Dozens of lightning-sparked California wildfires grew rapidly on Thursday, forcing tens of thousands of people to evacuate homes in the San Francisco Bay Area.

The state was hit by nearly 11,000 lightning strikes in 72 hours during a record heatwave, causing more than 360 fires. This is the worst lighting storms ever hit California in nearly two decades.

California Governor Gavin Newsom declared emergency and requested help from neighboring states. He has requested 375 fire crews from out of state as California prisoners who normally fight fires were locked down for COVID-19 or released from prison to slow the spread of the virus.

The fire has caused two deaths. A PG&E utility worker died on Wednesday helping first responders, after the death of a firefighting helicopter pilot in a crash earlier.

South of San Francisco, a cluster of lightning-strike fires doubled in size to 40,000 acres in San Mateo and Santa Cruz counties, injuring three first responders, forcing 22,000 to evacuate and destroying 20 structures, wildfire authority CalFire reported.

Big Basin Redwoods State Park, California's oldest state park with redwood trees up to 2,000 years old, suffered extensive damage to historic buildings, the state parks department said.

As the fire moved south, the University of California Santa Cruz called for voluntary evacuations from its campus on the northern flank of the coastal city.

To the north, at least nine fires raced through hills in California's wine country about 35 miles (56 kilometers) southwest of Sacramento, destroying over 105 homes and other structures.

Collectively known as the LNU Complex Fire, they have doubled in size to 131,000 acres since Wednesday, forming a "megafire" 10 times larger than New York's Manhattan island.

Wildfires have become more frequent and bigger in California in recent years in part driven by climate change.

Another group of 20 fires, called the SCU Lightning Complex, expanded by nearly a third to around 140,000 acres on Thursday some 20 miles east of Palo Alto.

Record-breaking heat baking the West Coast is caused by a dome of high pressure over the desert east of California that is siphoning off moisture and causing precipitation to evaporate before it reaches the ground, sparking dry lightning.

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