Analysis of how weather affected the spread of the Marshall Fire in Colorado

The Denver office of the National Weather Service has released an analysis of the weather that created the conditions on December 30, 2021 that allowed the Marshall Fire southeast of Boulder, Colorado to be turned into a blast furnace that within a few hours ran five miles to the east burning 6,000 acres and destroying nearly 1,100 homes with a total value of more than $500 million.

The NWS described the winds the day of the fire:

High winds developed in the mid morning hours on Thursday, December 30th, 2021, the result of a mountain wave that developed as very strong westerly winds raced over the Front Range Mountains and Foothills and crashed down onto the plains.  The mountain wave remained nearly unchanged through the rest of the day, resulting in very persistent and extremely high winds. Mountain waves are usually focused very close to the base of the foothills and adjacent plains. On this day, sustained winds of 50 to 60 mph with gusts of 80 to 100 mph were felt along Highway 93 and points east to around Superior and at times, Louisville.

On the map below, the final fire perimeter is outlined by light purple (upper center of the plot images), while the city of Boulder is located in the northwest corner. The black numbers are temperature (F), red numbers are wind gusts, and the wind barbs point to the direction the wind was from.

The surface plot for 12 PM MST above shows some of the strongest winds from this wind event. A peak gust of 115 mph was reported at the base of the foothills, just east of the intersection of Highway 93 and Highway 72. Note the 85 mph gust in south Boulder, and a 100 mph gust along Highway 93 (very bottom of the image below) at about noon.

After a very wet first half of the year that resulted in a lush, tall crop of grass, the six months leading up to the fire in late December were the driest in recorded history, by far. Drought affects moisture in the vegetation — the fuel moisture. The lower is it, the more easily and more intensely it burns in a wildfire. There were many areas in the fire with light vegetation, such as grass, that in December after it has cured would be more affected by recent rain (or the lack thereof) and relative humidity than long term drought. The relative humidity was in the mid-20s that day. But the National Weather Service said larger fuels such as shrubs and trees were plentiful in and around the affected subdivisions. Those fuels would be heavily affected by the historic drought and would have low fuel moistures in the live and dead vegetation.

Drought can also affect the home ignition zone. If gutters on homes are not kept clean of leaves, they can be ignited during an ember shower even if the fire is thousands of feet away. If the gutters have leaves during normal weather, especially in December near Boulder, they could also have water that is trapped by the leaves keeping them wet for weeks or months. But with historic drought, it is possible the water evaporated, making them susceptible for ignition by embers. A fire in a gutter can spread to the structure.

The lawns that in late December would often be covered by snow, were most likely brown and dry, making it possible for an ember to ignite the grass which could spread to homes. Mulch, such as bark or wood chips placed around ornamental plants and near structures would also be much drier than normal, making that fuel available. And remember, the winds were 50 to 60 mph with gusts of 80 to 100 mph.

As structures burned, millions of additional burning embers were lofted into the air with many of them igniting susceptible fuels out in front of the main fire. A burning home that in many cases was only 15 to 20 feet away from other houses could easily ignite through convective or radiant heat the neighboring residence.

Author: Bill Gabbert

After working full time in wildland fire for 33 years, he continues to learn, and strives to be a Student of Fire. View all posts by Bill Gabbert