Hurricanes are daunting enough to deal with no matter which area of the Globe a cyclone is impacting. Tornadoes embedded within a hurricane make these storms even more daunting. Here are some facts about tornadoes that occur within hurricanes.
The Difference Between A Tropcial Cyclone and A Tornado
Tornadoes have diameters on the scale of 100s of meters and are produced from a single convective storm (i.e. a thunderstorm or cumulonimbus). The strongest tornadoes – those of Fujita Tornado Damage Scale 4 and 5 – have estimated winds of 207 mph [333 kph] and higher. Tornadoes require substantial vertical shear of the horizontal winds (i.e. change of wind speed and/or direction with height) to provide ideal conditions for tornado genesis. Tornadoes are primarily an over-land phenomena as solar heating of the land surface usually contributes toward the development of the thunderstorm that spawns the vortex (though over-water tornadoes have occurred). Tornadoes typically last on the scale of minutes. The roughly 1000 tornadoes that impact the continental U.S. each year cause about ten times less – about $500 million in total. Tornadoes, in contrast, tend to be a mile or smaller in diameter, last for minutes and primarily cause damage from their extreme winds.
A tropical cyclone has a diameter on the scale of 100s of *kilometers* and is comprised of several to dozens of convective storms. The strongest hurricanes – those of Saffir-Simpson Hurricane Scale 4 and 5 – have winds of 131 mph [210 kph] and higher. Tropical cyclones require very low values (less than 10 m/s [20 kt, 23 mph]) of tropospheric vertical shear in order to form and grow. Tropical cyclones are purely an oceanic phenomenon – they die out over-land due to a loss of a moisture source, and have a lifetime that is measured in days Hurricanes tend to cause much more destruction than tornadoes because of their size, duration and variety of ways to damage items. The destructive circular eyewall in hurricanes (that surrounds the calm eye) can be tens of miles across, last hours and damage structures through storm surge, rainfall-caused flooding, as well as wind impacts. Hurricanes in the continental U.S. cause on average about $3 billion per landfall and about $5 billion annually.
Why Do Tropical Cyclones Spawn Tornadoes And How Often Does It Happen?
Tropical cyclones spawn tornadoes when certain instability and vertical shear criteria are met, in a manner similar to other tornado-producing systems. However, in tropical cyclones, the vertical structure of the atmosphere differs somewhat from that most often seen in midlatitude systems. In particular, most of the thermal instability is found near or below 10,000 feet altitude, in contrast to midlatitude systems, where the instability maximizes typically above 20,000 feet. Because the instability in TC’s is focussed at low altitudes, the storm cells tend to be smaller and shallower than those usually found in most severe mid latitude systems. But because the vertical shear in TC’s is also very strong at low altitudes, the combination of instability and shear can become favorable for the production of small supercell storms, which have an enhanced likelihood of spawning tornadoes compared to ordinary thunderstorm cells (Novlan and Gray 1974, Gentry 1983, McCaul 1991).
Almost all tropical cyclones making landfall in the United States spawn at least one tornado, provided enough of the TC’s circulation moves over land. This implies that Gulf coast landfalling TC’s are more likely to produce tornadoes than Atlantic coast TC’s that “sideswipe” the coastline. The rate at which TC’s produce tornadoes (waterspouts) over the ocean is unknown, although Doppler radars have identified many cases where storm cell rotation suggestive of the presence of tornadoes was observed over water, and there have been a number of cases where TC-spawned waterspouts have been witnessed from shore, with some of these coming ashore as tornadoes (McCaul, 1991).
What Parts of Tropical Cyclones Are Most Favored For Tornado Formation?
In the northern hemisphere, the right-front quadrant (relative to TC motion) of recurving TCs is strongly favored. In the southern hemisphere, the left-front quadrant presumably is favored, although there is little research on this point. Most of the tornadoes form in outer rainbands some 50-300 miles from the TC center, but some have been documented to occur in the inner core, or even in the TC eyewall (Novlan and Gray, 1974; McCaul, 1991).
How Long After Landfall Are Tropcial Cyclone-Spawned Tornadoes A Threat?
TC’s may spawn tornadoes up to about three days after landfall. Statistics show that most of the tornadoes occur on the day of landfall, or the next day. However, many of the largest outbreaks have occurred two days after TC landfall, as the TC remnants interact with mid latitude weather systems. The most likely time for tornadoes is during daylight hours, although they can occur during the night too (McCaul, 1991).
Are Tropical Cyclones Tornadoes Weaker Than Mid-Latitude Tornadoes?
In general, it appears that TC tornadoes are somewhat weaker and briefer than midlatitude tornadoes. During the period 1948-1986, the percentage of TC tornadoes that reached F2 or greater intensity on the Fujita scale was 26% (McCaul et al., 2004), while during a roughly comparable period (1950-1976), the corresponding percentage for all U.S. tornadoes was 36% (Kelly et al., 1978)(Gentry 1983; McCaul 1991).
In Florida, in particular, the most significant tornadoes tend to occur with “hybrid” cyclones or tropical cyclones with some hybrid influence. This usually means greater westerly shear in the storm environment which is believed to be favorable for stronger, long-lasting tornadoes. Hurricane Agnes in 1972 was a minimal category 1 hurricane with considerable hybrid influence and it produced the most F2 and greater tornadoes in a single day in Florida history (Hagemeyer 1997; Hagemeyer and Spratt, 2002).
Why Are Tropical Spawned Tornadoes Especially Difficult To Deal With?
TC tornadoes are often spawned by unusually small storm cells that may not appear particularly dangerous on weather radars, especially if the cells are located more than about 60 miles from the radar. In addition, these small storms often tend to produce little or no lightning or thunder, and may not look very threatening visually to the average person. Furthermore, the tornadoes are often obscured by rain, and the storm cells spawning them may move rapidly, leaving little time to take evasive action once the threat has been perceived. (McCaul et al. 1996, Spratt et al. 1997).
What is the Largest Known Outbreak of Tropical Cyclone Spawned Tornadoes?
2004’s Hurricane Ivan caused a multi-day outbreak of 127 tornadoes, with the bulk of the tornadoes on 17 September in the mid-Atlantic region, some two days after Ivan’s landfall in Alabama. State-by-state tornado counts from Ivan include Florida with 22, Georgia 25, Alabama 8, South Carolina 7, North Carolina 4, Virginia 40, West Virginia 3, Maryland 9, and Pennsylvania 9. There were 26 tornadoes on 15 September, 32 on 16 September, 63 on 17 September, 2 on 18 September, and 4 on 19 September. At least 7 people were killed and 17 injured by these tornadoes.
The previous record was during Hurricane Beulah, which spawned a reported 115 tornadoes in southeast Texas during the first several days after its landfall in September 1967 (Orton 1970). Frances of 2004 is close behind in third place, with 106 tornadoes, and Rita of 2005 is in fourth place with 92.
While it is difficult to predict which TCs will produce large tornado outbreaks, there is evidence suggesting that the likelihood of a major outbreak increases for TCs that are large, intense, are recurving and entering the westerlies, have forward speeds from about 8-18 mph, and are interacting with old, weakened frontal boundaries. In addition, the TC’s right-front quadrant must receive significant exposure to land, and this strongly favors TCs making landfall on the Gulf coast as opposed to those grazing the Carolinas (McCaul, 1991; McCaul et al., 2004).
What is the Single Deadliest Tropical Cyclone Spawned Tornado?
One of the tornadoes spawned in October 1964 by Hurricane Hilda killed 22 people in Larose, LA (Novlan and Gray 1974).
What is the Single Costliest Tropical Cyclone Spawned Tornado?
One of the tornadoes produced by Hurricane Allen in 1980 did about $50 million damage (1980 dollars; about $127 million damage in 2005 dollars) in the Austin, TX, area. More recently, Hurricane Cindy spawned a strong tornado that damaged the Atlanta Motor Speedway and other nearby areas to the tune of some $71.5 million in July 2005.
What States Are Most Vulnerable for TC Tornadoes?
Historical records show that the largest and most intense TC tornado outbreaks have occurred in states bordering the Gulf coast and the Atlantic coast from Virginia southward. The biggest outbreaks have occurred (starting from west to east, not in order of outbreak size or severity) in Texas (from Carla in 1961, Beulah in 1967, Allen in 1980, Alicia in 1983, and Gilbert in 1988), Louisiana (Audrey in 1957, Carla in 1961, Hilda in 1964, Andrew in 1992, and Lili in 2002), Mississippi (Audrey in 1957, Andrew in 1992, and Rita in 2005), Alabama (Audrey in 1957, Danny in 1985, Georges in 1998, Cindy in 2005, and Rita in 2005), Georgia (Ivan in 2004, Cindy in 2005, Katrina in 2005), Florida (Agnes in 1972, Opal in 1995, Josephine in 1996, Charley in 2004, Frances in 2004, and Ivan in 2004), South Carolina (Beryl in 1994, Frances in 2004, Jeanne in 2004), North Carolina (Floyd in 1999, Frances in 2004), Virginia (Gracie in 1959, David in 1979, Frances in 2004, Gaston in 2004, and Ivan in 2004). The Gulf coast states tend to have the most frequent and significant TC tornado events, partly because of their tendency to have at least one state fully exposed to the right-front quadrant of the TC when landfall occurs there (McCaul 1991). However, the mid-Atlantic states can also get major outbreaks if the parent TC moves far enough inland during recurvature.
What About Florida? They get the Most Hurricane Strikes, do They Also Get A Lot of TC Spawned Tornadoes?
Florida is no stranger to significant TC tornado activity. Among the larger outbreaks in recent Florida history are those produced by Agnes in 1972 (Hagemeyer 1997; Hagemeyer and Spratt 2002), Opal in 1995 (Sharp et al., 1997), and Charley, Frances and Ivan in 2004. Florida also gets many tornadoes from subtropical storms or TCs having hybrid characteristics, such as Josephine in 1996.
Brooks, H. E., and C. A. Doswell, III, 2001: Normalized damage from major tornadoes in the United States: 1890-1999. Wea. Forecasting , 16, 168-176.
Jarrell,J.D., M. Mayfield, E.N. Rappaport, and C.W. Landsea, 2001: “The Deadliest, Costliest, and Most Intense United States Hurricanes from 1900 to 2000 (and other Frequently Requested Hurricane Facts)”NOAA Technical Memorandum NWS/TPC-1