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The wildfire was strengthened by wind earlier in the day and was out of control after growing to 122 acres, Boulder Fire-Rescue spokesperson Marya Washburn said.
While the boisterous weather has since subsided, authorities believe they will battle the blaze for several more days, said Boulder Fire-Rescue Wildland Division Chief Brian Oliver.
The fire is spreading near the same area where a 2021 blaze destroyed more than 1,000 homes.
The new blaze grew to a sizable threat around 2 p.m. Saturday, when it spread to protected wildland near the National Center for Atmospheric Research, police said. As a result, authorities have called it the NCAR fire.
There is no yet known cause for the fire.
The Associated Press contributed to this report.