I've got concerns with the fire weather conditions today and there is a Red Flag Warning in effect for much of western SD. Will we technically reach RFW criteria (RH < 15% and gusts > 25 mph) in all locations that are under the warning? Probably not.
So why the concern? I am going to preface this post with a note about the Auburn fire. On Monday we saw the Auburn fire grow to nearly 1000 acres on the north side of Rapid City. Wind gusts peaked at 23 mph (Ellsworth AFB observations). The RH bottomed out at 11%.
1) Dry conditions
The Auburn fire exhibited large wildfire growth near a major city in western SD. Why does location matter? Resources! The "world" was sent to the Auburn fire to assist in suppression. There were 3 SEATS, one air tanker, helicopters, engines from numerous VFDs, USFS and other federal assets, and leadership/resources by the RCFD and SDWF (obviously this is not a complete list of resources). These resources were on scene very quickly because of the proximity to town yet the fire still grew. This tells me that the fuels conditions are ripe for large fire growth. Short-range spotting with nearly a 80%+ PIG is exceptionally problematic and will resist suppression efforts. Since then, we've seen 48 additional hours of very dry conditions and very poor RH recovery during the nights. The fuels are more dry today than they were on Monday and that's kind of a scary thought.
2) Humidity
I get it, the RH today will be higher than the past two days. But this is a pretty negligible difference. There really is no functional difference to fine fuel moisture, and its effect on fire behavior, between an RH of 11% and 15-20% (forecast for today).
3) Wind
The wind is key. The potential for gusts from 30 - 40 mph on the prairies to the east and to the north of the Black Hills is concerning. These forecast winds are much stronger than what we saw on Monday and the functional difference between a gust of 23 mph and 33 mph is significant when it comes to fire behavior. The wind makes today a far different day than Monday was. And it makes the fire danger that much worse. I computed the Grassland Fire Danger Rating for the conditions last Monday and for the expected conditions for today. The GFDI breakpoints are:
