Moderate to heavy rain showers could potentially lead to excessive runoff and flash flooding later this week in parts of western Montana.
National Weather Service forecasters in Missoula say an unusual amount of moisture originating from the Gulf of Mexico will move into the region by early Thursday.
The heaviest rainfall will be along and south of I-90.
West-central and southwestern valleys have about a 50% chance of receiving a half inch of rain. The higher terrain could see an inch or more of precipitation.
The system is expected to migrate to the northwest part of the state on Friday, officials said.
Forecasters say recent burn scars and flood-prone urban areas are at higher risk of runoff-related impacts.