Short-term forecasting of tornadic environments in the complex terrain of western North Carolina

On the morning of May 6, 1999, an F1 tornado struck the city of Asheville, NC in the predawn hours. Tornadoes are a rare occurrence in the mountainous terrain of western North Carolina, where sharply varying elevations span some 4000 feet. Accurately forecasting the storm environment that will exist over such complex terrain when convection is expected is quite challenging. The rugged terrain tends to play havoc with the shear profile, rapidly altering the low-level wind field experienced by developing storms. Numerical models generally lack the resolution to predict the surface and boundary layer wind flow in such topography, and often struggle with the static instability. The May 6, 1999 Asheville tornado case was examined to look for any short-range model trends toward a tornadic environment which may have been evident. Comparison of certain features of the 00 UTC May 6, 1999 Eta model run to the subsequent 06 UTC Rapid Update Cycle (RUC) model run was carried out to look for developing features that would support the formation of mesoscale convective systems and non-supercellular tornadoes. It was seen that the later RUC model run suggested a forecast of much stronger instability for the time of convection- as seen in the vertical lapse of equivalent potential temperature. Consideration was also given to the unique north-northwest to south-southeast orientation, and the relatively wide expanse, of the French Broad River Valley in which Asheville sits. This orientation permits southwesterly low-level synoptic-scale gradient flow to back to the southeast as it flows through the valley. This backing in the low-level flow generally increases the storm-relative helicity experienced by a northeastward moving storm, and may yield a sudden, but sustained, increase in streamwise vorticity. It is suggested that a close examination of short-range model trends combined with a detailed knowledge of the complex terrain can assist a mesoscale forecaster to better anticipate storm type and preferred locations for tornadogenesis in the high country of North Carolina.