Hurricane Ivan Tornado Outbreak - Meteorological Synopsis

Meteorological Synopsis

See also: Meteorological history of Hurricane Ivan

On September 13, 2004, as Category 5 Hurricane Ivan moved through the Yucatan 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 250 mi (400 km) 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 encounted 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. Less than an hour after the watch was issued, the first tornado of the outbreak touched down in Escambia County, Florida. Over the course of the next 20 hours, the National Weather Service (NWS) office in Tallahassee monitored over 50 mesocyclones that had sufficient vorticity to spawn tornadoes. The high number of potential storms led to the issuance of 130 tornado warnings, of which only 20 later verified. One of the mesocyclones had a "nearly textbook appearance" and produced four tornadoes between 8:15 p.m. and 9:50 p.m. as it traveled nearly 75 miles across the Florida Big Bend. 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 touched down across three states.

With the tornado threat gradually moving northward, new tornado watches were issued further north into much of South Carolina. During the first half of September 16, several F0 tornadoes touched down across the Southeast. By the afternoon, downstream subsidence limited thunderstorm activity; however, as the day wore on, sufficient warm, moist air from the Gulf Stream and breaks in cloud cover allowed for CAPE values to rise. 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. The majority of the activity took place along a warm front that developed east of Tropical Storm Ivan within the shear maxima.

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.