An Analysis of the Environmental and Physical Processes That Led to a Nocturnal Tornado in a Highly Stable Boundary Layer

On the evening of 9 July 2018, discrete supercells impacted northeast Montana and northwest North Dakota, producing large hail up to tennis ball size and measured wind gusts as high as 75 kts. Despite several environmental parameters being favorable for tornadogenesis prior to sunset, no tornadoes were observed during this time. At 0545 UTC 10 July 2018, as the lone remaining supercell was in the process of transitioning to a quasi-linear convective system with bowing segments, an EF-2 tornado occurred at Watford City, North Dakota, causing significant damage to an RV park. Twenty-eight people were injured and there was one fatality of a 6-day old infant. This tornado developed in an environment characterized by extreme low-level storm-relative helicity but also high convective inhibition. Furthermore, Watford City is located in a region where the minimum radar beam height is greater than 3,300 m above the surface, rendering velocity-based detection of rapid tornadogenesis nearly impossible. This presentation will seek to identify the mesoscale processes that caused this event and how these findings can be applied to similar scenarios, especially in areas with poor radar coverage. Focused attention will be given to hypothesizing why tornadogenesis occurred in an increasingly stable boundary layer and how storm mode may play an important role in this process.