LOS ANGELES (AP) — Big new wildfires challenged California firefighters Tuesday even as they increased containment of earlier blazes that erupted as dry north winds arrived over the weekend.

Evacuations were ordered after the Aero Fire erupted Monday and spread over more than 8 square miles (21 square kilometers) near Copperopolis, a small community in Calaveras County, about 100 miles (161 kilometers) east of San Francisco in the state's historic Gold Country region.

Authorities set up three evacuation centers, but the number of evacuees was unknown.

Three structures were destroyed and one was damaged, said the California Department of Forestry and Fire Protection, or Cal Fire. A decrease in winds and a rise in relative humidity helped firefighters gain 23% containment.

The Aero Fire is among the latest blazes to erupt in California in a matter of days as a quiet start to fire season suddenly became active, with flames consuming drying grasses and brush encouraged by back-to-back wet winters. Most of the fires have been kept small, but a handful have charred thousands of acres.

In the northern Central Valley, a blaze dubbed the Sites Fire erupted Monday afternoon and spread over nearly 16 square miles (41 square kilometers) of rural Colusa County about 105 miles (169 kilometers) north of San Francisco. It was 5% contained as of Tuesday evening.

In an optimistic development, remaining red flag warnings for fire weather conditions in the Central Valley were expected to expire by midday, the National Weather Service said.

In the mountains of northern Los Angeles County, the size of the 4-day-old Post Fire was unchanged at just over 24 square miles (62 square kilometers), and containment increased to 31%.

Firefighters were building and reinforcing fire control lines and protecting infrastructure such as power facilities, oil lines and recreational areas. They have made significant progress on the northern side of the blaze, said Landon Haack, a section chief with Cal Fire.

“The crews have been working really hard out here,” Haack said during a Tuesday afternoon briefing. “They’ve got a lot of it cleaned up, mopped up, putting out the smoke.”

Firefighters were focused on the fire’s southern edge near Pyramid Lake, a popular boating destination that has been closed as a precaution since Sunday.

Officials said firefighters were being ferried across the water and then climbing into steep terrain to prevent flames from reaching the lake’s northern side.

One commercial property has been destroyed, and one injury has been reported.

In Sonoma County, north of San Francisco Bay, the 3-day-old Point Fire was 40% contained and remained at less than 2 square miles (5 square kilometers) in size due to favorable firefighting conditions, Cal Fire said. Two structures have been destroyed, and one injury has been reported.

About 60 miles (97 kilometers) northeast of Los Angeles, the nearly 2-square-mile (5-square-kilometer) Hesperia Fire was 86% contained. The fire erupted Saturday and forced road closures and evacuation warnings in San Bernardino County.

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Associated Press writer Christopher Weber in Los Angeles and radio reporter Julie Walker in New York contributed to this report.