Dry conditions around the area

It's been a while since we've seen our last precipitation event across much of South Dakota. Most areas haven't seen 0.10" of rain or snow-water equivalent in over two weeks. Precipitation totals for the past 14 days are found below.

There are plenty of dry swaths across the state and we are not expecting a major precipitation event until after the Christmas holiday (and by the way we could see a high-impact winter storm in the 12/26 - 12/27 time frame). So why does this matter?

The fine fuels are cured this time of the year and so the only way they will resist fire spread is if they are wet from precipitation. And you guessed it, if we don't have precipitation (or at least high humidity) those fuels will continue to dry. Find a warm day with relatively low RHs and a nice breeze and the stage is set for large fire growth across the prairies.

On December 17th, this exact scenario played out near Fairburn, SD where a 45 acre fire occurred. Interestingly, a new tool called the HDW index predicted that the 17th would have higher fire potential. The HDW looks at vapor pressure deficit (and measure of dryness) and wind speeds to calculate a value. This value is the compared to the climatological HDW for that day. If you start to get above the 90th percentile, you are getting closer to a historically warm/dry/windy day. Combine that dry fuels and you get good fire growth conditions. Below is the HDW forecast for today with the previous 6 days of HDW values and an ensemble forecast for the next 7 days. Tomorrow could be another one of those high fire danger days. My concern level is still relatively low because the warm/dry conditions will precede the windy conditions but it is something we need to pay attention to.

You can find real-time HDW data here: https://eamcweb3.usfs.msu.edu/HDW/index.html

Strong winds tonight

The National Weather Service has issued a High Wind Warning for portions of western SD tonight. Wind gusts from the northwest could exceed 60 mph.

The wind, however, will be accompanied by snow across the higher elevations of the Black Hills which is a very good thing. Good because over the past week hundreds to thousands of piles have been ignited across the area. This includes giant "machine piles" which can burn for several weeks. Piles are lit when sufficient snow cover exists but that snow cover may melt or sublimate before the pile is fully extinguished. This can allow the fire to spread beyond the pile and into the surrounding landscape.

The past few days have been warm and dry and some south-facing hillsides might have lost their snow cover. I have some limited concerns for pile escapes with the wind tonight if the precipitation is not observed. I doubt we'll have large issues but it is something to keep in mind.

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. 

Dry and warm air aloft this morning

An impressive subsidence inversion is in place over western SD this morning. The UNR sounding shows warming and drying with height just above the surface.

Current dew points (below top) and RH (below bottom) show this dry air mass in place over the region with the driest air at higher elevations.

This dry and warm air will inevitably remove some of the snowpack over the Black Hills. If sufficient snow is melted or sublimated, we may start to expose bare ground bringing a risk for piles escapes. I encourage all land owners and managers to closely monitor their local snow conditions to assess potential for fires to creep away from the piles. This concern will amplify through the week as little precipitation is expected.