MONTANA — An upper level area of low pressure moved over the Northern Rockies to the High Plains region from Saturday through Monday, bringing heavy rain and accelerating snowmelt that together posed a flood threat across Montana. The system tapped into an exceptionally deep plume of moisture over western Montana, leading to heavy precipitation accumulations in a short period.
Multiple inches of rain, falling on top of ongoing spring snowmelt, elevated flood risks through Monday. A reasonable worst-case scenario projected 3.5 to 4 inches of rain along and west of the Interstate 15 corridor across north-central Montana, according to the FOX Forecast Center. Some locations, including Missoula and Great Falls, were expected to receive a month’s worth of rain from this single storm system. Montana typically receives 2 to 3 inches of rain per month in May and June. Two-inch rainfalls across a two-day period occur approximately every 2 to 5 years in the Great Falls area, according to the FOX Forecast Center.
Flash Flood Watches were issued for vulnerable burn scar areas into Saturday evening, specifically those affected by the Elk, Robertson Draw, and East Side wildfires. These previously burned regions faced a heightened risk of flash flooding due to the heavy rainfall, as denuded soil and lack of vegetation reduce the land’s ability to absorb water.
Snowmelt further compounded flood concerns. While spring temperatures had already melted much of the seasonal snowpack in regional mountains, some areas along the Continental Divide, particularly in Glacier National Park, still held snow. The combination of melting snow and intense rainfall increased runoff into rivers and streams.
Current river forecasts predicted the Saint Mary and Sun River would reach flood stage late Sunday into Monday. However, officials noted that these forecasts were heavily dependent on actual precipitation amounts and remained subject to change as the storm progressed.