The True Prairie (TP) is a large area in the central U.S. which was a tall grass prairie for thousands of years prior to its conversion to crop land. An analysis of climatically controlling factors indicated that the tall grass prairie is favored by the ratio of warm season precipitation to potential evapotranspiration exceeding 0.75 (west boundary), cold season precipitation less than 38 cm (south boundary), high cold-season frequency of thunderstorms (north boundary), and high drought frequencies (entire region). A “climate year” approach was used to assess the temporal and spatial variability of these conditions during the 20th Century. This analysis did not reveal any long-term trends in most climate year types, although there were significant decadal-scale fluctuations, most notably a high frequency of drought-type years in the 1930s and 1950s. However, the observed upward trend in precipitation is manifested in an increasing frequency of one climate year type characterized by above normal cold season precipitation in the southern border area of the TP.