On September 13, 2004, as
Category 5 hurricane Ivan moved through the
Yucatán Channel, the
Storm Prediction Center noted the possibility of isolated tornadoes for parts of
Louisiana and the
Florida Panhandle three days out. Though the storm weakened while approaching the United States, strong low-level
wind shear along its northeastern periphery allowed for the development of potentially tornadic
supercells. A
National Oceanic and Atmospheric Administration Hurricane Hunter aircraft intercepted Ivan during the afternoon and evening of September 15. During the mission,
dropwindsondes were deployed off the Gulf Coast to assess the
rainband environment. By this time, an intense band of thunderstorms, with embedded supercells, developed about east of the hurricane's center. A
thermodynamic sounding around 1:00 p.m.
EST from
Tampa Bay, Florida revealed unusually favorable conditions for deep, rotating
convection. Ahead of the band,
convective available potential energy (CAPE) levels reached 2,500
J/
kg and significant
helicity. The significant levels of CAPE, accompanied by a dry air intrusion between the rainband and the main convection of Ivan, were the primary cause of the outbreak. A comparison between Ivan and
Hurricane Jeanne revealed that both storms encountered similar conditions near landfall; however, CAPE values were lower during Jeanne and the system produced far fewer tornadoes. In light of the increased tornado threat, the first
tornado watch was issued around 1:00 p.m.
EST for the Florida Panhandle and southern
Alabama. By then, "miniature" supercell thunderstorms began approaching the coastline. Data from
Doppler weather radar indicated
mesocyclones within several of the cells, though mostly while over water. At the onset of the outbreak, three cells exhibited signature characteristics of supercell thunderstorms: cyclonic
updraft and a
hook-like appendage. Similar to storms over the
Great Plains, a mid-level
rear flank downdraft was present; however, this feature did not reach the surface. Two of these tornadoes reached
F2 intensity and, collectively, they killed four people, injured eight more, and left $5.5 million in damage. By midnight on September 15, 26 tornadoes had touched down across three states. During the first half of September 16, several F0 tornadoes touched down across the
Southeast. Extensive convective banding to the north and east of the storm continued to encounter extreme levels of helicity. Subsequently, numerous rotating thunderstorms developed over the region, especially across
Georgia and
South Carolina. Throughout the afternoon, 22 tornadoes touched down across the two states, one of which, an F2, killed one person. Following a lull in activity during the overnight hours between September 16 and 17, conditions once more became increasingly favorable for tornadoes across
North Carolina and
Virginia. Strong shear associated with the weakening tropical cyclone and the destabilization of air ahead of the main rainband and consequently, a tornado watch was issued for portions of both states around 10:00 a.m. ==Tornado event==