On the weather types that shape the precipitation patterns across the U.S. Midwest

Abstract

The U.S. Midwest is an area that has been plagued by heavy and persistent precipitation leading to frequent flood events. The improved understanding of the types of weather conditions and settings associated with heavy precipitation can provide basic information to improve our preparation for and response to these events. Here 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 Midwest. Weather type 1 (WT1) features a zonally-aligned wave train propagating from the North Pacific to North America. Overall, WT2 is characterized by a wave train pattern with high (low) pressure in the western (eastern) United States. WT3 features a unique pattern with a high pressure system over the continental United States except for the northwestern United States, similar to the La-Nina forced responses. WT4 is characterized by a wave train moving from the Pacific Northwest to the North Atlantic with a strong ridge over the western United States, while WT5 features a positive geopotential height anomaly originating from the Arctic, probably influenced by the Arctic Amplification. 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 WT1 exerts the strongest impacts on precipitation, accounting for up to 40% of the total precipitation over the Midwest, followed by WT5. Moreover, we detect a significant upward trend in the number of WT1 and WT5 events for 1948–2017 and their persistency, suggesting a rising risk of heavy and long-lasting precipitation across the Midwest. Overall, the weather types during summer and winter are consistent with those obtained from the analysis of the entire year, although the weather types during winter have a larger magnitude in the geopotential height anomaly. WT1 accounts for the largest contribution to total precipitation in the Midwest during summer and winter.