Changes in Large-Scale Fall Extreme Precipitation in the Mid-Atlantic and Northeast United States, 1979–2019

Abstract

Abstract Global Historical Climate Network (GHCN) data are used to characterize changes in large-scale fall extreme precipitation in the mid-Atlantic and the northeast United States. Days with the highest regional extreme precipitation total [extreme precipitation (EP) days] are sorted into weather types based on tropical cyclone (TC), atmospheric river (AR), and extreme integrated water vapor transport (IVT) influences. Increased cumulative precipitation from EP days is attributed primarily to three sources. First, over the mid-Atlantic states, Pennsylvania, and eastern New England, large increasing trends are found in precipitation occurring on EP days attributed to TC-related weather types. These increases are due to a late-1990s increase in TC frequency, which manifests primarily as increased TC remnants in the mid-Atlantic area. Second, over New York State and central and northern New England, there are increasing trends in EP day precipitation from AR-related weather types. Finally, there is evidence of increasing extreme IVT-related precipitation in the absence of ARs and TCs. However, when taking into account prior TC influences, it is found that a combination of TC-related weather types accounts for much of the increasing EP day precipitation trend. These trends are then compared to EP day synoptic changes relating to atmospheric moisture content and transport. Results indicate that fall EP days have become warmer and moister, but that this does not necessarily translate to higher IVT because wind speeds have stayed the same or slowed. This is consistent with fall climatological changes during the 1979–2019 analysis period, including higher atmospheric water content and slowed westerlies in the vicinity of the mid-Atlantic. Significance Statement This paper sorts days with high extreme precipitation totals in the mid-Atlantic and northeast United States fall into categories based on the presence of factors such as tropical cyclones (TCs), atmospheric rivers (ARs), and other extreme moisture transport. These days have become more frequent between 1979 and 2019, in part due to events with TC influences. However, the synoptic environment on extreme days has also changed, becoming warmer and moister. Many of the most extreme events involve combined AR and TC influences. This is important because it shows that 1) large-scale extreme precipitation has become more common and has changed synoptically and 2) interactions between tropical and non-tropical systems are a key factor for extreme events in this part of the country. Future work might investigate how changes in TC frequency, moisture transport, and jet stream patterns will affect these events in a future climate.