A large fire started on the eastern fringes of the town of Hot Springs, SD on Saturday, August 11, 2018 around 1600 hrs local time. Hot Springs is in far northern Fall River County which is in the southwestern corner of SD bordering both Nebraska and Wyoming. As of today, this fire has burned 560 acres making it the largest fire in the Black Hills Forest Fire Protection District so far this year. Here I'll examine both the climate conditions and weather conditions that helped (or hindered) wildfire growth.
Climate Conditions:
Historically, large wildfires that have occurred within the Black Hills during August have seen 1 to 3 months of dry conditions and at least 1 month of above average temperatures prior to the fire start. Drought conditions are only weakly correlated to wildfire growth in the region.
This year was actually a very wet year for the area with 30-day and 60-day precipitation prior to the fire start from 1-3 and from 3-5 inches above average for the area, respectively. Temperatures were also much colder than average over the prior 30 days. In short, the climatic conditions did not favor large fire growth for this area of South Dakota. Image credits: High Plains Regional Climate Center.
Weather Conditions
The weather conditions that favor large wildfire growth during the month of August within the Black Hills include low relative humidity (and poor overnight humidity recovery), winds in excess of 20 mph, and above average temperatures. Additionally, if 7 or more days pass without a wetting rain, the local fire potential can be enhanced.
To examine the weather conditions, we'll look at both the Red Canyon and Wind Cave/Elk Mountain Remote Automated Weather Stations (RAWS) as these are the closest RAWS stations to the wildfire. Below lists temperature, min/max RH, wind, and precipitation from each station for the 7 days prior to and for the day of the fire start.
Both the Red Canyon and Wind Cave RAWS sites did not see any precipitation in the 4 days leading up to August 11. In this same time frame, temperatures were at or above seasonal averages. Overnight RH recovery was poor at best through this same period. This allowed a short-term drying of the fuels from August 6 through August 11, as shown by the minimum 10-hr fuel moisture at Red Canyon. On both August 10 and August 11, temperatures were in the low 90s, minimum RH dipped to or below 21%. Then on August 11, breezy southerly to southwesterly winds exceeding 20 mph were seen. This, in combination with favorable terrain, aided fire movement across the landscape.
It should be noted that precipitation in August is typically convective in nature. A thunderstorm may move over a weather station and not over the area of a future fire start. This wildfire did start in an area on the fringe of previous precipitation systems as seen from the precipitation image below which includes data from Doppler radars. It goes to show that location is key. Image credit: NWS
Lastly, on August 11, the skies were clear (outside of regional smoke haze) as seen in the MODIS imagery below. This allowed the solar radiation to heat the fuels and warm southern aspects to temperatures higher than the surrounding areas. Left of center is the Black Hills (dark in color) and the fire began at the southern end of the Black Hills. Image credit: NASA Worldview.
Summary
There are obviously a multitude of factors that contributed to both the fire spread and to the ability for firefighters to limit the spread of the fire to less than 600 acres (at least as of today). This includes the tireless efforts of all of the interagency firefighters that came together to manage this wildfire. But we must also recognize that both weather and climate conditions played a role in how the fire evolved across the landscape. The weather conditions were mainly favorable for fire spread, and recognize that these are not "classic" Red Flag Conditions (RH < 15%, winds gusting >25 mph). However, climate conditions were not nearly as favorable for large wildfire growth and this likely helped to keep the Vineyard Fire from becoming a much more significant blaze.
