Seasonal and Regional Variations of U.S. Trends in Extreme Precipitation Frequency

Numerous studies have documented increases in U.S. heavy precipitation during the latter part of the 20th Century. Recent studies have also revealed that event frequencies were quite high early in the 20th Century, nearly as high as in the 1980s and 1990s. This suggests that natural variability may be quite large and perhaps the recent increases in the U.S. have a large natural component. The meteorological reasons behind the observed major decadal-scale variations in heavy precipitation have not been investigated. Have there been secular changes in the frequency, intensity, and other characteristics of the meteorological phenomena producing heavy precipitation? Can we attribute these changes to hemispheric or global trends in circulation, SSTs, etc.? Are the recent increases primarily a result of increases in atmospheric water vapor concentrations? Heavy precipitation events occur in a variety of meteorological situations/types that are seasonally and regionally variable. The seasonal and regional differences in trends can provide important insights into possible causes of decadal-scale variations in the frequency and intensity of extreme events. An investigation of such variations has revealed that in those cases where regional monthly trends are statistically significant, the trends are overwhelmingly upward. The great majority of these statistically significant upward monthly trends occur in the warm season (May-October), with the most widespread increases occurring in August. The central part of the U.S. from the Gulf Coast northward into the Great Lakes in particular has experienced statistically significant increases in many warm season months. The timing and locations of the observed increases suggest that a variety of phenomena could be contributing, including tropical cyclones, mesoscale convective systems, extratropical cyclones, and increased water vapor transport from the Gulf of Mexico/western Atlantic. More detailed investigation is required to unravel the causes.