Miniature supercells observed in an offshore outer rainband of Hurricane Gustav (2008)

Tornadoes spawned within the outer rainbands of landfalling tropical cyclones regularly pose a threat to coastal regions. These tornadoes, while less intense than their Great Plains counterparts, can still cause considerable loss of live and property damage. Much of the challenge faced by forecasters is to identify which convective cells will become tornadic. 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 the structure of offshore convective cells, in particular miniature supercells, before they move onshore and spawn tornadoes.

Hurricane Gustav made landfall along the Louisiana coast on the morning of 1 September 2008 as a Category 2 hurricane. Over 40 tornadoes were reported as Gustav crossed the lower Mississippi Valley region. The most significant tornadic activity occurred in the afternoon and evening of 1 September when an intense outer rainband developed ~300 km east of the storm center and produced 22 tornadoes over southern Alabama, Mississippi, and Louisiana.

This study documents the three-dimensional structure of convective cells embedded within Hurricane Gustav's outer rainband when the cells were more than 80 km offshore. An extensive observational dataset composed primarily of airborne NOAA WP-3D Doppler radar data and then supplemented with flight-level, dropsonde, rawinsonde, buoy, and WSR-88D radar data is used. The three-dimensional winds and convective structure within three deep convective cells are documented via dual-Doppler analyses. Each cell exhibited supercell characteristics (i.e., rotating updrafts) well offshore. The local environment was characterized by moderate low-level helicity and CAPE with relatively dry mid-levels. A synopsis of our preliminary results and their comparison with previous studies will be presented at the conference.