An observationally based hypothesis for significant tornadogenesis in mountain environments

Since low-relief terrain characterizes the topographic environments beneath the great majority of tornadic supercell thunderstorms, mountainous terrain is widely regarded as a strong inhibitor of tornado occurrence. In the Rocky Mountains and Appalachians of North America, the European Alps and other mid-latitude regions characterized by complex topography, the potential is large for landforms to disrupt low-level flows and preclude tornadogenesis within synoptic environments that might otherwise yield tornadoes over flat terrain. On rare occasions, however, large, intense (F2-F4) and long-lived tornadoes have been documented to form from supercells propagating over regions characterized by high topographic relief. Two such events in the Northeastern United States are examined in companion papers (by Bosart et al. and LaPenta et al.). The present study aims to identify common characteristics of these and other comparable events towards developing a hypothesis explaining the rare process of major tornadogenesis within mountain environments.

Long-lived and intense tornadoes that occurred over hilly terrain amid landforms exhibiting topographic relief ?150 m have occurred in the United States roughly once every two years over the past two decades. The two case studies detailed provide compelling evidence that rather than acting as inhibitors, terrain influences actually play a deterministic role in significant mountain tornado occurrence. We hypothesize that three contributing factors must coexist to overcome the topographic inhibition that usually precludes major tornadogenesis: (1) A mesoscale environment supportive of supercell thunderstorm development according to conventional indicators of wind shear and static stability; (2) Modifications to the low-level wind field by topographic configurations in favored locations such as valley confluences that create local orographic enhancements to tornadogenesis potential; (3) Arrival of a channeled outflow surge, originating from either the supercell or other nearby convection, beneath the mesocyclone of a mature supercell that provides the catalyst to overcome frictional disruptions by topography for tornadogenesis.