Nearly 11,000 lightning strikes have been recorded in California over 72 hours, in the heaviest spate of thunderstorms to hit the state in more than a decade.
A total of 367 individual fires were ignited, with more two dozen growing into major blazes, authorities said.
A helicopter pilot was killed after the aircraft crashed while on a water-dropping mission in Fresno County, about 160 miles south of San Francisco, the California Department of Forestry and Fire Protection (CalFire) said.
Multiple fires raced through northern California’s wine country, shutting down the major Interstate 80 motorway at Fairfield, about 35 miles southwest of Sacramento.
Flames leapt across the motorway, trapping motorists caught in a hectic evacuation.
Four people suffered burns but survived, although the severity of their injuries was not immediately known, according to officials.
CalFire spokesman Will Powers said thousands of residents were under mandatory evacuation orders.
Fires burned across some 46,000 acres of hills and mountains around Fairfield and the neighbouring town of Vacaville late on Tuesday and into Wednesday.
At least 50 homes and other structures were destroyed and another 50 properties damaged, CalFire said.
Diane Bustos said her husband abandoned their car as it caught fire and then blew up in Vacaville.
She lost both her shoes while she and her family ran for their lives.
“I made it, God saved me,” Ms Bustos told local television station KPIX.
Dead livestock was pictured among torched properties and some animals were seen wandering loose.
“We are experiencing fires the like of which we haven’t seen in many, many years,” California Governor Gavin Newsom told a news conference.
Mr Newsom said he had requested 375 fire engines from outside the state to assist in battling the blazes.
He declared a statewide fire emergency on Tuesday.
The last time California experienced dry lightning storms of such devastating proportions was in 2008, CalFire said.
Fanned by high winds, the fires are racing through vegetation parched by a record-breaking heatwave that began on Friday.
Meteorologists have said the extreme heat and lightning storms were both linked to the same atmospheric weather pattern – an enormous high-pressure area hovering over America’s desert South West.
A temperature of 54.4C (129.9F) was recorded at Furnace Creek in California’s Death Valley on Sunday, in what could be the hottest reading ever reliably taken on the planet.