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.
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