89th American Meteorological Society Annual Meeting

Monday, 12 January 2009
The mega—tornado outbreak in a high—density built environment : An under—appreciated hurricane hazard: a Florida worst—case scenario
Hall 5 (Phoenix Convention Center)
Bartlett C. Hagemeyer, NOAA/NWS, Melbourne, FL
Major hurricane hazards include extreme wind, storm surge, flooding from extreme rainfall, and tornadoes. In the wake of relatively recent major disasters such as Hurricanes Katrina and Floyd, the potential for storm surge devastation and coastal and inland flooding is tragically all too clear. The threat from tropical cyclone tornadoes, especially in a high population density coastal state like Florida, is well known, but the risk for a major tornado disaster is under-appreciated. The author reinvestigated the tornado outbreak from Hurricane Agnes in June of 1972 that had previously been undocumented in the historical record. It was found that while Agnes had a small impact from surge and wind in the western Florida Panhandle where it made landfall, it caused the worst tropical cyclone tornado outbreak in Florida history.

In September 2004, major Hurricane Ivan made landfall in the far western Florida Panhandle. Outer rainbands in the right front quadrant impacted the Florida Panhandle and Big Bend inland to Georgia on September 15 and 16 for over 24 hours, causing at least 24 tornadoes that killed 8 people. Although there was one well-documented killer tornado in Panama City, most of the strong outer rainbands and tornadoes affected sparsely populated areas. Hurricane Ivan presented a unique societal issue in Florida for the first time: the continuous warning for dangerous tornadoes over a small area for a long period of time. From the afternoon of September 15 through the early morning of the 16 over 130 tornado warnings were issued over a small portion of Florida and adjacent Georgia. The warning effort was enabled by the technology of the WSR-88D radar network implemented in the 1990's across the nation. All of the tornado warnings were justified by the meteorological interpretation at the time, but what of the public response? The risk of desensitization to tornado watches has been a concern as they are generally issued for almost all landfalling tropical cyclones and kept up for many hours. However, the risk of tornado warning desensitization and fatigue during the 12 to 24 hours when people are traditionally focused on final preparation for hurricane landfall was exposed during Ivan for the first time in the modern era.

The great untold story of Hurricane Ivan is that as bad as Ivan's tornado outbreak was over a small area, the outer rainbands preceding Ivan missed the Florida Peninsula and rotated harmlessly over the Gulf of Mexico until striking the Panhandle and immediately producing tornadoes on the ground. If Ivan had been just a few degrees in longitude further east, the tornadic outer rainbands would have moved first across the Florida Peninsula. It would likely have been the greatest tornado outbreak in Florida history with nearly continuous, multiple tornado warnings in effect for over a day in high density urban areas. A simulation of this worst case scenario will be presented. The simulation raises many issues about societal preparation and response and policy concerns in the complex hurricane arena. The first step is to realize that such a worst-case scenario is indeed possible, and then to begin thinking about how to deal with it.

Supplementary URL: