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.