Increasing Frequency of Flood Events across the Central United States: A Weather-Type Perspective

The frequency of flood events has increased across large areas of the central United States since the second half of the 20^th century; these increasing trends have been largely related to changes in precipitation. The aim of this presentation is to provide insights into the possible reasons responsible for these changes, providing basic information that may enhance our capability of predicting and projecting these changes.

This study highlights the role of weather types in explaining the observed changes in precipitation and, consequently, in the frequency of flood events. More specifically, we identify five weather types from daily 500-hPa geopotential height using the k-means cluster analysis. Consistent with their distinct large-scale atmospheric patterns, these weather types exert different effects on precipitation in the central United States. Because of the strong moisture transport, strengthened low-level jet stream and wavy upper-level polar jet stream located in the western United States, among the five weather types weather-type 1 exerts the strongest impacts on precipitation, accounting for up to 40% of the total precipitation over the study region, followed by weather-type 5. Moreover, we detect a significant upward trend in the number and persistency of these two weather types for 1948-2017, suggesting a rising risk of heavy and long-lasting precipitation across the central United States.