The Role of Stratosphere-Troposphere Coupling in Subseasonal Forecasts of the North American 2017-2018 Cold Air Outbreak

During the end of December 2017 and early January 2018, a potent cold air outbreak (CAO) impacted the Central and Eastern United States (US). This extreme weather event produced record low temperatures, travel disruption, power outages, and unfortunately loss of life. For example, a strong extratropical cyclone tracked along the east coast from as far south as Florida toward the end of this cold event, resulting in widespread winter storm warnings and blizzard warnings across the Eastern US, especially New England. Given the difficulty in predicting extreme events on the subseasonal-to-seasonal (S2S) timescale of two weeks to two months, it is important to understand the critical factors that may improve prediction capability. One such factor is extratropical stratosphere-troposphere coupling dynamics, which prior studies have linked to extreme CAOs across Europe and North America. In particular, the North American CAO of 2017-18 featured robust vertically propagating Rossby waves from Siberia and the North Pacific days before the event, resulting in wave reflection within the stratosphere. However, while using reanalysis to diagnose precursors can be insightful, S2S model experiments are important in understanding how well-represented and relevant such dynamical mechanisms are in an operational S2S forecast model setting. In this presentation we will outline the event evolution of the December 2017 CAO and assess the impact of the stratosphere on the event forecast. To do so, we use the Community Earth System Model 2 (CESM2) Community Atmosphere Model 6 (CAM6) S2S real-time model to perform three different stratospheric nudging experiments up to 3-4 weeks in advance of the CAO, where the mid-to-lower stratospheric temperature and zonal wind are nudged in the zonal mean: (1) a free run (FREE) , (2) a run where the stratosphere is nudged to the European Centre for Medium Range Weather Forecasts (ECMWF) fifth reanalysis (ERA5) event evolution (ERA5-NUDGE), and (3) a run where the stratosphere is nudged to ERA5 long-term climatological conditions (CLIMO-NUDGE). Initial results find that all experiments demonstrate a shift toward colder Great Plains 2m temperature (T2M) anomalies with each initialization, but the ERA5-NUDGE experiment indicates a tendency to shift forecast T2M anomalies into lower terciles (i.e., more negative anomalies) compared to the FREE and CLIMO-NUDGE experiments. Additionally, the coldest ensemble members of the ERA5-NUDGE experiment demonstrate a North American circulation pattern closer to that of the true evolution during the event at subseasonal lead times, indicating the potential influence of stratospheric wave reflection in setting up this event. Assessing the impact of the stratosphere through such nudging experiments may provide information about stratospheric processes that forecasting communities can use to improve forecasts, while drawing a parallel to the experimental methods outlined in the Stratospheric Nudging And Predictable Surface Impacts (SNAPSI) protocol.