BOZEMAN – The USGS has studied the Bridger Foothills Fire burn scar from September of 2020 and is projecting a high percentage for possible flash flooding and debris flows this Spring and Summer with the right weather conditions.
Typical summer thunderstorms can produce locally heavy rain and if that occurs over the steep terrain of the Bridger Foothills fire burn scar flash flooding and debris flows will impact the urban interface.
There are several areas of this burn scar that have a 60-80% (red) chance for debris flows with a rainstorm or thunderstorm producing 24 mm/h in 15 minutes. The entire burn scar west of the highway has a 40-60% chance of debris flows.
In a major wildfire the aftermath of all the burned fuels produces what is called a “hydrophobic layer”. This acts like a layer of cement and repels water and doesn’t allow absorption. The combination of steep terrain and this hydrophobic layer will accelerate runoff and this will allow debris, rocks, burned logs etc to flow downhill.
This is typical of all burn scars but when that burn scar is in the urban interface the danger to life and property is a greater concern.
For those living in the Bridger Canyon north of Bozeman there could be several to numerous flash flood warnings issued as spring rains occur and especially when afternoon thunderstorms develop over the region.
The National Weather Service is planning on installing some remote weather sensors to help monitor possible heavy rain events to give as much lead time as possible for warning statements.
Unfortunately, there is very poor low level radar coverage in this region. Heavy rain or snow events could be missed, and lead times greatly reduced. Residents of the Bridger Canyon should be hypersensitive when moderate to heavy rain occurs and be prepared to evacuate. Have a plan ready before the storms impact your area.
The Billings National Weather Service and Great Falls National Weather Service plan on a media virtual meeting to discuss the threat of debris flows in burn scar areas of the Bridger Foothills Fire and the Bobcat Fire Monday April 26th and we will have more information to share with you after this meeting.