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.

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.

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.”

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 with 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.

“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.”

The weekend’s lightning storm combined with extremely dry conditions to spark about 840 separate blazes from the Big Sur area of Monterey County to Del Norte County on the Oregon border.

By contrast, 574 lightning-sparked fires blackened about 86 square miles in Northern California in all of 2007.

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-thirds 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.