One Year Later: The Legion Lake Wildfire

The Legion Lake Wildfire started on the morning of December 11, 2017 within Custer State Park during a period of very strong winds, warm temperatures, and critically low relative humidity. By the week's end, this fire had burned 54,023 acres. The fire required a Type II Incident Management Team and is the third-largest wildfire on record in the the Black Hills. What makes this fire especially notable is the fact that it occurred in December--a month not typically associated with large wildfire activity.

NASA Worldview. December 12, 2017

Conditions on both December 11 and December 12 were very favorable for large wildfire growth over southwestern SD. The weather conditions from the Custer State Park RAWS station are found in the Table below. Prior 30-day accumulated precipitation was less than a 0.1 inch which led to very dry fuels that were not covered in snow. Temperatures were above average through the first few days of the fire and on December 12, the temperature reach 60 degrees F while the minimum humidity dropped to 11%. But the biggest issue was the wind: gusts exceeded 30 mph for much of the day allowing the fire to spread throughout Custer State Park.

Although specific burned acreage numbers are not available, it was assumed that this fire burned ~2,500 acres on the first day alone. But conditions on December 12 were more conducive to large wildfire growth. And the fire has gotten established with some very remote and inaccessible areas of Custer State Park. According to the 209 Incident Status Summary (ICS-209) filed on December 13, the fire had burned 47,312 acres, the majority of which burned late in the afternoon and evening of December 12. This implies that the fire made a ~45,000 acres run, in both timber and grass fuel models, in a matter of hours. And did I mention this was December?

Legion Lake Wildfire. December 12, 2017. 

Fires have not historically gotten this large in December. In fact, the 5 largest previous wildfires since 2000 have all be near or less than 20 acres.

And if we look at the yearly wildfire distribution, the month of December is known for being the slowest month for wildfires for the entire year (data is pre-Legion Lake). July and August are the months with the most wildfire activity.

So what made December 2017 so special? It was really the combination of very favorable weather (as discussed above) and climatic conditions. Southwestern SD was under drought conditions with substantial soil moisture deficits. Furthermore, Custer State Park was running precipitation deficits over the previous 120 days.

Dry weather and precipitation deficits take their toll on the fuels. The 1000-hr fuel moisture was in the 11-15% range while 100-hr fuel moisture ranged from 6-10%. And the fine fuels were completely cured as is typical for December in this area of the US. On December 12, the day of the ~45,000 acre run, the Grassland Fire Danger Index was at the top-end of the Extreme Category.

In summary, this fire started on the perfect day and under the perfect climatic conditions to lead to exceptional growth. December wildfires might be historically rare but the Legion Lake fire proved that December fires can be just as destructive as their warm season counterparts. We shouldn't be surprised by these types of events--under the right conditions, they should be expected.