Low temperatures and persistent precipitation combined with an unusually high June snowpack to set the stage for historic flooding in southwest and south-central Montana this week. At least five rivers in Park, Carbon and Stillwater counties set all-time records for high flows, wiping out bridges and roads and sending entire buildings downstream in the process.

Ed McIntosh, a meteorologist with Billings news station Q2, said a recent cycle of precipitation from the West Coast brought a significant amount of rainfall to southern Montana, where it went to work melting an already ample snowpac...