FORESTHILL (CBS13/AP) – Smoke fills the skies in the Sierra as firefighters work to get the upper-hand on several wildfires.

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The National Weather Service issued a fire weather warning Monday after thunderstorms and erratic winds were added to the forecast, making battling the fire difficult and increasing the potential for new fires. The conditions are expected to last until Wednesday.

Lighting strikes overnight sparked several fires, according to a CAL FIRE spokesperson. He says crews are focusing on the northern and southern ends of the fire where flames are closest to communities.

Credit: CBS

Credit: CBS

American Fire

The American Fire has burned 14,765 acres in the Tahoe National Forest and is 49 percent contained. A total of 1,851 firefighters, including 51 hand crews, 75 engines, 42 water tenders, 36 dozers and 12 helicopters are joining in the battle. Air tankers are being used as they are available.

Officials estimate the fire will be fully contained on August 28 at midnight.

Local residents hope the storms also bring much-needed rain prior to the lightning.

The cause of the fire is under investigation.

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A firefighter staging area has been set up in the town of Foresthill.

Swedes Fire

The Swedes Fire burning in Butte County is threatening more than 400 structures and has forced hundreds of people and animals to evacuate.

California Department of Forestry and Fire Protection Capt. Scott McLean said Monday that in addition to the winds, dry lightning may also possibly affect a nearly 2,000-acre fire in Butte County.

Firefighters have contained 45 percent of the blaze that began on Friday.

McLean says the 900 firefighters in Butte County are among 5,000 battling 10 wildfires burning across California.

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(The Associated Press contributed to this report.)