By Terence Chea
The Associated Press

SAN FRANCISCO — Fire crews from Nevada and Oregon have arrived to help California firefighters battle hundreds of blazes that are darkening skies over the San Francisco Bay area and Central Valley, causing public health officials to issue air-quality warnings.

The lightning-caused fires have scorched tens of thousands of acres and forced hundreds of residents to flee their homes, though few buildings have been destroyed, said Daniel Berlant, a spokesman for the California Department of Forestry and Fire Protection.

"It's just extremely, extremely dry," Berlant said Tuesday. "That means any little spark has the potential to cause a large fire. The public needs to be extra cautious because we don't need any additional wildfires."

More than 800 wildfires were set by an electrical storm that unleashed nearly 8,000 lightning strikes across Northern California over the weekend.

The storm was unusual not only because it generated so many lightning strikes with little or no rain over a large geographical area, but also because it struck so early in the season and moved in from the Pacific Ocean. Such storms usually don't arrive until late July or August and typically form southeast of California.

"You're looking at a pattern that's climatologically rare. We typically don't see this happen at this time of summer," said John Juskie, a science officer with the National Weather Service in Sacramento. "To see 8,000, that's way up there on the scale."

The lightning storm struck California when the state was experiencing one of its driest years on record. Earlier this month, Gov. Arnold Schwarzenegger declared a statewide drought and directed agencies to speed up water deliveries to drought-stricken areas. Many communities have adopted strict conservation measures.

Areas hit the hardest by the weekend thunderstorm include Mendocino County, where 131 fires have burned more than 13,000 acres and threatened about 500 homes; Butte County, where 25 fires have burned more than 3,900 acres and threatened 400 homes; and the Shasta-Trinity Forest, where more than 150 fires have burned about 8,000 acres and threatened 200 homes.

Firefighters continue to battle the state's largest blaze, a 58,000-acre fire that began more than two weeks ago in a remote region of the Los Padres National Forest in southern Monterey County. A separate 8,500-blaze was burning in the forest's Big Sur area.

Even before the lightning struck, California had already seen an unusually large number of destructive wildfires that had burned nearly 90,000 acres, compared with 42,000 acres during the same period last year, according to CalFire officials. The fire season typically does not peak until late summer or early fall.

"This doesn't bode well for the fire season," said Ken Clark, a meteorologist in Southern California with AccuWeather.com. "We're not even into the meat of the fire season at this point, and the brush is extremely dry. It's not going to get any better, it's going to get worse."