A day after the National Weather Service in Great Falls issued its initial Flash Flood Watch, forecasters updated and extended the warning Friday morning, September 19, keeping it in effect through midnight Saturday for a portion of central Montana. The geographic scope narrowed compared to Thursday's watch, focusing on the Big Belt, Bridger, and Castle Mountains; the Elkhorn and Boulder Mountains; and the Little Belt and Highwood Mountains.
The continued concern centers on the same combination of factors flagged Thursday: heavy rainfall interacting with burn scars left by the summer 2026 fire season, particularly in the Horse Gulch area. On burned terrain, water that would normally soak into the ground runs off rapidly, picking up debris and channeling into drainages with unusual speed and force. Forecasters noted that heavy rainfall over burn scars can trigger debris flows even from storms that would pose little risk on unaffected slopes.
The Friday update kept the watch active through the evening, meaning Saturday outdoor plans in the affected mountain ranges carry elevated risk. Residents and recreationists in or near the named ranges should check NWS Great Falls for the latest conditions before heading out. Those living in drainages below burned areas should have a plan in place and stay alert to changing conditions through Saturday night.