Tropical cyclones (TCs) may spawn tornadoes either during or prior to landfall, when outer convective bands move over land. The synoptic and climatological conditions associated with TC tornadoes have been studied in the past for over 50 years. Examination of the characteristics of individual mesocyclones that produce tornadoes is, however, more recent. Doppler radar studies of TC mesocyclones since the mid-1990's have discussed their detailed features and enriched understanding of their morphology, contributing to the National Weather Service (NWS) warning process. Radar studies of TCs Gordon, Allison, Earl, Frances and Floyd have documented persistently high reflectivity regions accompanied by strong lateral shears within the outer bands, associated with mesocyclogenesis and tornado occurrence. High variability existed from one TC tornado case to the next for several relevant parameters, including maximum reflectivity, spatial extents of the mesocyclone circulations, vertical vorticity, storm relative helicity and Convective Available Potential Energy (CAPE).

Recently we have incorporated initial synoptic conditions of Earl in MM5 simulations for 2 September 1998 (1200 UTC) to produce short-term, mesoscale forecast fields across nested, two-way interactive grids. The largest MM5 grid, with a horizontal resolution of 81 km, covered the entire US; while 27, 9 and 3 km resolution grids handled Florida and South Carolina features associated with the tornadic eastern periphery of Earl. Many mesoscale processes were evident in the 9-km and 3-km grids. Confluence at 925 mb appeared in the short-term (6 h) forecasts near observed mesocyclone origins. High vertical vorticity values, on the order of 10-3 s-1, were registered at mesocyclone locations, and the mesocyclones’ horizontal dimensions were comparable to those observed. The forecast direction of motion of the mesocyclones also agreed with that observed on radar. Model forecast CAPE values were high around tornadic supercells.