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.

Beneficial Precipitation over SD

Remember Thanksgiving Day when I issued a notice about heightened fire weather conditions? Well part of that forecast was made based upon the lack of precipitation over portions of south-central and southwestern SD. Luckily, that is exactly where the majority of the snow fell this past weekend! Good news for those areas as we finish out the last week in November.

Some areas over south-central SD (from Winner/Presho east towards the Missouri River) missed out on the moisture but the weather forecast looks seasonal this week and no fire weather issues are foreseen.

Heightened fire danger for Thanksgiving

We are starting to see some hints that Thursday could bring higher fire danger to areas along and south of I-90, west of the Missouri River. Strong warm air advection across the area will push temperatures into the 60s while southwesterly (downsloping) winds from 10-20 mph and gusts to 25 mph bring dry air into the region. Minimum RHs may fall below 25% (graph below).

In addition, west-central and south-central SD have seen little in the way of precipitation over the past 3 weeks and those areas missed out on much of the snowfall this past weekend (graph below). The fine fuels are cured and have been exposed to dry conditions which will make them more susceptible to ignition sources.

Historically during November, southwestern and south-central SD see large fire growth when the RH is less than 30%, winds have southerly component in excess of 15 mph, temperatures are 10+ degrees above average, and it has been 20 or more days since the last wetting rain. As it stands now, we will be meeting all of the criteria on Thursday. Just keep it in mind as we get to Thanksgiving Day.

Very High Grassland Fire Danger... or not.

Today's Grassland Fire Danger Index (GFDI) is in the High to Very High categories across western SD. But let's discuss whether or not we should be concerned. Here is the GFDI for today:

The GFDI incorporates wind speed, relative humidity (RH), temperature, and a fuel curing percentage as inputs. I've calculated the GFDI for today assuming a peak temperature of 50 F, a minimum RH of 50%, and wind speeds of 34 mph. These are approximately what we are observing over the Very High fire danger in the map above. I've calculated a range of GFDI values based upon a range of fuel curing percentage values so you can see how the sensitivity of the GFDI to the curing percentage.

In the case where the curing percentage is 90% or 100%, we can see that the GFDI calculation results in a Very High fire danger. However, if we run the same calculation for a wind speed of 25 mph, which is more representative of the sustained winds today, the results only give us a Moderate to High fire danger rating for a spectrum of curing percentages ranging from 80-100%, as shown in the table below.

You can see that the GFDI is quite sensitive to changes in both curing percentage and wind speed.

The two aspects of the GFDI that are not included within the calculation though are antecedent moisture and cloud cover. It's cloudy outside which implies no sun shine thus the fuel temperatures will remain low throughout the day. And this area of SD has recent decent precipitation, including snow, over the past 10 days which will lessen the fire potential. And let's face it, historically, we have not seen large fire growth on days with RHs at or above 50%.

To summarize, my overall concern for large fires today is much lower than what the GFDI calculations are showing.

A borderline critical day

My two cents on today's wildfire potential:

The wildland fire danger across South Dakota is elevated today due to strong winds and RHs below 30% in many areas.

But conditions aren't nearly as critical as they could be because of the recent precipitation. Every reporting weather station (ASOS and RAWS) across the state has received a wetting rain within, at most, the last 10 days. If the exact same meteorological conditions as we are seeing today occurred after 20+ days since the last wetting rain, I'd be far more concerned about the potential for large fire growth.

It's definitely windy out there (and a bit dry) but historically we just don't see big fires this soon after widespread precipitation events.

Strong winds expected on Wednesday

A strong cold front is expected to move through the state on Wednesday, October 3. This mainly-dry front will bring strong northwest winds to the area. Winds gusts from 40-50 mph will be common over western SD.

I do expect gusts from 50-60 mph over portions of southern Meade County, and central/eastern Pennington County as the wind accelerates locally around the eastern edges of the Black Hills.

Minimum RHs will be in the 30-40% range which, although not critical, are likely low enough to allow for large fire growth.

We have recently received sufficient moisture to wet the fuels across the area. However, the fine fuels are expected to dry with the warm temperatures and sunshine today. The curing percentage of the fine fuels is in the 70-90% cured range. This will lead to Grassland Fire Danger Indices ranging from High to Very High over western SD.

Stay alert out there. I don't anticipate any major fire issues (due to recent precipitation and dew) but if a wildfire does get started, it has the potential to race across the landscape. Remember too that strong winds can lead to downed powerlines and the potential for increased ignitions.

Fire weather this weekend

Widespread rain was seen over a large portion of SD over the past two days. However, southwestern SD stayed relatively dry (precipitation images below). Thus this forecast will focus on the issues over southwestern SD through the weekend.

Saturday:
High temperatures will be in the mid-80s with minimum RH in the 8-18% range. Westerly winds from 5-15 mph are expected with some gusts to 25 mph possible. Look for mostly clear skies although some altostratus or cirrus are possible. The GFS is showing a very dry lower troposphere and nearly unidirectional westerly winds up the column. There are some 30-knot winds just above the mixed layer and if we can tap into that momentum, we may see some strong gusts at the surface.

Saturday night:
Minimum temperatures will range from 45-55 across the lower elevations and from 55-65 over the higher elevations (yes, it will be warmer over the higher elevations of the Black Hills). Maximum RH recovery will range from 40-50% over those cooler lower elevations and only from 15-25% over the warmer higher elevations. This will precondition the fuels for Sunday.

Sunday:
This will be a pre-cold front day and the forecast will depend on the timing of the frontal passage. But for now, it looks like the front won’t pass southwestern SD until Sunday night which will lead to hot and dry conditions during the day. The forecast as of now calls for temperatures to range from 85-95, minimum RH from 8-18%, and southwest winds from 10-20 mph while gusts to 25+ mph are possible.  Red Flag Conditions are possible. A deep well-mixed layer is forecast which may transport some higher momentum to the surface bringing stronger wind gusts.

Just because we’ve seen drizzle and rain over the past two days over much of SD does not mean it has been wet everywhere! Keep your situational awareness up through the weekend.

Much needed rain on the way

The 104 acre Anada fire burned yesterday, 9/18, roughly 25 miles east of Rapid City. This man-caused fire was started from a combine and burned through 2-ft tall grass and was pushed by a 10-mph wind. Peak temperature were in the mid-70s and the minimum RH was near 40%. These conditions led to flame lengths on the head of the fire of 6-8 ft. The weather conditions were far from critical but the fire behavior showed that the fuels were definitely dry and susceptible to rapid fire spread.

The number of days since the last wetting rain is related to the potential for large grass fires. During September if two or more weeks have passed since the last wetting rain, the potential for prairie fire dramatically increases. And as of yesterday, nearly all West River stations hadn't reported a wetting rain in 20+ days with some stations not seeing a wetting rain in over a month. It had been 18 days, 21 days, and 27 days since the last wetting rains at Rapid City Regional Airport, Ellsworth Air Force Base, and Pinnacles RAWS site, respectively. These are the closest stations to the Anada fire and at least represent the conditions on the ground there.

Luckly, rain is on the way. We've already seen a few hundreths of an inch of drizzle/rain over and south of the I-90 corriodor of western SD. Areas farther north of I-90, west of the Missouri River, have seen from 0.20 to 0.50 inches of rain.

The forecast calls for even more precipitation over the next 36 hours. The convection-resolving HRRR weather model shows more precipitation on the way (image below).

In short, this is good news. However, the forecast for precipitation over far southwestern SD does not look nearly as optimistic. And remember that this is an area that has been quite dry over the past 50 days. Critical conditions are not expected in the near future, but if they do develop, we'll have to watch this area of the state for higher fire potential.

Critical Fire Weather Conditions Possible on Wednesday (9/12) and Thursday (9/13)

Look for a return of hot, dry, and breezy conditions tomorrow and Thursday which will likely lead to critical fire weather conditions over southwestern SD. Wednesday does look like the most critical of the two days.

Temperatures will be in the upper 80s to mid-90s across the area, minimum RHs will drop to below 20%, and south to southwest winds will be gusty from 15-30 mph.

Wednesday weather forecast images:

Furthermore, these areas of southwestern SD have not seen much for rain over the past 2 weeks with some locales not seeing a wetting rain in over 3 weeks:

The VegDRI product shows much of this dryness across the region.

The primary threat will be over southwestern SD as this is where the hottest, driest, and breeziest conditions are forecast to be. The largest 5 fires since 2000 (table below) show similar conditions to what we have seen over the past 2+ weeks and to what we expect to see tomorrow. In short, pay attention and don't get complacent around new ignitions.

Sunday/Monday (Aug. 26/27) fire danger to increase

Both the NAM and the GFS (weather prediction models) are showing hot, dry, and windy conditions developing over southwestern SD on Sunday, August 25. Furthermore, the GFS is again painting hot, dry, and windy conditions for Monday over the same area while the NAM model keeps those conditions farther south in Nebraska.

Custer and Fall River Counties (circled below) have not seen the precipitation over the past 7 days that the surrounding areas have seen. The Red Canyon RAWS and the Wind Cave/Elk Mountain RAWS have each reported a scant 0.02" of rain during this period. We are not expecting widespread rain over these areas today or tomorrow and therefore we anticipate the fuels will continue to dry.

Sunday:
Critical fire weather conditions may develop over southwestern SD on Sunday. If the forecast holds, expect a local enhancement in the fire danger, especially over Custer and Fall River Counties where a recent dry spell has been seen. Look for temperatures in the 90, min RH < 20%, and southwest winds gusting from 25-35 mph. A few thunderstorms are possible over central/northern SD but will not likely co-locate with areas of critical fire weather. All of western SD will be hot and relatively dry, but the wind looks to be strongest over those noted southwestern areas.

Monday:
Critical fire weather conditions may continue through Monday for southwestern SD; however, high uncertainty remains in the forecast. The primary uncertainty is in the timing of a frontal passage on Monday; if the front arrives earlier, more humid conditions will occur. However, if the front arrives later (like the GFS model is predicting), the dry, hot, and windy conditions (similar to Sunday's weather) will spread across southwestern SD. And one last notable point: with the front comes the potential for scattered thunderstorms which may imply lightning and ignitions.

Land and fire managers will need to keep an eye on the forecast to see how the situation unfolds.

GFDI possibilities:
Let's look at a couple of different possible weather scenarios and the associated Grassland Fire Danger Indices.

1. A temperature of 93, an RH of 20%, winds of 25 mph, and a curing percentage of 80% yields a Grassland Fire Danger Index of "Very High."
2. A temperature of 96, an RH of 15%, winds of 30 mph, and a curing percentage of 90% yields a Grassland Fire Danger Index of "Extreme."

Recent rains help but other areas of SD remain dry

Recent rainfall across southwestern and west-central SD has dramatically lessened the wildfire potential, at least in the short term. Areas of most concern were Haakon County, Stanley County, eastern Pennington County, Jones County, and northern Jackson County. All of these areas have received at least 0.25 inch of rain, if not substantially more. Fall River County (where the Vineyard Fire occurred) has seen widespread areas of 0.75+ inch of rain. Image below (1-day precipitation totals) does not include rainfall totals from the ongoing precipitation event over central and south-southcentral SD.

Northwestern and northcentral SD, however, do remain dry. Buffalo in Harding County hasn’t seen a wetting rain in over two weeks with a 30-day accumulated rainfall of only 0.86 inches. The Bear Creek RAWS in Dewey County hasn’t had a wetting rain in 28 days with 30-day precipitation totals at a scant 0.45 inch. The Shadehill RAWS in Perkins County also hasn’t had a wetting rain in 28 days. The Grand River RAWS in Corson County hasn’t had a wetting rain in 17 days. Mobridge in Walworth County hasn’t had a wetting rain in 24 days. Thus my focus for enhanced fire potential shifts to these areas.

No critical conditions are seen in the forecast for the next 7 days but scattered thunderstorms across the state this week could bring new ignitions. The 5-day accumulated precipitation forecast is below.

Weather/climate conditions for Vineyard Fire

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. 

Critical fire weather possible this weekend

The forecast for the weekend could bring critical fire weather conditions to parts of western South Dakota. High temperatures on Saturday and Sunday are looking to be in the 90s to lower 100s over the western SD prairies while minimum RHs each day will range from 13-20%. Wind will be the critical factor, however. Wind gusts from 10-20 mph are expected Saturday:

But Sunday winds are forecast to gust from 20-30 mph:

When combined with the hot temperatures and low humidity, the stronger winds on Sunday could bring critical fire weather conditions to west-central SD where fuels conditions are quite dry as noted earlier this week.

Wildfire potential also increasing over northwestern SD

Harding County, tucked away in northwestern SD, is also seeing dry conditions similar to those found across the west-central SD prairies. Buffalo, SD has only received 0.87" of precipitation over the past 30 days and fuel curing percentage is roughly 75% there. The notable part of Harding County is the areas of Ponderosa Pine contained within the Custer National Forest. It's not all prairie across the region and the timber is drying out as well. The photo below is from the Sheep Draw fire from late March in 2015 showing the terrain with stringers of Ponderosa Pine.