FAQ: Hurricanes Versus Tornadoes
How are tropical cyclones different from tornadoes?
Contributed by Chris Landsea
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 have
occurred). In contrast, tropical cyclones are purely an oceanic phenomena -
they die out over-land due to a 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.
An interesting side note is that 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)
Why do tropical cyclones spawn tornadoes?
Contributed by Bill McCaul
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 midlatitude 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).
What percentage of tropical cyclones spawn tornadoes?
Contributed by Bill McCaul
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 (Novlan and Gray 1974,
Spratt et al. 1997).
What parts of a tropical cyclone are most favored for
tornado formation?
Contributed by Bill McCaul
In the northern hemisphere, the right-front quadrant (relative to TC 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 TC center, but
some have been documented to occur in the inner core, or even in the TC eyewall
(
Novlan
and Gray 1974,
McCaul et al. 1996,
Spratt et al. 1997).
How long after landfall are TC tornadoes a threat?
Contributed by Bill McCaul and Bart Hagemeyer
TC's 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 TC tornadoes occurs on the day of
TC 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 shpe, many of the tornadoes
occure as the outer rainbands reach the state, well prior to the landfall of
the TC center (
Hagemeyer
and Hodanish 1995, Hagemeyer 1997).
Last updated August 13, 2004
Are TC tornadoes weaker than midlatitude tornadoes?
Contributed by Bill McCaul and Bart Hagemeyer
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 F4's. There have,
however, been numerous F3's, 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).
Last updated August 13, 2004
What is the largest known outbreak of TC tornadoes?
Contributed by Bill McCaul, Chris Vaccaro and the National Hurricane Center
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 TCs will produce large tornado outbreaks,
although there is some indication that the likelihood of a major outbreak
increases as TC size and intensity increase.
What is the deadliest single TC-spawned tornado?
Contributed by Bill McCaul
One of the tornadoes spawned in October 1964 by Hurricane Hilda killed 22
people in Larose, LA (
Novlan and Gray 1974).
What is the most damaging single TC-spawned tornado?
Contributed by Bill McCaul
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).
Why are TC tornadoes especially difficult to deal with?
Contributed by Bill McCaul
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).
Which states are most vulnerable to TC tornadoes
outbreaks?
Contributed by Bill McCaul
Historical records show that the largest and most intense TC tornado outbreaks
have occured 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 quadrent of the TC when landfall occures there (
McCaul
1991).
Last updated August 13, 2004
What about Florida? They get the most hurricane strikes; do they also get a
lot of TC tornadoes?
Contributed by Bill McCaul
Florida is no stranger to significant TC tornado activity. Perhaps the largest
TC tornado outbreak in recent Floirda history was that spawned by Hurricane
Agnes in 1972 (
Hagemeyer
1998, Hagemeyer and Spratt 2002) . Hurricane Opal in 1995 also
generated many tornadoes in the Florida panhandle when coming ashore (
Sharp 1997). Florida also gets many tornadoes from subtropical
storms or TCs having hybrid barotropic/baroclinic characteristics, such as
Josephine in 1996 (
Spratt et al. 1998).
Last
updated August 13, 2004
How does the damage from hurricanes compare to tornadoes?
Contributed by Chris Landsea
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.
References:
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.