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Hurricanes and Tornadoes
1. How are tropical cyclones different from tornadoes ?
While both tropical cyclones and tornadoes are atmospheric vortices, they have little in common. Tornadoes have diameters on the scale of 100s of meters and are produced from a single convective storm (i.e. a thunderstorm or cumulonimbus). A tropical cyclone, however, has a diameter on the scale of 100s of kilometers and is comprised of several to dozens of convective storms. Additionally, while 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, 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. These vertical shear values are indicative of the horizontal temperature fields for each phenomenon: tornadoes are produced in regions of large temperature gradient, while tropical cyclones are generated in regions of near zero horizontal temperature gradient. 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, or waterspouts do occurr). In contrast, tropical cyclones are purely an oceanic phenomena - they die over land due to the loss of a moisture source. Lastly, tropical cyclones have a lifetime that is measured in days, while tornadoes typically last on the scale of minutes. For more information on tornadoes you can go to the Storm Prediction Center's FAQ maintained by Roger Edwards.
Tropical cyclones at landfall often provide the conditions necessary for tornado formation. As the tropical cyclone makes landfall and begins decaying, the winds at the surface die off quicker than the winds at, say, 850 mb. This sets up a fairly strong vertical wind shear that allows for the development of tornadoes, especially on the tropical cyclone's right side (with respect to the forward motion of the tropical cyclone). For the southern hemisphere, this would be a concern on the tropical cyclone's left side - due to the reverse spin of southern hemisphere storms. (Novlan and Gray 1974).
2. Why do tropical cyclones spawn tornadoes?
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 tropical cyclones is focused at low altitudes, the storm cells tend to be smaller and shallower than those usually found in most severe midlatitude systems. But because the vertical shear in tropical cyclones 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).
3. What percentage of tropical cyclones spawn tornadoes ?
Almost all tropical cyclones making landfall in the United States spawn at least one tornado, provided enough of the tropical cyclones circulation moves over land. This implies that Gulf coast landfalling tropical cyclones are more likely to produce tornadoes than Atlantic coast tropical cyclones that sideswipe the coastline. The rate at which tropical cyclones produce 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 (Novlan and Gray 1974, Spratt et al. 1997).
4. What parts of a tropical cyclone are most favored for tornado formation ?
In the northern hemisphere, the right-front quadrant (relative to tropical cyclone motion) 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 50-200 miles from the tropical cyclone center, but some have been documented in the inner core, or even in the eyewall (Novlan and Gray 1974, McCaul et al. 1996, Spratt et al. 1997).
5. How long after landfall are tropical cyclone tornadoes a threat ?
Tropical cyclones may spawn tornadoes from a day or two prior to landfall to up to three days after landfall. Statistics show that most of the tornadoes occur on the day of landfall, or the next day. The most likely time for TC tornadoes is during daylight hours, although they can occur during the night, too (Novlan and Gray 1974, McCaul 1991) .
Although statistically the largest number of tropical cyclone tornadoes occurs on the day of landfall, some of the biggest and most damaging outbreaks have taken place 1 or 2 days after landfall, as with Beulah in 1967, Danny in 1985, and Beryl in 1994. In the case of Florida, with its peninsular shape, many of the tornadoes occur as the outer rainbands reach the state, well prior to the landfall of the tropical cyclone center (Hagemeyer and Hodanish 1995, Hagemeyer 1997).
6. Are tropical cyclone tornadoes weaker than midlatitude 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). In addition, there have been no F5-rated TC tornadoes since reliable records commenced in 1950, and only two F4s. There have, however, been numerous F3s, and some of these have caused many casualties and much damage. Of course, we cannot rule out the possibility that a future TC might spawn an F5 tornado (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, longer-lasting tornadoes. Hurricane Agnes in June 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 1998, Hagemeyer and Spratt 2002).
7. What is the largest known outbreak of tropical cyclone tornadoes?
2004's Hurricane Ivan caused an outbreak of 117 tornadoes - with the bulk of the tornadoes on 17 September - which developed over a 3 day period in the United States, including 37 in Virginia, 25 in Georgia, 18 in Florida, 9 in Pennsylvania, 8 in Alabama, 7 in South Carolina, 4 in both Maryland and North Carolina, 3 in West Virginia, and 2 in Maryland. There were 26 tornadoes reported on 15 September, 32 tornadoes on 16 September, 57 tornadoes on 17 September, and 2 tornadoes (in Maryland) on 18 September. At least 8 people were killed and 17 injured by the 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). These outbreaks of tornadoes from Ivan and Beulah represent two of the largest tornado outbreaks of any kind in the U. S. tornado climatology. It is difficult to predict which tropical cyclones will produce large tornado outbreaks, although there is some indication that the likelihood of a major outbreak increases as TC size and intensity increase.
8. What is the deadliest single TC-spawned tornado ?
One of the tornadoes spawned in October 1964 by Hurricane Hilda killed 22 people in Larose, LA ( Novlan and Gray 1974).
9. What is the most damaging single TC-spawned tornado ?
One of the tornadoes produced by Hurricane Allen in 1980 did about $100 million damage, in recent dollars, in the Austin, TX, area (Gentry 1983).
10. Why are tropical cyclone tornadoes especially difficult to forecast?
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).
11. Which states are most vulnerable to TC tornadoes outbreaks ?
Historical records show that the largest and most intense TC tornado outbreaks have occurred in Texas (Hurricane Carla in 1961, Beulah in 1967, Allen in 1980, and Gilbert in 1988), Louisiana (Hurricane Audrey in 1957, Carla in 1961, Hilda in 1964, and Andrew in 1992), Mississippi(Hurricane Audrey in 1957 and Andrew in 1992), Alabama (Hurricane Audrey in 1957, Danny in 1985, Andrew in 1992, and Georges in 1998), South Carolina (Tropical Storm Beryl in 1994), and Florida (Hurricane Agnes in 1972, Opal in 1995, and Tropical Storm Josephine in 1996). 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 tropical cyclone when landfall occurrs(McCaul 1991).
12. How does the damage from hurricanes compare to tornadoes?
Even though winds from the strongest tornadoes far exceed that from the strongest hurricanes, hurricanes typically cause much more damage individually and over a season. The strongest tornadoes - those of Fujita Tornado Damage Scale 4 and 5 - have estimated winds of 207 mph [333 kph] and higher, while the strongest hurricanes - those of Saffir-Simpson Hurricane Scale 4 and 5 - have winds of 131 mph [210 kph] and higher. Hurricanes in the continental U.S. cause on average about $3 billion per landfall and about $5 billion annually (Pielke and Landsea 1998). The roughly 1000 tornadoes that impact the continental U.S.each year cause about ten times less - about $500 million in total ( Brooks and Doswell 2001). The top 30 most damaging hurricanes in the last 100 years (normalized to account for higher population, wealth and inflation) have each caused over $2.9 billion (Jarrell et al. 2001). In comparison, only the most damaging tornado in the last 100 years or so - if it hit today - would cause about $2.9 billion in damage: the May 1896 St. Louis tornado (Brooks and Doswell 2001).
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. Tornadoes, in contrast, tend to be a mile or smaller in diameter, last for minutes and primarily cause damage from their extreme winds.
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.