Wildfire an increasing concern for the summer

A month ago I was hopeful that we would see low wildfire potential over our summer wildfire season. However, the conditions that we have been experiencing recently are changing my thoughts.  

[But first, I need to say that I am really referring to western SD here. Summer brings a peak of wildfire activity to this area. This is opposed to eastern SD where wildfire activity peaks in March/April.]

Precipitation:

April and May are two of the four wettest months of the year over western SD. For Rapid City, Custer, and Philip these two months account for 30%, 28%, and 33% of the annual average precipitation, respectively. This is also the time of year when herbaceous plants (fine fuels and the carrier of wildfire) need the moisture to begin greening up. If greenup is delayed, stunted, or doesn't occur, the plant material is more susceptible to carrying fire.  

Large precipitation deficits have been observed over western SD during this period. Many stations have only seen 50-70% of the average precipitation. 

Temperature:

Hot temperatures create more evaporative demand which pulls moisture from the soils and plants. The big thing we haven't seen over the past two months is hot temperatures. In fact, temperatures have actually been cooler than average by several degrees. 

Even though we haven't had much for precipitation, the evaporative demand has been lower due to cooler temperatures which then reduces the stress on the plants. We can track this through the Evaporative Demand Drought Index (EDDI). The EDDI values for South Dakota are not showing any issues with dryness. 

So you could say that less precipitation than average combined with cooler than average temperatures has the net effect of "average" with regards to our evaporative demand. This is a good thing and it is the primary factor that is keeping me from being overly concerned about our summer wildfire potential at this point. 

Fuel moisture:

One metric to characterize drought is the 1000-hr fuel moisture. The southwestern Black Hills (Red Canyon), southeastern Black Hills (Custer State Park), and the central Black Hills (Baker Park) RAWS stations all show a 1000-hr fuel moisture that is drier than average. Thanks to Jay Wickham from SDWF for putting these together. 

So the big dead fuels in at least the central/southern Black Hills are relatively dry. 

Soil Moisture:

Yes, the past two years have been very wet across the region and soils still remain wet over South Dakota. But by looking at the change in soil moisture anomalies, we can see that things are drying out considerably. Without additional rain, evaporation and transpiration can tap out that residual soil moisture and we are heading in that direction. 

Greenup:

How has greenup been progressing so far? It seems to be slower than average. The Vegetation Drought Response Index (VegDRI) combines a variety of metrics to measure the vegetation conditions. For South Dakota, is is showing dry over most of the state. 

One of the variables going into VegDRI is the NDVI which measures the 'greeness' of the vegetation as compared to an average. I hypothesize that our fuels are more 'brown' than usual due to the lack of recent precipitation and the cooler-than-average temperatures that may be delaying greenup. Contrast the image above to the one from this time last year when things were very wet and very green. 

It is a very different picture in 2020 than it was in 2019. 

So what is in the forecasts?

The Climate Prediction Center (CPC) creates monthly outlooks for both temperature and precipitation. These forecasts become a cornerstone of the forecast process that wildfire meteorologists use to create seasonal outlooks. If the CPC forecasts leans towards cool and wet, we tend to hedge our bets towards lower wildfire potential. If the opposite is true, it might mean more wildfire activity. (Of course there are many other factors at play when considering wildfire potential.) The CPC is forecasting good odds at a hot June with no signal one way or the other for precipitation:

 

Again, I go back to evaporative demand. If this CPC forecast verifies, the hot temperatures will increase evaporation leading to more stress on the plants. Without accompanying moisture, we could see earlier curing of the fuels. 

Summary:

My research has shown that positive temperature anomalies during June combined with precipitation deficits in April-June favor large wildfire activity in June and July. I must caveat this, however, with the fact that long-term drought (as given by the Palmer Modified Drought Index [PMDI]) is also well-correlated to big fires during the same months and our current PMDI values remain quite wet. 

Back in late April, I was hopeful that May/June were going to bring ample rains which would reduce our wildfire potential over western SD. Since we have not seen those rains materialize, our threat for bigger fires has increased. Furthermore, the forecasts for a hot June tilt the scales towards more fire activity as well. 

I know 2018/2019 were two the of the wettest years on record in SD but things have changed in 2020. And we need to recognize that and plan for it. Be ready and don't let this unfolding situation catch you off guard.