A Comparison of Two Different Tornadic Environments: Bertha ‘96 and Danny ‘97 Vs. Dennis ‘99 and Floyd ‘99

Abstract: During the years 1994 through 1999, the National Weather Service (NWS) in Wakefield, Virginia has experienced the effects of 6 tropical storms and hurricanes, in addition to several remnants of tropical systems. In addition to flooding and wind, one of the primary hazards associated with these storms have been tornadoes. Bertha ‘96, Danny ‘97, Dennis ‘99 and Floyd ‘99 each produced tornadoes, some with significant damage, in the NWS Wakefield, Virginia county warning area.

Prior research from NWS Wakefield, Virginia (Stuart and Cobb 1997, Cobb and Stuart 1998) has included radar observations and analysis of tornadic thunderstorms within the well-defined spiral bands of Bertha ‘96 and Danny ‘97. The spiral bands were characterized by a distinct dry slot in WSR-88D imagery, along the centerward edge of the precipitation. Lines of rotating thunderstorms were situated along the centerward edge of the spiral band. The dry slot and line of rotating thunderstorms within a dominant rainband have also been documented in other tropical systems (McCaul 1987, Spratt et al. 1998).

During Dennis ‘99 and Floyd ‘99, the distinct dry slot and line of rotating thunderstorms were absent. However, Dennis ‘99 and Floyd ‘99 produced a total of 3 tornadoes, one of which was rated F2 on the Fujita Scale.

This study will compare tornadic storm environments characterized by the dry slot and line of rotating storms, to environments characterized by isolated tornadic thunderstorms within a broad area of rain. Thermodynamic and shear characteristics of the environments will be compared through upper air data, SKEW-T and HODOGRAPH analyses of upper air soundings in the region, area WSR-88D VAD Wind Profiles, surface plots and surface observations. The goal of this study is to identify typical values of thermodynamic indices and wind shear for tornadoes with and without distinct dry slots as indicated by radar imagery.

Stuart, Neil A., Hugh D, Cobb, and Gregory J. Stumpf, 1997: A Comparison of the Storm-Scale Vortex Detection Capability, Between the WSR-88D Mesocyclone Detection Algorithm and the National Severe Storms Laboratory Mesocyclone Detection Algorithm During Tropical Storm Bertha. Preprints, 28th Conference on Radar Meteorology, Austin, TX, American Meteorological Society, 28, 361-363.

Cobb, Hugh D., and Neil A. Stuart, 1998: Characteristics of Tornadic Mesocyclones Associated with Tropical Cyclones in the Middle Atlantic Region as a Guide to Developing a Strategy for Warning Decision Using the WSR-88D. Preprints, 19th Conference on Severe Local Storms, Minneapolis, Minnesota, American Meteorological Society, 19, 124-127.

McCaul, Eugene W., 1987: Observations of the Hurricane "Danny" Tornado Outbreak of 16 August 1985. Mon. Wea. Rev. 115, 1206-1223.

Spratt, S.M., D. Sharp, F. Alsheimer, C. Paxton, A. Sandrik, and Pat Welsh, 1997: A WSR-88D Assessment of Tropical Cyclone Outer Rainband Tornadoes. Wea. Forecasting, 12, 476-498.