Stratospheric Influence on Large‐Scale Precipitation in the Eastern United States in Late January 2019

Abstract Using ensemble forecasts from the Global Ensemble Forecast System and controlled integrations with the Weather Research and Forecasting (WRF) model, we investigate how the North American stratospheric sub‐vortex, closely linked to the 2019 sudden stratospheric warming (SSW), influences a large‐scale precipitation event over the eastern United States in late January 2019 and further quantify its contribution to the precipitation anomalies of the event. This precipitation event is directly driven by a tropospheric cyclone and a downstream anticyclone, which jointly transport warm, moist air from the Gulf of Mexico into the eastern U.S. Through a series of WRF experiments, we indicate that a nudged run (NOBS), in which the stratospheric circulation is nudged toward the observed evolution represented by the ERA5 reanalysis data set, successfully captures the evolution of the tropospheric cyclone and anticyclone, whereas a parallel run (NCLM) nudged toward the ERA5 winter climatology fails to capture them. The NOBS run accurately simulates the sub‐vortex over North America, which facilitates the southward invasion of cold air. Moreover, in the NOBS experiment, the well‐captured stratospheric sub‐vortex enhances downward coupling through the phase‐locking mechanism, directly promoting the development of the tropospheric cyclone and indirectly influencing the downstream anticyclone. Our results indicate that the stratospheric anomalies contribute to approximately 37% of the total precipitation during the event. This finding underscores the importance of the sub‐vortex's geometry in the lowermost stratosphere for driving this precipitation event and highlights the need to focus on polar vortex geometry to fully understand regional stratosphere‐troposphere coupling.