Large-Scale Dynamics associated with Extreme Precipitation in the Northeast United States

Extreme precipitation poses significant risks to life and property throughout the United States. While the frequency of extreme precipitation events has increased across the nation in the past few decades, research shows the largest increase occurs in the Northeast US. While detailed studies have investigated the climatology and projected changes in extreme precipitation in the Northeast, the causal-mechanisms embedded in the large-scale dynamics that historically induce extreme precipitation events are not well-established. We use gauged-based precipitation observations and ERA-5 reanalysis data to analyze the synoptic-scale atmospheric circulation patterns that favor extreme precipitation in the summer and fall from 1979-2021. To differentiate the large-scale dynamics that promote extreme versus non-extreme precipitation events, we stratify our daily precipitation thresholds into 80th-90th, 90th-99th, 99th, and 99.9th+, and composite the anomalous large-scale circulations for each threshold. Here we show the most extreme precipitation days (99th and 99.9th+) are onset by a negatively tilted and high amplitude trough-ridge axis across the east coast of the US. The negatively tilted trough and accompanying strong anticyclone provide the northwestward moisture transport from the Atlantic into the Northeast region necessary for extreme precipitation development. In contrast, we find a positively tilted trough axis and a decrease in moisture transport in weaker precipitation thresholds (80th-90th and 90th-99th). The waviness of the jet stream determines the orientation of the anomalous trough-ridge axis, which dictates the amount and direction of moisture transport into the Northeast for extreme precipitation development. Follow-up analyses include examining whether the historical trend in extreme precipitation over the Northeast US is connected to the change in the waviness of the jet stream.