Trends and projections of extreme precipitation over the contiguous United States

Historical and projected trends in contiguous United States (CONUS) extreme precipitation events were examined in CMIP5 models, using, as a metric, the Extreme Precipitation Index (EPI) and a similar method based on the EPI. An increasing trend over the CONUS was found in the EPI, with variation among seven sub-regions. Model median of CMIP5 simulations catches the trends in observations, although with a decreased magnitude. Model spread is large and in most cases bigger than the model signal itself. Most statistically significant increasing trends in the observation based EPI occur over Midwest and Eastern regions, while most of the decreasing trends occur over the western regions. When compared to observations, at least some of the ensemble members for most models show correlation coefficients greater than 0.5. However, many models also have ensemble members with negative correlation coefficients. Projections of extreme precipitation events for two Representative Concentration Pathway (RCP) scenarios (RCP 4.5 and RCP 8.5) show increases in EPI anomalies from 2006-2100. For CONUS both scenarios show a steady increase in extreme precipitation event frequency throughout the period while the RCP 4.5 projection signal is of a smaller magnitude compared to that of RCP 8.5. While there is variability among regions, overall RCP scenarios show an increase across all regions with the exception of some variability between decades in some regions for RCP 4.5. Model spread is not larger than the signal for either scenario for the CONUS and most of the sub-regions showing overall agreement among models of a future increase in extreme precipitation event frequency