Mesoscale environments and radar characteristics of three late evening Appalachian tornadic supercells in early May 2009

Over a five day period in early May 2009, three separate tornadic supercells impacted the Appalachian mountains of southwest Virginia (VA) and northwest North Carolina (NC). Each tornado occurred at an elevation of approximately 750-800m MSL. This is not only an unfavorable geographic location for tornadic activity based on long term historical records, but two of the tornadoes occurred at atypical times of day for this region; late evening hours (2130-2245 local standard time) which is outside the strong diurnal signal favoring the afternoon and early evening. The tornado that occurred latest in the evening (8 May 2009) was also the strongest (EF2), resulting in several injuries, and was the first documented tornado in Alleghany County NC since a brief F0 tornado in 1973. In fact, only one other tornado had ever been reported since 1950 in the two mountain counties to the north and south, Grayson County VA and Ashe County NC respectively.

The environments associated with these three events shared some common meteorological features, including a mid level short wave trough approaching from the west, strong low-level wind shear (0-3km storm-relative helicity greater than 250 m2s-2) but weak instability (surface-based convective available potential energy less than 500 Jkg-1), and low Lifting Condensation Level heights (below 800 m AGL). Differences in the environments were related to the orientation of the steering flow, the presence of a boundary (in at least one of the cases), and potential influences of local terrain in the tornadogenesis process. Radar characteristics will also be examined briefly, but many observed differences between the storms were due more to sampling at different ranges from the radar. Each parent storm contained notable supercell structures, with the 3 May 2009 and 6 May 2009 tornadic storms arguably being characterized as “mini-supercells”. The deeper 8 May 2009 storm was a longer-lived classic supercell, with at least one brief tornado touchdown in the western slopes of the Appalachians, before crossing higher ridges and finally producing the stronger tornado (with an 8km track) as the storm began to collapse on the eastern edge of the Appalachians.

The motivation for this study is first to document these climatologically atypical events, which were also unique in that they all occurred within a few days of each other. Secondly, it serves as a reminder for local forecasters of the importance in recognizing environments highly favorable for supercell and tornado formation, regardless of the location and time of day. Armed with a heightened situational awareness, including the potential impacts of orography, even relatively small supercells with subtle radar-based signatures can be more easily recognized and warned for given careful and continuous scrutiny. Still, these kinds of events remain particularly challenging to anticipate and detect in an operational environment, especially considering the relative data paucity within the Appalachians, and the degree to which complex terrain may impact storm morphology and tornadogenesis, as well as radar sampling.