EXTREME WEATHER EVENTS RESULTING FROM THE CUMULATIVE IMPACTS OF MULTIPLE- STORM SETS: CASE STUDY OF THE 500-YEAR 1999 FLOYD FLOOD IN NORTH CAROLINA

Extreme-tropical and extra-tropical storms are not frequent in North Carolina (NC). But clusters of smaller-scale storms that produce mega-storm impacts are common, and in concert with human modifications, can produce cumulative impacts resulting in extreme weather events. NC has a long history of extreme weather events without the occurrence of any category 5 storms and only a couple of historic storms estimated to be category 4 events that were actually part of storm sets. The compounding and cumulative impacts of multiple storms during at least 7 time-periods between 1899 and 1999, produced enormous human catastrophes, as well as substantial geologic change in the evolution of NC’s coastal system. These record 7 storm years include 1999, 1996, 1955, 1954, 1944, 1933, and 1899.

Case Study: The Sept.-Oct. 1999 Floyd Flood was a 500-year storm event. Hurricane Floyd itself was a relatively minor category 2 storm at landfall. The occurrence of 6 different weather events during a 6-week period produced the cumulative impact of an extreme weather event causing extensive barrier island destruction and 2 months of inland flooding in NC. The events included Hurricane Dennis, a category 2 storm that diminished to a tropical storm (TS) and spent 5 days off the NC coast building a sea state that caused extensive overwash and shoreline recession of the barrier islands. TS Dennis finally made landfall at Cape Lookout, along with a subsequent frontal system, dropped over 6 to 20 inches of rain on different parts of the Coastal Plain, filling the rivers well above flood stage. Hurricane Floyd then made landfall in the Cape Fear area with a 6 to 10 foot storm surge and another 12 to18 inches of rain as it slowly tracked slightly inland and shore-parallel. The result was severe erosion and damage to all of NC’s already weakened barrier islands. The Floyd storm was followed by an unnamed tropical depression and another frontal system that delivered enough rain to cause the river levels to crest and stay well above flood stage to October 10. The rivers were receding when Hurricane Irene arrived with enough rain to put the rivers back above flood stage which persisted until the end of October. The cumulative impact of these multiple events resulted in $6 billion in damage to the barrier islands and adjacent flooded lowlands.