A firefighting helicopter pilot was killed in a crash and scores of homes burnt in California on Wednesday as hundreds of lightning-sparked blazes forced tens of thousands of people to flee their dwellings.

Nearly 11,000 lightning strikes were documented during a 72-hour stretch this week in the heaviest spate of thunderstorms to hit California in more than a decade, igniting 367 individual fires.

Almost two dozen of them had grown into major conflagrations, authorities said.

Multiple fires raced through hills and mountains adjacent to Northern California’s drought-parched wine country, shutting down Interstate 80 at Fairfield, about 56km southwest of Sacramento, as flames leapt across the highway, trapping motorists caught in a hectic evacuation.

Police in the nearby town of Vacaville reported that advancing flames had prompted the evacuation of a state prison there and a medical facility for inmates.

Four residents whose communities were overrun by flames hours earlier in the same area suffered burns but survived, though the severity of their injuries was not immediately known, Will Powers a spokesperson for the California Department of Forestry and Fire Protection, said.

He said thousands of residents were under mandatory evacuation orders in a four-county area stricken by a cluster of nine wind-driven fires collectively dubbed the LNU Complex, triggered by lightning on Monday.

In central California, a helicopter had been on a water-dropping mission in Fresno County about 258km south of San Francisco when it crashed, killing the pilot, a private contractor, CalFire said.

As of Wednesday night, the LNU complex of fires had burnt largely unchecked across 50,000ha, with zero containment, destroying at least 105 homes and other structures and leaving another 70 damaged, CalFire said.

Several of the fires had merged by nightfall.

There were social media accounts of people trapped in the blaze, but CalFire’s Powers said authorities had no reports of anyone missing.

“We are experiencing fires the like of which we haven’t seen in many, many years,” California governor Gavin Newsom told a news conference, adding he had requested 375 fire engines from out of state to help.

He declared a statewide fire emergency on Tuesday.

The last time California experienced dry lightning storms of such devastating proportions had been in 2008, CalFire spokesperson Scott Maclean said.

Fanned by “red-flag” 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 are both linked to the same atmospheric weather pattern — an enormous high-pressure area hovering over America’s desert southwest. — Reuters