Surface map on the morning of June 9, 1972
Dense Fog Moves In
As the front moved slowly southwest during the morning, strong easterly winds brought additional moisture to the area. Dewpoint temperatures, a measure of the moisture in the air, were in the middle to upper 60s°F, about ten degrees higher than usual for early June. Upslope clouds formed over western South Dakota by the process of orographic lifting; when terrain forces air to rise, the air becomes saturated as it cools, and clouds develop.
Idealized depiction of orographic lifting
500 millibar map for the morning of June 9, 1972 (approximately 18,000 feet above sea level)
Two areas of thunderstorms developed over the Black Hills during the afternoon as the approaching low pressure made the atmosphere more unstable. Rain over the northeastern corner of the Black Hills spread southward toward the Nemo area. Showers over the southern Black Hills and southeast of Rapid City moved northwest and intensified over the central Black Hills. By 6 p.m., a nearly continuous line of thunderstorms covered the eastern Black Hills. The strong easterly winds along the ground and light winds above the hills kept the storms anchored in place.
High Volumes of Water
Around 12:30 a.m. June 10, the storms began to dissipate and move slowly to the east over the plains with no precipitation indicated by the Ellsworth Air Force Base weather radar by 6:30 a.m.
Precipitation totals (inches) at three sites: 5 miles south-southwest of Sturgis, Pactola Dam, and near Johnson Siding (5 miles west of Rapid City)