Colorado and the storm of urban fires in the middle of winter
Northwest of Denver peak gusts reached 185 kilometers per hour, the equivalent of a Category 3 hurricane. Those winds generated intense fires of grass and bushes in south of Boulder and brought them east to the cities of Superior and Louisville, developing a firestorm. By the time it was over, nearly 1,100 houses had been destroyed or damaged, two people were reported missing and thousands were displaced.
The Marshall fire is now the most destructive in state history. Four of the five largest wildfires on record in Colorado occurred between 2018 and 2021.
Unlike many of the megafires in the western United States in recent years, which generally occur in forests and wilderness, the Marshall fire traveled rapidly to densely populated neighborhoods and went from a wildfire to an urban conflagration.
Tens of thousands of residents were evacuated As flames flew through the streets and dead ends.
The fire was led by what climate scientist and Boulder resident Daniel Swain called “a storm of embers“. Powered by hurricane force winds, embers leapt from house to house, burning many from the inside out, while burning trees, lighting commercial buildings and blowing up a highway.
The natural color image above was acquired just hours after the fire started on December 30 by MODIS on NASA’s Aqua satellite. At the time, the plume of smoke, which was also visible on radar, stretched about 100 kilometers over the eastern plains of Colorado. The fire also generated its own time: the increase in heat created an area of low pressure that drew surface winds into the fire from all directions.
The next day brought in much needed moisture as a cold front moved in and left falling more than 10 inches of snow, which dampened the fire but also complicated the response. As of January 3, 2022, the 6,200-acre fire perimeter was fully contained.
High winds and wildfires are not uncommon in the Front Range, but a forest fire in December is; the normal fire season lasts from May to September. A recent study found that increases in extreme fire weather are being driven by decreasing atmospheric humidity and rising temperatures.
In 2021, Colorado experienced an unusually warm summer and fall, along with record dryness. The hot, dry period followed an unusually wet spring, which reduced wildfires during the summer and stimulated vegetation growth, which then dried up and provided abundant tinder for the December fire.
At the time of the fire, eastern Boulder County was classified as extreme drought., according to the US Drought Monitor The map above shows the Standardized Precipitation-Evapotranspiration Index (SPEI) for Colorado for the month of December 2021. It represents both precipitation and temperature. (Does not include the New Years snowfall that helped put out the fire.) According to the Colorado Climate Center, SPEI values greater than minus 2 are very rare and are indicative of extreme hot and dry conditions in eastern Colorado.
The map also shows the contrast between the extreme drought in the eastern part of the state, where the fire occurred, and snowpack in the western part of the state, where heavy snowfall in December brought snowpack near or above average. Denver, which normally has 30 inches of snow at the end of December, did not record its first winter snowfall until December 10, its last.
Images of NASA Earth Observatory by Joshua Stevens, using MODIS data from NASA EOSDIS LANCE and GIBS / Worldview, and PRISM data courtesy of the West Wide Drought Tracker. Text by Sara E. Pratt.
NASA Earth Observatory