fires on the Tahoe National Forest.

The forest dispatched 14 engines, one water tender, seven crews, three helicopters, one air tanker and two air attack ships to combat the remote fires still burning at less than one acre.

“We have strong initial attack forces on these fires to keep the acreage small. We are optimistic regarding our progress but all depends on the predicted afternoon winds,” said Jeanne Pincha-Tulley, Tahoe National Forest fire chief.

On Sunday, one fire burned in the Yuba River Ranger District about five miles west of Graniteville in Nevada County.

Eight fires burned in the American River Ranger District south of Interstate 80 in Placer County.

Thunderstorms are common in the Sierra Nevada during the summer months. Last year, lightning sparked hundreds of fires across California. Thousands of acres burned around Nevada County foothill towns and heavy smoke compounded poor air quality conditions already suffering from high ozone levels.

A hazardous weather outlook that forecasted isolated thunderstorms in the Sierra Nevada for Sunday was expected to lift by today, according to the National Weather Service.

Clouds that brought cooler temperatures, muggy conditions, some rain sprinkles locally and mountain thunder showers over the weekend was expected to drift away today.

Much warmer weather along with lower humidity will return this week as high pressure builds over the region.