Composite Analysis of the Mesoscale Environments Associated with Significant Nocturnal Tornadoes in the Plains

The danger of significant (EF-2 or greater) tornadoes to residents of the United States has been well documented. Of these, nocturnal tornadoes (defined here as occurring at least 1 hour after sunset) are a relative rarity in the Plains due to the decoupling of boundary layer winds after sunset, but are 2.5 times more likely to be deadly than their more frequent daytime counterparts. Because of the low frequency of nocturnal tornadoes in the Plains, forecasting and warning operations for these events can be particularly challenging. This study investigates the kinematic and thermodynamic parameters associated with significant nocturnal tornadoes occurring in the Plains from 1990 to 2007, with a total of 115 of these tornadoes in 76 separate events included in the analysis. Composite analyses are generated using North American Regional Reanalysis (NARR) data in order to investigate mesoscale parameters associated with the low-level near-storm environment in which the significant tornadoes occurred. WSR-88D data are also utilized to assess storm morphology in order to differentiate between tornadoes occurring with isolated cells and those occurring in mesoscale convective systems. Composite analysis was then used to identify common mesoscale features associated with these nocturnal events, and the mechanisms that allowed the boundary layer to remain coupled. An examination of the mesoscale environments resulted in the development of a conceptual model that Plains forecasters may utilize to better anticipate these events allowing for improved forecasts and warnings for nocturnal tornadoes in the Plains.