Strong winds and critical fire weather conditions helped the Greenwood Fire in Superior National Forest explode to nearly 20,000 acres, essentially doubling in size in a period of only 24 hours.
Tuesday's update from the U.S. Forest Service estimates the wildfire in Lake County, Minnesota, to be an astonishing 19,493 acres. The fire was just over 10,000 acres in the forest service's 6 p.m. update Monday.
Growing to nearly 20,000 acres makes the fire's footprint approximately 31 square miles, which is just 10 square miles smaller than the size of St. Cloud, and 12 square miles larger than Mankato.
Fueling the fire on Monday were southwest/westerly winds at 20-25 mph, with gusts to near 30 mph. Afternoon humidity levels also dipped to dangerous levels, and when combined with temps in the 80s, it allowed the fire to roar.
The forest service notes that a pyrocumulus fire cloud developed Monday afternoon, resulting in "rapid fire spread and lightning."
Pyrocumulus are smoke plumes that rise vertically like a thunderstorm, resulting in stronger, erratic winds that lead to extreme fire behavior, according to the Los Angeles Times, which has long covered the nation's worst wildfires in California.
While Tuesday will feature more moderate fire behavior thanks to higher humidity and lower temps, more critical fire weather is expected Wednesday as the humidity will again drop and be coupled with breezy northwest winds.
The Greenwood Fire now tops the Oxcart Fire – that burned around 13,000 acres in northwest Minnesota in March – as the largest wildfire in Minnesota this year. It's also the largest wildfire in Minnesota since the Wannaska Complex fire torched 32,239 acres between Sept. 29 and Oct. 2 2012, though that was a combination of four fires in the Warroad area.
In 2011, a lightning strike ignited a fire that wound up burning 93,000 acres in the Boundary Waters Canoe Area.