Paul Sakuma, Associated Press
Firefighters keep an eye on wildfire in Big Sur, Calif., Tuesday. Fires are burning across northern California.

SAN FRANCISCO — In less than a day, an electrical storm unleashed nearly 8,000 lightning strikes that set more than 800 wildfires across northern California — a rare example of "dry lightning" that brought little or no rain but plenty of sparks to the state's parched forests and grasslands.

The weekend storm was unusual not only because it generated so many lightning strikes 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."

Thousands of firefighters battled the blazes Tuesday from the ground and air. 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. "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."

Despite the many lightning strikes that hit the ground on Saturday alone, the weekend thunderstorm brought little precipitation because the rain evaporated in hot, dry layers of the atmosphere before it hit the ground, Juskie said.

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.

From San Francisco to Los Angeles, cities have only seen a tiny fraction of the rainfall they normally receive at this point in a typical year. In the Central Valley, the cities of Sacramento, Modesto, Stockton and Red Bluff have recorded their driest March-to-May periods since at least the 19th century, according to the weather service.

"A combination of lightning and very dry fuels will spark fires," said Mark Strobin, a weather service meteorologist in Monterey. "It doesn't take much nowadays especially with how dry it is."

Even before the lightning struck, California had already seen an unusually large number of destructive wildfires with about 140 square miles burned, compared to about 66 square miles during the same period last year, according to state officials. The fire season typically does not peak until late summer or early fall.

One of the state's worst wildfire years occurred in 2001, when more than 2,000 lightning-caused blazes burned about 289 square miles, according to the National Interagency Fire Center.

Areas hit the hardest by the weekend thunderstorm include Mendocino County, where 131 fires have burned more than 20 square miles and threatened about 500 homes; Butte County, where 25 fires have burned about 6 square miles and threatened 400 homes; and the Shasta-Trinity Forest, where more than 150 fires have burned about 12 square miles and threatened 200 homes.

The biggest fire burning in California started more than two weeks ago in a remote region of the Los Padres National Forest in southern Monterey County. That 91-square-mile blaze was about two-third contained Tuesday.

The weather service has said more dry thunderstorms could strike Northern California later this week.

Several wildfires also were burning in New Mexico, where about 150 children were evacuated from a youth camp southeast of Albuquerque on Tuesday as a blaze in the Manzano Mountains closed in.

Associated Press Writer Alicia Chang contributed to this story from Los Angeles.