Officials say the Garnet Fire is 99% contained after burning 60,000 acres in the Sierra National Forest.
"We noticed that the thunderstorm got worse and then there were black ashes raining upon us. At this point we made the decision to head back," said Villarosa.
Fire damaged the 118-year-old park's headquarters, historic core and campgrounds. But the towering trees are still standing.
Red flame retardant doesn't actually extinguish fires, but it does make it harder for trees and leaves to combust.
When it broke out in July, the Museum Fire charred three square miles and forced the evacuation of two dozen homes near Flagstaff, Arizona.
Some 44,000 troops will be available for "unprecedented" operations to put out the fires that have swept the region.