The Elements of Runoff - Plains snow water equivalent for Upper Missouri River Basin Runoff - plains snow melt is typically complete by mid April. Mountain snow begins to accumulate in October, peaks in April and snowmelt is usually complete by mid-July. The May 1 runoff forecast is 26.9 million acre feet. The runoff forecast has improved thanks to late season plains and mountain snow in the upper basin and a continued precipitation trend may provide much needed moisture for the region.