Hill and Blaine counties are under a flood watch, according to the National Weather Service.
Rain is expected to continue through Sunday morning, causing snow melt in higher elevations.
In the valley, soil is already soggy and will be unable to hold the increased moisture, the weather service said.
Streams will swell, especially in creeks that come from mountainous areas, causing some flooding.
Residents who live in areas that are usually prone to flooding should be prepared to precautionary measure, the weather service said.
A more serious flood warning was issued for Chouteau, Cascade, Fergus and Judith Basin counties.