Firefighters scrambled Friday to quash fires ignited by lightning as thunderstorms with mostly small but welcome amounts of rain rumbled across drought-stricken Northern California, where forests have been burning for weeks.

Firefighters were diverted from the huge Caldor Fire south of Lake Tahoe to fight multiple overnight lightning fires throughout El Dorado County, according to the local unit of the California Department of Forestry and Fire Protection.

The largest of those fires erupted in heavy brush in the steep terrain of Kanaka Valley. Rain from the storm cell helped firefighters and the fire’s spread was stopped at less than 7 acres (2.8 hectares), Cal Fire said.

Another fire believed to have been sparked by lightning was burning in Mendocino County, north of San Francisco Bay. The Press Democrat reported it covered 8 acres (3.2 hectares) Friday morning.

Lightning blitzes can have disastrous outcomes in parched California. Last year’s record amount of land burned included huge Northern California fires that were ignited when remnants of a tropical storm unleashed thousands of bolts.

A cluster of 2020 lightning fires known as the August Complex burned more than 1,615 square miles (4,182 square kilometers) and is considered the largest California wildfire on record.

The National Weather Service said there were more than 1,100 cloud-to-ground lightning strikes in California between Thursday evening and Friday morning, including 110 in the Bay Area. Cloud-to-cloud flashes were too numerous to count.

Rainfall from the thunderstorms was no drought-buster but was welcome anyway.

The official downtown San Francisco rain gauge recorded 0.01 inch (0.25 millimeters) of rain by 5 a.m., marking the first time it has rained there on Sept. 10 since 1978, the weather service said. The most recent measurable September rain in the city happened in 2019.

In the Central Valley, Sacramento Executive Airport received 0.05 inch (1.27 millimeters) of rain by 8 a.m. The last time at least that much rain fell was 175 days earlier on March 18, the weather service said.

Thunderstorms with downpours also moved through parts of Southern California on Thursday and early Friday, triggering flash flood concerns for burn scars of past wildfires.

The threat of new lightning-sparked fires came as more than 13,000 firefighters were working to rein in 13 major fires and more than 12,700 residents were still waiting to return to evacuated homes, according to state agencies.

The Caldor Fire, the 15th-largest in state history, was 53% contained after burning more than 341 square miles (883 square kilometers) and destroying more than 1,000 structures, including hundreds of homes.

In the northern Sierra Nevada and southern Cascades, the Dixie Fire covered more than 1,485 square miles (3,846 square kilometers). Second in size to the August Complex, it has destroyed more than 1,300 structures.

Cal Fire, meanwhile, announced that a fire that destroyed 142 structures in the Sierra last month was human-caused but that investigators were still working on details. The River Fire began Aug. 4 in a Placer County campground and burned 4 square miles (10.4 sq.km.) before it was contained Aug. 13.

A historic drought and recent heat waves tied to climate change have made wildfires harder to fight in the American West. Scientists say climate change has made the region much warmer and drier in the past 30 years and will continue to make weather more extreme and wildfires more frequent and destructive.

President Joe Biden will survey fire damage during a visit to California next week in which he will also campaign for Gov. Gavin Newsom, a Democrat who faces a recall election on Tuesday.