On 12/30/21, during a downslope windstorm, at least two wildfires were reported in Boulder County, Colorado. The Middle Fork Fire ignited sometime after 10 a.m. north of the city of Boulder, near Middle Fork Road and Hwy 36, and was quickly extinguished.

The Marshall Fire, reported around 11 a.m., originated south of Boulder near Hwy 93 and Eldorado Springs Dr. It spread quickly through grass, reaching the communities of Superior and Louisville in approximately an hour, transitioning into an urban fire that consumed structures throughout the day. Its spread diminished with the cessation of the downslope winds and transition to weak easterlies, increase in humidity, and finally, snowfall.

This CAWFE simulation shows the evolution of the Marshall Fire throughout 12/30, along with the fire extent (inset) derived from active fire detection imagery (where red indicates active fire, brick red – previous active fire, blue – a water body, and gray – cloud) near midnight local time. While the heat signature of burning grass on clay may diminish rapidly, lingering active fire may indicate areas with the longer-burning structures.