Secular trends in the size of extreme precipitation events over the eastern United States

The United States has experienced an exceptional number of extreme precipitation events over the last decade. Recent research reveals that the frequency of heavy and extreme precipitation events is indeed increasing. It is unclear, however, what types (e.g. local vs. large scale) of precipitation events are increasing and what physical mechanisms are responsible. In this study, these questions are addressed by examining the spatial aspects of precipitation events for the period 1950-1996. To carry this out, daily precipitation totals from the cooperative observer network are spatially interpolated onto a fine scale (10 by 10 km) grid over the eastern two-thirds of the United States. An automated algorithm is developed to: 1) identify precipitation regions for two day periods across a range of spatial scales (2500 km2 to 500,000 km2); 2) link nearby regions together to form events; 3) assess the relative heaviness of the precipitation over each scale. Daily weather maps are examined in order to characterize the synoptic environments associated with the most extreme events (i.e. those having a recurrence interval of one year or more at a given spatial scale). Secular trends are identified in the frequency of extreme events over different spatial scales and the synoptic environments associated with these events.