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A Northern California wildfire that forced evacuation orders for several communities covered nearly 21 square miles (54.3 square kilometers) early Tuesday, authorities said.

The Siskiyou County Sheriff’s Office issued evacuation orders Monday for the Lake Shastina, Juniper Valley and the Mount Shasta Vista subdivision as the fire exploded in size.

The total number of people under evacuation orders was unclear. The Sacramento Bee reported that nearly 3,000 people live in Lake Shastina and as many as 8,000 others live in the area to tend thousands of marijuana grows.

Ignited by lightning on June 24, the Lava Fire is burning brush and timber in the Shasta-Trinity National Forest north of the town of Weed, about 250 miles (402 kilometers) north of San Francisco.

Nearly 500 firefighters were battling the flames in rough and rocky terrain.

The region was under excessive-heat warnings, with afternoon high temperatures expected to range from 100 degrees (37.7 Celsius) to 110 degrees (43.3 Celsius).