Extreme Winter Precipitation Regimes in Eastern North America: Synoptic-Scale and Thermodynamic Environments

Abstract

Abstract We define extreme precipitation regimes (EPRs) during the eastern North American winter based on widespread and persistent heavy precipitation, using ERA5 precipitation data from 1979 to 2020. We find 62 EPRs and analyze their synoptic-scale and thermodynamic environments. EPRs impact most of eastern North America with heavy precipitation, especially from Louisiana to Quebec, and generally last for 5–8 days. They are associated with an anomalously strong 500-hPa trough–ridge over western–eastern North America that travels slowly eastward, favoring intrusions of moist, tropical air into eastern North America, and a strong baroclinic zone from the central United States to Atlantic Canada. They are also characterized by high frequencies of cyclones in the midwestern United States, anticyclones over eastern Canada and the subtropical Atlantic, and atmospheric rivers (ARs) in eastern North America. Precipitation is maintained by large moisture influxes, primarily from the Gulf of Mexico and Caribbean Sea, from the EPR start to the time midway through the EPR period. The influxes are often associated with ARs feeding into cyclones, where the moisture falls as precipitation. We also categorize EPRs based on the spatial anomaly correlation (AC) of synoptic-scale weather patterns between individual EPRs and the EPR composite. High AC EPRs have similar but stronger 500-hPa features over North America, greater moisture flux from the Gulf of Mexico and inland precipitation over eastern North America, farther inland cyclone track, higher frequency of subtropical Atlantic anticyclones, and lower EPR-to-EPR variability than low AC EPRs. Significance Statement Cool-season extreme precipitation regimes (EPRs) often lead to flooding and other impacts and represent a significant forecast challenge. We define and analyze EPRs during the eastern North American winter to obtain a better understanding of their associated meteorological conditions. We also categorize EPRs into two distinct categories to capture the variability among EPRs. EPRs generally last 5–8 days and are associated with slowly moving large-scale weather patterns favoring intrusions of moist, tropical air into eastern North America, a strong temperature contrast, and frequent cyclones in the midwestern United States with anticyclones to the north and south. The intrusions of moist, tropical air are often associated with atmospheric rivers (ARs) that deposit their moisture in cyclones as precipitation.