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“This has been a fire that has moved extremely quickly," Forest Supervisor Rachel Smith told The Associated Press. "Just in a matter of a couple of minutes yesterday afternoon the fire grew from just 50 acres (20.2 hectares) to nearly 1,500 acres (607 hectares). This is the kind of growth that historically we have not experienced on our forest prior to the last couple of years.�

An overflight late Tuesday measured the fire at 4.2 square miles (10.8 square kilometers), slightly smaller than initial estimates after it grew rapidly in just a few hours. A forest statement said fire behavior also decreased during the night.

Firefighters were working to protect homes near the confluence of the Scott and Klamath rivers, a very lightly populated area about 20 miles (32.2 kilometers) from the California-Oregon state line and about 50 miles northwest of Mt. Shasta.

There weren't any immediate reports of injuries or homes burned Tuesday night. However, the Siskiyou County Sheriff’s Office issued evacuation orders for several areas, including one south of Hamburg, a riverside community of around 100 people. Additional areas were warned to be ready to evacuate.

State Route 96 was also closed, along with a section of the Pacific Crest Trail north to the Oregon border. Smith said there were dozens and possibly hundreds of hikers on the trail.

“We're asking those folks to leave it as quickly as they can and we're providing resources to get them off the trail,� Smith said.

The Head Fire was burning near the site of , which began on July 29 of last year. That fire started in the Klamath ´ºÉ«Ö±²¥ Forest and exploded in size when a thunderstorm created winds up to 50 mph (80 kph). It reduced much of Klamath River, a scenic community of about 200 people, to ash and , including two who may have been trying to flee the flames. Their bodies were found inside a charred vehicle in the driveway of a home.

Forecasters said weather would continue to be hot and dry but with instability caused by moist air being pulled into the region, bringing the threat of afternoon and evening dry thunderstorms with strong outflow winds. That pattern was expected to break down on Thursday, with cooler and calmer weather entering the weekend.

Klamath ´ºÉ«Ö±²¥ Forest sprawls over more than 2,650 square miles (6,863 square kilometers) in Northern California and southern Oregon.

A slew of other lightning-caused fires were reported Tuesday in Northern California, including in Mendocino County, Shasta-Trinity ´ºÉ«Ö±²¥ Forest and the Tahoe area, although most were small and quickly contained, fire officials said.