The National Weather Service office in Great Falls issued a Flash Flood Watch Wednesday morning covering portions of central Montana, including the Big Belt, Bridger and Castle Mountains, with the watch running from Thursday afternoon through Thursday evening. The primary concern is the Horse Gulch Burn Scar, where bare, hydrophobic soils left by wildfire can turn moderate rainfall into fast-moving debris flows with little warning.
Burn scars are among the most dangerous terrain during summer storm season in Montana. Vegetation that would normally absorb or slow runoff is gone, and the scorched soil often repels water rather than soaking it in, sending rainfall racing downslope in concentrated channels. The Horse Gulch fire left that landscape primed for exactly this kind of response, and forecasters noted that heavy rainfall is expected to arrive during the watch period.
Residents and recreationists in the Big Belts and surrounding ranges should monitor National Weather Service updates and avoid drainages and canyon bottoms below the burn scar during and immediately after afternoon thunderstorms Thursday. Even storms that appear to be falling some distance away can send flash floods and debris flows downstream faster than people can move out of the way. The watch expires at midnight Thursday.