WCD-19. Predictability Considerations of the 10 August 2020 Midwest Derecho

A derecho struck parts of the Midwest on 10 August 2020. Iowa and Illinois saw the brunt of the impacts, which included extreme surface winds. The highest measured gusts exceeded 120 mi hr-1 (104 kt), but damage-based wind speeds were estimated at as high as 140 mi hr-1 (122 kt). The highest winds and most significant damage occurred along the U.S. Highway 30 corridor between Ames, IA and Cedar Rapids, IA. Anecdotal reports suggested that winds exceeding severe criteria (~58 mi hr-1, 50 kt) persisted for an extended period after the passage of the leading convective line, so the duration of the high winds was another impressive aspect of this event. The High-Resolution Rapid Refresh (HRRR) model (3 km grid spacing) is one of the best tools forecasters have for forecasting extreme weather events such as this one. The operational version at the time of the event, version 3 (HRRRv3), did not provide a strong indication of this derecho until a few hours before it occurred. Version 4 on the other hand, which replaced version 3 as the official operational version in December 2020 and was available for this event in real time, provided a higher certainty of the derecho, and turned out to be more accurate. Retrospective forecasts of the event using the same model as HRRRv4 but with 500 m grid spacing provided enhanced perspectives on the optimal performance of the event given better forcing (initial and boundary conditions). Advanced radar data assimilation techniques were also used as another means of assessing the maximum potential accuracy of the forecasts and as a stress test on the utility of the particular data assimilation technology. It has already been noted that whether a given NWP forecast produces a derecho is highly dependent on the quiescence in the warm sector ahead of a pre-existing cold front, which appeared to trigger the evolution of initial elevated convection to the robust surface-based convection that evolved into the mesoscale convective system that spawned the derecho. This work investigates other environmental and/or internal-storm processes that either enabled or precluded the derecho from being forecast. This work will be useful in determining the ability of sophisticated NWP to correctly forecast future extreme convective weather events.