Several studies based on observational records found increasing trends over the central United States. Recently, Villarini et al. (2013) found a large increase in the number of rainfall days exceeding the 95th percentile of the rainfall distribution over the Upper Mississippi River Basin, and a much weaker signal in the Lower Mississippi River Basin.
Villarini, G., E. Scoccimarro, and S. Gualdi. “Projections of Heavy Rainfall over the Central United States Based on CMIP5 Models,” Atmospheric Science Letters, 14, 3, pp. 200–205, 2013.