Extended-range Predictability of Early December 2021 Severe Weather Events from the operational Global Ensemble Forecast System

The first half of December 2021 was an exceptionally active period for severe thunderstorms east of the Rockies, which was punctuated by two significant severe weather outbreaks on 10 and 15 December. On 10 December, the deadliest tornado outbreak on record for December occurred in the mid-Mississippi and Ohio valleys and resulted in 89 fatalities, 675 injuries, and $4.1 billion in damage. In all, long-track supercell thunderstorms produced 67 tornadoes, with 23 of those tornadoes rated as significant or violent (rated >= EF2 on the Enhanced Fujita Scale). Less than a week later, a serial derecho and tornado outbreak occurred in the central Great Plains and upper Midwest on 15 December. This severe weather outbreak resulted in 1 fatality, 2 injuries, and $1.9 billion in damage. In addition to 583 reports of severe winds, with widespread measured gusts of 70–90 kts, 119 tornadoes were confirmed, with 33 rated EF2.

The significant socioeconomic impact of two multi-billion dollar disasters occurring within 5 days of each other underscores the need to predict these extreme weather events at longer lead times to allow stakeholders to adequately prepare. Therefore, the aim of this presentation is to explore the medium-to-extended-range predictability of the December 2021 severe weather outbreaks. Specifically, we will diagnose factors that contributed to the favorable synoptic scale evolution preceding these events. For example, the recurvature of Typhoon Nyatoh and subsequent strong interaction with the North Pacific jet led to flow amplification and the excitation of a Rossby wave packet downstream. The triggering of the Rossby wave packet resulted in retraction of the North Pacific jet, followed by a series of cyclogenesis events in the west Pacific between 7–11 December. In all, these events resulted in a westward phase shift of a longwave trough from over the eastern U.S. to the western U.S., which favored the movement of several shortwave troughs from the western U.S. to the Great Plains and Mississippi valley on 10 and 15 December. Next, we assessed to what degree this evolution was predictable in extended range forecasts from operational NCEP Global Ensemble Forecast System version 12 (GEFS). The results show that the GEFS forecasts initialized prior to 0000 UTC 30 November showed significant uncertainty in the 500 hPa geopotential height pattern in the western in central U.S. for the second week in December. This uncertainty was significantly reduced as forecast probabilities of a 500 hPa trough in the western and central U.S. increased starting with the GEFS forecast initialized at 0000 UTC 30 November. It appears that an improved initialization and track/intensity forecast of Typhoon Nyatoh contributed to the improved forecasts in the U.S. in the extended range.