Severe Convective Weather Outbreaks on 10 and 15 December 2021: Large-Scale Antecedent Conditions

Abstract The first 2 weeks of December 2021 were exceptionally active for severe convective storms across the central and eastern United States. While previous work has indicated that this was related to the existence of a negative phase of the Pacific–North American pattern, we demonstrate that such a pattern was configured via dynamical linkages between multiple extratropical cyclogenesis events in the western North Pacific, the recurvature of Typhoon Nyatoh, and the subsequent phase evolution of the North Pacific jet. These processes were found to aid in the excitation of Rossby wave packets and the amplification of upper-level flow downstream over the Pacific, ultimately configuring synoptic-scale weather regimes supportive of anomalous high-frequency and high-intensity severe convective weather in the contiguous United States. In addition, abnormally warm Gulf of America/Gulf of Mexico sea surface temperatures, aided by a period of antecedent synoptic-scale subsidence, played a critical role in enhancing convective instability in the surface warm sector. This work underscores the importance of cataloging these events for purposes of examining (and potentially enhancing) predictability. Significance Statement The first half of December 2021 recorded one of the most active cool-season severe weather periods in the United States, resulting in two billion-dollar convective outbreaks on 10 and 15 December. This study links these extreme events to upstream dynamical processes over the North Pacific, including extratropical cyclogenesis, the recurvature of Typhoon Nyatoh, and the retraction of the North Pacific jet. These processes amplified downstream flow and configured synoptic environments favorable for severe weather across the United States. Additionally, anomalously warm Gulf of America/Gulf of Mexico sea surface temperatures enhanced convective instability. By identifying these key precursors, this work highlights the potential for improved anticipation of extended-range severe weather likelihood—particularly during the cool season—when such events remain rare but highly impactful.