The 60-acre Maxwell Fire in Lefthand Canyon was caused by an unattended campfire, authorities said Tuesday.
Forest Service spokeswoman Elsha Kirby said fire officials found a still smoldering campfire ring 50 feet north of Lefthand Canyon Drive.
Containment of the fire, which started Sunday afternoon, increased at 80 percent, Kirby said.
Officials were concerned that temperatures soaring into the 90s and relative humidity dipping below 15 percent Tuesday will make it harder to contain the fire.
"There will be high risk of dehydration for firefighters," she said.
Winds are expected to be light, but there could be gusts of up to 20 mph in the afternoon.
"The weather conditions will be the major factor that drives the fire behavior today," she said.
All this could fan flames and generate smoke plumes visible around the metro area.
145 firefighters were battling the blaze Tuesday. Firefighters are actively monitoring the line along Lefthand Canyon Drive to continue protecting Boulder Heights.
One light helicopter was conducting reconnaissance and water drops. Two single-engine air tankers also are available.
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