A climatology of heavy midlatitude rainfall across the United States from 1949–2000

The increasing regularity and intensity of heavy precipitation events in the United States during the latter part of the 20th century is widely recognized and many climate models predict continued growth in the proportion of precipitation from extreme rainfall events concurrent with the projected increase in average global temperatures. To evaluate heavy precipitation events on a regional scale, the principal component analysis (PCA) technique is applied using the expanded version of the evenly-distributed Richman-Lamb dataset. The PCA technique identifies climatologically similar regions with respect to rainfall in the United States east of the Rocky Mountains between 1949 and 2000. Results show regions of coherent precipitation patterns across the eastern two-thirds of the Continental U.S.

A subset of observations from the U.S. Historical Climatology Network for the same domain and new regions helps identify heavy, significant, and extraordinary rainfall events. Precipitation values within 500 km of tropical events are not used. These event definitions are based on the 90th or higher percentile of total precipitation events with at least 25 mm (1 inch) of rainfall as well as the number of heavy events in each PCA-defined region. Further decadal, regional, and seasonal event categorizations uncover meaningful trends in the heavy, significant, or extraordinary event classifications.