Despite withering summer heat, thousands of firefighters aided by planes slowly gained ground Wednesday against California wildfires (search) that have burned more than 18,500 acres of brush and forest and caused hundreds of people to evacuate homes.

Wet weather headed into the region, bringing hope of relief but also raising fears of flash flooding and new lightning-caused fires.

The Federal Emergency Management Agency (search) authorized funds for some of the blazes, which were burning in Los Angeles County, to the east in Riverside County, to the south in San Diego County and in Yosemite National Park (search) in the Sierra Nevada.

One of the most difficult fires, 4,700 acres in Angeles National Forest on the edge of the Mojave Desert, was 46 percent contained. The fire in Pine Canyon prompted voluntary and mandatory evacuations of more than 500 homes Tuesday, and an outbuilding and motor home were destroyed.

"The fire has been unpredictable, but now firefighters are trying to make a stand at Highway 138 -- just north of the fire -- to attempt to cut it off there," said Los Angeles County fire Inspector John Mancha.

In Riverside County, a 3,698-acre blaze on the edge of San Bernardino National Forest (search) was 50 percent contained as it burned toward unpopulated terrain. It previously posed a threat to the mountain communities of Idyllwild, Pine Cove and Garner Valley.

Elsewhere in the county, a 350-acre fire southwest of Lake Elsinore was 90 percent contained and people who had evacuated from the Bundy Canyon area returned home.

In eastern San Diego County, an 8,500-acre blaze sparked by illegal fireworks was expected to be fully surrounded by nightfall Wednesday.

Two outbuildings were destroyed and four firefighters were treated for minor injuries, said California Department of Forestry Firefighter Tyler Ashton.

The fire prompted evacuation of about 100 rural homes and a Boy Scout camp Tuesday, but structures or communities were no longer threatened, Ashton said.

In the Sierra, hikers were evacuated and trails were closed in part of Yosemite National Park after a lightning-sparked wildfire grew to 1,300 acres Tuesday. The fire was one of nine fires burning in the park from lightning strikes two weeks ago, the National Park Service said.

In Nevada, a television reporter and several firefighters were injured by a wildfire that blackened 300 acres and destroyed several homes in the hills just west of Carson City.

Carson City Fire Chief Dan Shirley said the fire was burning with increasing unpredictability after sending up a huge plume of smoke over the community.

"It's very scary looking," Forest Service spokeswoman Christie Kalkowski said.

Investigators said the fire was human-caused.

In Arizona, fire officials hoped to decide later this week when residents evacuated from two mountain communities can return to their homes.

Firefighters fended off flames that threatened the 74-cabin community of Turkey Flat, the 15 cabins in Columbine and the $200 million Mount Graham International Observatory.

Fire officials remained concerned about the possibility of flash flooding and continued heavy firefighter traffic, he said.