Medano Creek Month-by-Month in an Average Year. April is the second snowiest month of the year at Great Sand Dunes, but there are also some sunny days with highs in the 60s. Spring is the windy season throughout the southwestern United States, especially afternoons, though mornings are usually calm. The creek begins to trickle down as the snow on the Sangre de Cristo Mountains melts. By late April the creek may be a few inches deep. Cottonwood trees and willows along the creek are not yet leafed out.