Wintertime cold air outbreaks (CAOs) in the Great Plains of the United States (US) have significant socioeconomic, environmental, and infrastructural impacts; the events of December 1983 and February 2021 are key examples of this. Previous studies have investigated CAOs in other parts of North America, particularly the eastern US, but the development of CAOs in the Great Plains and their potential sub-seasonal to seasonal (S2S) predictability have yet to be assessed. This study firstly identifies 37 large-scale CAOs in the Great Plains between 1950 and 2021, before examining their characteristics, evolution, and driving mechanisms. These events occur under two dominant weather regimes at event onset: one set associated with anomalous ridging over Alaska and the other set associated with anomalous pan-Arctic ridging. Alaskan ridge CAOs evolve quickly (i.e., on synoptic timescales) and involve stratospheric wave reflection. Conversely, Arctic high CAOs are preceded by weak stratospheric polar vortex conditions several weeks prior to the event. Both categories of CAOs feature anomalous upward wave activity flux from Siberia, with downward wave activity flux over Canada seen only in the Alaskan ridge CAOs. The rapid development of the Alaskan ridge CAOs, also linked with a North Pacific wave train and anomalous wave activity flux from the central Pacific, suggests that these events could be forced by tropical modes of variability. These findings present evidence that different forcing mechanisms, with contrasting timescales, may produce distinct sources of predictability for these CAOs on the S2S timescale.

The February 2021 Great Plains cold air outbreak (CAO) was a high-impact weather event involving numerous power outages, traffic incidents, widespread winter storm warnings, and various cold-related fatalities (BBC News, 2021;NWS, 2021a). Texas was impacted particularly hard due to the failures of power generation, natural gas supply, and water infrastructure (Doss-Goslin et al., 2021). Based on certain metrics, this event was the second most severe Great Plains CAO since 1950(Millin et al., 2022. Given the high degree of severity, there is great importance in understanding potential sources of atmospheric predictability in both observations and models that could improve forecasts of such events on the subseasonal-to-seasonal (S2S) timescale (i.e., 2-8 weeks; e.g., Vitart & Robertson, 2018).The stratospheric polar vortex, which is the boreal wintertime cyclonic circulation 10-50 km high in the atmosphere near the North Pole, has links to Northern Hemisphere CAO development. If vertically propagating planetary-scale waves break in the stratosphere, the stratospheric polar vortex will weaken and warm; if the westerly circulation reverses then a major sudden stratospheric warming occurs (