BUTTE — A slow-moving storm system over the Pacific Northwest will drift east on Friday, bringing an increasing chance of showers and thunderstorms throughout the day. Cold air aloft will create enough instability to fuel a few stronger storms, with localized wind shear creating a minor risk for hail. However, slow-moving storms capable of producing heavy rain remain the primary threat.
As the system moves closer Friday night into Saturday, upper-level energy and lingering instability will drive an extended period of steady rainfall. The highest totals will focus south of a line from Helena to Lewistown down toward Dillon and Big Sky. Given the duration of the rain, localized flooding and excessive runoff are significant concerns, particularly across urban areas and recent burn scars. Along the I-90 corridor from east of Butte through Bozeman, there is roughly a 90% chance of reaching at least 1 inch of rainfall by Sunday morning, with lower odds to the north and south.
Precipitation will gradually clear out from west to east late Saturday night into Sunday morning as weak high pressure builds in through Monday. While most of next week looks dry, a few minor disturbances could trigger brief, spotty showers at times.
