 The Missouri 's drainage basin has highly variable weather and rainfall patterns , Overall , the watershed is defined by a Continental climate with warm , wet summers and harsh , cold winters . Most of the watershed receives an average of 8 to 10 inches ( 200 to 250 mm ) of precipitation each year . However , the westernmost portions of the basin in the Rockies as well as southeastern regions in Missouri may receive as much as 40 inches ( 1 @,@ 000 mm ) . The vast majority of precipitation occurs in winter , although the upper basin is known for short @-@ lived but intense summer thunderstorms such as the one which produced the 1972 Black Hills flood through Rapid City , South Dakota . Winter temperatures in Montana , Wyoming and Colorado may drop as low as − 60 ° F ( − 51 ° C ) , while summer highs in Kansas and Missouri have reached 120 ° F ( 49 ° C ) at times . 
