* WHAT...Flash flooding caused by excessive rainfall is possible. * WHERE...A portion of southwest Montana, including the following areas, Gallatin and Madison County Mountains and Centennial Mountains, Northwest Beaverhead County and Ruby Mountains and Southern Beaverhead Mountains. * WHEN...From Wednesday afternoon through Wednesday evening. * IMPACTS...Excessive runoff may result in flooding of rivers, creeks, streams, and other low-lying and flood-prone locations. Flooding may occur in poor drainage and urban areas. * ADDITIONAL DETAILS... - Slow moving thunderstorms capable of producing heavy rain are expected Wednesday afternoon and evening. - http://www.weather.gov/safety/flood