Tornadoes spawned within the outer rainbands of landfalling tropical cyclones regularly pose a great threat to coastal regions. These tornadoes, while less intense than there Great Plains counterparts, can still cause considerable loss of live and property damage. In fact, between 1970 and 2000, tornadoes were responsible for ~5% of all deaths and ~13% of the total insured damage caused by landfalling hurricanes along U.S. coasts. Much of the challenge faced by forecasters is to identify which convective cells will become tornadic out of the hundreds that often move onshore. If common distinguishing features of the tornadic cells can be identified while the cells are still offshore and nontornadic, then forecasters may be able to increase their lead time and better forewarn the public. The objective of this study is to document defining characteristics of tornadic convection prior to tornadogenesis within the rainbands of landfalling tropical cyclones.

This study utilizes an extensive observational dataset composed primarily of airborne NOAA WP-3D Doppler radar data and then supplemented with surface (both onshore and offshore), rawindsondes, GPS dropsondes, and aircraft flight-level data from Hurricanes Ivan and Jeanne of 2004. The three-dimensional wind fields of deep convective cells within offshore hurricane rainbands are documented via dual-Doppler analyses. The thermodynamic structure is inferred from flight-level, GPS dropwindsonde, and surface (buoy) observations. Several cells in an outer rainband in Ivan exhibited supercell characteristics (i.e., rotating updrafts and hook echoes) well offshore. The local environment was characterized by moderate low-level helicity and CAPE with relatively dry mid-levels. These cells two produced tornadoes soon after moving onshore. These cells will be contrasted with cells within one of Jeanne's rainband that did not produce tornadoes after moving onshore. A synopsis of our preliminary results and their comparison with previous studies will be presented at the conference.