Numerous studies have documented increases in U.S. heavy precipitation during the latter part of the 20th Century. In recent years, newly available digital daily data extending back in time to the late 19th or early 20th Century have 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 are not understood. 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 variations in trends can provide important insights into possible causes of decadal-scale precipitation variations. A preliminary investigation of such variations has revealed certain key features. Regional monthly trends that are statistically significant at the 90% level of confidence are overwhelmingly positive. The observed increase in the annual number of heavy events is primarily driven by increases 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.

We can speculate on the implications of these findings. 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.