A fast-moving wildfire quickly grew to 18,000 acres in a rural Northern California county, prompting thousands of evacuations.

Dubbed the Camp Fire, the blaze began at about 6:30 a.m. Thursday. It quickly led to road closures and evacuations in Pulga and Paradise, two unincorporated communities in the foothills of the Sierra Nevada, about 170 miles north of San Francisco, according to the California Department of Forestry and Fire Protection.

At least 26,000 people had been ordered to evacuate Thursday morning, said Lynne Tolmachoff, a spokeswoman for Cal Fire. The fire began in the Feather River Canyon and quickly expanded, pushed along by 40 mile-per-hour winds, she said. “It grew very rapidly and spread rapidly into these towns,” she said.

Fire advisories, known as red flag warnings, were issued for the area given extreme dryness and high winds. Pacific Gas & Electric PCG, -2.05%   warned on Twitter that it might pre-emptively shut down power to Northern California areas including Lake, Napa, Butte, Plumas, Yuba, Sierra, Placer and Nevada counties.

An expanded version of this report appears on WSJ.com.

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