Tornadoes in the United States

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Tornadoes in the United States 1950-2019
A tornado strikes near Anadarko, Oklahoma. This was part of the 1999 Oklahoma tornado outbreak on May 3, 1999.

Enhanced Fujita Scale—occur more often in the United States than in any other country.[3]

Most tornadoes in the United States occur east of the

Mississippi Valley and the southern United States are all areas that are vulnerable to tornadoes. They are relatively rare west of the Rockies and are also less frequent in the northeastern states. Tornado Alley is a colloquial term for an area particularly prone to tornadoes. There is no officially defined 'Tornado Alley' – at its broadest this area stretches from northern Texas to Canada with its core centered on Oklahoma, Kansas and northern Texas. Another highly significant region – colloquially known as Dixie Alley – is the southern United States and particularly the northern and central parts of Alabama and Mississippi. Florida
is one of the most tornado-prone states. However, Florida tornadoes only rarely approach the strength of those that occur elsewhere.

Although favorable conditions for tornadoes in the United States can occur at any time, they are most common in spring and least common in winter.

landfalling tropical cyclones, which usually occur in late summer and autumn. In the United States, thunderstorms capable of producing tornadoes usually form when the temperature is at its highest, typically from 4:00 p.m. to 7:00 p.m.[6][7]

Although the period in which most tornadoes strike ("

tornado season") is March through June, tornadoes – including violent tornadoes and major tornado outbreaks – have been documented in the United States during every month and day of the year.[8] Two examples of this are when a series of tornadoes hit the state of Indiana on November 22, 1992, and injured at least nine people.[8] Another notable non-season tornado was where a tornado struck the area of McLean County, Illinois. Even though the tornado was during a winter month, it blew 20 railroad cars off their tracks, and hauled a camper over 100 yards (91 m).[8]

During the winter months of the year, tornadoes have been known to hit the

May 31, 1985 outbreak, which killed 76 across Ohio and Pennsylvania, as well claiming 12 victims in Ontario, Canada. It was also the deadliest outbreak in both Tennessee and Kentucky since the 1974 Super Outbreak.[10]

Usually, tornadoes hit specific areas of the United States in specific seasons. During the winter months, tornadoes are usually spotted in the Southern area of the country, as well as states near the

Gulf Coast.[11] As spring comes, hot air progressively moves back into the Gulf Coast. This pushes the mass of colder air forward out of the Gulf States and into the Southeastern states, where tornado frequency is highest in April.[11]

As spring passes and summer begins, the mass of warm moist air moves northwest into the

Bermuda High sitting over parts of the United States.[11] Though there may be some thunderstorms, they don't often become severe enough to spawn tornadoes.[11]

Tornadoes may be formed out of season, especially during the months of hurricane season in the Gulf Coast states and Southeastern states. Because these areas are prone to hurricanes, they may be struck with tornadoes that are spawned from hurricanes.[12] Tornadoes are most likely to form in the right-front quadrant of the hurricane, but can also form in rain bands associated with the storm.[12] This is caused by the large amount of vertical wind shear to the right of the storm.[13] Tornadoes are also spawned from U.S. hurricanes due to the moistness of the air at the landfall of the storm, which makes conditions favorable for a supercell storm to develop within the hurricane.[14]

Regional activity

Average annual tornado reports in the United States

Tornadoes have been documented in every U.S. state (not including the non-state territories of

hurricanes that strike the state.[18] The state with the highest number of strong tornadoes per unit area is Oklahoma, with the state's Cleveland County having the most tornadoes per square mile.[19] States such as Oklahoma and Kansas have lower population densities, so tornadoes in those states, while severe, may cause somewhat less property damage.[20]

Midwest

The Midwestern states are very prone to tornado activity, as they are part of "Tornado Alley."[21] Some states in the area that may be hit by tornadoes frequently include Oklahoma, Kansas, Illinois, Indiana, and Iowa.[21] The Midwestern states are often hit by tornadoes during the late spring and early summer months, especially the months of May and June.[11] This is due to the aforementioned air mass moving northward into the Midwest and combining with another air mass that move southward from Canada.[11] This creates unstable air, creating the potential for storms to strike the most during these months.[11] The frequency begins to decrease in the middle of the summer, as the air mass moves northward into the Great Lakes states.[11]

Doppler On Wheels unit observing a tornado near Attica, Kansas

A very notable recent storm that has hit this area was the Northwood, Kansas tornado, in the

Enhanced Fujita Scale (the highest possible rating). Based on the damage produced, winds inside the tornado were estimated to have been in excess of 205 mph (92 m/s; 330 km/h).[23]

New Richmond Tornado of May 1899 and the Flint, Michigan tornado of June 1953 also rank amongst the deadliest tornadoes in US history.[26] The region was badly hit by the Palm Sunday Tornado Outbreak in April 1965 and by the Super Outbreak of April 1974. According to NCDC figures for the 1950 to 2006 period, Nebraska reported 2,440 tornadoes followed by Iowa (2,185), Illinois (2,086), Missouri (1,922), South Dakota (1,487), Minnesota (1,477), Indiana (1,327), North Dakota (1,216), Wisconsin (1,185), Michigan (981), and Ohio (916).[15]

South

The Southern United States has suffered more tornado fatalities than any other part of the country.[27] Some areas experience repeated damaging tornado events, such as the Tennessee Valley and in northern Alabama. The state of Alabama is tied for the most reported F5 tornadoes.[28] For the period 1950 to 2006, three hundred and fifty eight people were killed by tornadoes in Alabama, ranking the state third nationwide behind Texas (521) and neighboring Mississippi (404).[15] Fourth is Arkansas (336) and fifth is Tennessee with 271 fatalities.[15]

Tornado disasters to affect the southern USA include the

Super Outbreak of 1974. The 2008 Super Tuesday tornado outbreak was the deadliest tornado outbreak in the US in 23 years.[26] 58 people lost their lives with Tennessee, Arkansas, and Kentucky particularly badly affected. The 2011 Super Outbreak
devastated much of the South, leaving 348 people dead.

Hurricanes and other tropical storms can also generate tornadoes. Louisiana and the coastal regions of Mississippi and Alabama are most at risk from these storms. The Atlantic seaboard states can be affected too. According to NCDC figures for the 1950 to 2006 period, Mississippi reported 1,787 tornadoes, followed by Louisiana (1,644), Alabama (1,608), Arkansas (1,579), Georgia (1,324), North Carolina (1,042), Tennessee (892), South Carolina (819), Kentucky (710) and Virginia (565).[15] West Virginia, by contrast, is one of the least vulnerable states of all with just 120 tornadoes reported over the period.[15]

East and Northeast

A waterspout near Florida

The Northeastern U.S. to the east of the Appalachian Mountains is much less vulnerable to violent tornadoes but is by no means immune. One of the most extraordinary tornadoes in history struck Worcester, Massachusetts. This F4 tornado struck the city on 9 June 1953 and killed 90 people.[29] The tornado was generated by the same storm system that hit Flint, Michigan with a devastating F5 tornado that killed 116 people.[29]

Areas further south – notably

District of Columbia (1).[15]
The worst tornado outbreak in the Northeast occurred in Pennsylvania on
May 31, 1985, and produced the only F5 tornado in the region to date.[30]

Hurricanes and other tropical storms can generate large numbers of tornadoes.[31] Non-supercell tornadoes are rarely as strong as supercell-generated storms.[32]

Florida's most violent tornadoes generally occur during the winter months, when the state is most vulnerable to invasions of cold air that help generate such storms.

2007 Central Florida Tornadoes.[34]

West and Southwest

Of the states around the Rocky Mountains, Colorado reports by far the greatest numbers of tornadoes. Eastern Colorado, both climatically and physically, has much more in common with the neighboring Plains states of Kansas and Nebraska than with the mountainous areas further west. Small cities such as Limon, Kit Carson, Thurman, and Flagler are some places in Eastern Colorado that have experienced dangerous tornadoes.[35] The same can be said, to a lesser extent, of eastern Wyoming. Tornadoes are less frequent in mountainous areas.[36] Of the states in this region – according to NCDC figures for the 1950 to 2006 period – Colorado reported 1,617 tornadoes, followed by Wyoming with 560, Montana (345), Idaho (175), and Utah (114).[15]

In the Southwestern United States, New Mexico reported 485 tornadoes during the 1950 to 2006 period (NCDC figures), California reported 355, Arizona reported 209, and Nevada reported 75.[15] New Mexico borders the notoriously tornado prone states of Texas and Oklahoma, hence the noticeably higher figures. Most of the tornadoes here happen in the very eastern part of the state. New Mexico's region and the desert landscape of the state help prevent the amount of twisters that happen in Oklahoma and Texas.[15] Arizona and New Mexico experience regular summer thunderstorms during their monsoon season. These are sometimes tornadic but rarely produce violent tornadoes.

The

deadly tornado on 5 April 1972. This storm was the deadliest to hit the United States that year.[37]

Intensity

A powerful tornado hits the town of Dimmitt, Texas.

Tornadoes' damage varies based on their wind speeds and where they strike. The

Enhanced Fujita Scale classifies a tornado by its damage and then uses that classification to estimate the tornado's wind speed. For example, if it causes very little damage, then it is classified as an EF0 tornado and likely had very low winds. If damage from the tornado is severe enough, then it may be rated and likely had winds exceeding 200 mph (89 m/s; 320 km/h). In the United States, tornadoes have been known to form at high and low intensities. The probability of a high intensity or violent tornado differs by location across the country.[38] For example, due to the high frequency of tornadoes in the "Tornado Alley" area mentioned before, a more violent tornado would be more likely to form there due to the strength of the thunderstorms produced by the two bordering air masses.[39] The states with the highest number of F5 and EF5 rated tornadoes since data was available in 1950 are Alabama and Oklahoma, each with seven tornadoes. Iowa, Kansas, and Texas each are tied for second-most with six.[28] The state with the highest number of F5 and EF5 tornadoes per square mile, however, was Iowa.[40] The state with the most number of tornadoes classified as "violent", or F4 and F5, is Kentucky, and the state with the highest average intensity ranking for tornadoes is Alabama.[40]

Tornadoes that are classified as EF4 and EF5 (or "violent tornadoes") on the

Enhanced Fujita Scale only account for an average of two percent of all tornadoes in the United States each year.[41] However, these high-intensity storms do account for an average of seventy percent of all tornado-related deaths in the United States each year.[42] These storms can have winds of over 200 miles per hour (320 km/h) and stay on the ground for over an hour.[43]

The United States has seen 59 F5 and EF5 (the highest intensity and damage ranking) tornadoes since records began in 1950.[28] No tornadoes prior to 1950 were officially ranked F5, due to inadequate engineering data and other information on the historical tornadoes. From 1950–1970 tornadoes were assessed retrogressively, primarily using information recorded in government databases, and newspaper photographs and descriptions. Beginning in 1971, tornadoes were rated by the NWS using on-site damage surveys.[44]

On May 3, 1999, a storm produced a violent tornado in the vicinity of Oklahoma City and was a part of the 1999 Oklahoma tornado outbreak, which also brought many violent storms to the states of Kansas, Arkansas, and Tennessee.[45] In total, the storm produced 66 tornadoes over four states.[45] The strong F5 storm took place near the town of Bridge Creek, Oklahoma, located in Grady County.[45] During the storm, a group of students in meteorology from the University of Oklahoma had chased the storm on a Doppler on Wheels (or DOW), which scans storms while attached to a car.[45] Doppler on Wheels data from the nearby storm revealed winds of 301 ± 20 miles per hour (484 ± 32 km/h), which was over the wind speed in the F5 classification of that time.[45] Though the wind speed was over the F5 maximum, the tornado was not named the first ever F6 storm, as there was no F6 classification.[45] Other scientists reviewed the DOW data taken by the students, and concluded that the estimated wind speed may have been inaccurate, but still over 300 miles per hour (130 m/s).[45] Also, the wind speed recorded was taken from over 200 feet (61 m) above ground level.[45] The National Weather Service also did a damage check, and found that the damage was that of an average F5 tornado.[45]

Impacts

Injuries and fatalities

In the United States over 80 deaths and 1,500 injuries are associated with tornadoes each year.

National Oceanic Atmospheric Administration, most tornado deaths are caused by people not following instructions on what to do the right way.[3] They also mention that some people are not even warned that a tornadic storm is coming, while others get a warning but do not believe that a tornado will hit their area.[3] In 2012, 68 people were killed by tornadoes in the United States.[47] Kentucky had the most fatalities, 22.[47] Kentucky was followed by Indiana (13), Illinois (9), Missouri and Oklahoma (6), Ohio (4), Alabama and Tennessee (3), and Florida and Kansas (1).[47]

Over 39 percent of all tornado-related deaths and many injuries come from residents of mobile homes.[48] It is a widespread myth that tornadoes are "attracted" to mobile home parks, and cause the most fatalities there because they hit there the most.[48] This is not true. However, mobile homes do often have poor construction and do not provide adequate protection during a tornado event.[48] An example to confirm this occurred in April 1991, when a very powerful tornado struck the town of Andover, Kansas. When the town was hit, eighty-four homes and fourteen businesses were destroyed by the tornado, but with no fatalities and only minor injuries.[49] After the tornado hit Andover, it swept through a mobile home park consisting of 223 trailers.[49] The park did have a tornado shelter, to which over 200 residents fled and survived without injuries.[49] Others that did not take shelter in the tornado shelter stayed in their mobile homes, and thirteen of them were killed by the tornado.[49]

Damages

On May 20, 2013, a large tornado of the highest category, EF5, ravaged Moore, Oklahoma.

The

Enhanced Fujita Scale
classifies tornado strength from weakest - an EF0 tornado - to strongest - an EF5 tornado - based on damage caused to property and infrastructure.

An EF0 tornado has estimated wind speeds from 65–85 mph (29–38 m/s; 105–137 km/h) which usually results in minor structural damage and broken tree limbs.

An EF5 tornado has estimated wind speeds of greater than 200 mph (89 m/s; 320 km/h) and can destroy reinforced concrete structures and well-built homes, reducing them to piles of rubble or sweeping them entirely off their foundations or slabs.

When a tornado cannot be rated because there is no evidence that it caused structural or other relevant damage (which often occurs when a tornado touches down in unpopulated rural areas), it is classified as an EFU - U meaning "unknown" - tornado.

Forecasting

Until recently there was little hope of forecasting these systems. Meteorologists could identify those conditions that were likely to produce severe weather and generate a watch. If a tornado was spotted a warning was issued. More recently Doppler radar allows investigators to see a circulation develop in the storm. Because of their size and intensity, there are few storm observations. Increasing information has been gathered in the field from those chasing the storm.

Research activities


See also

Notes

  1. ^ (Douglas 2007, p. 76)
  2. ^ Perkins, Sid (2002-05-11). "Tornado Alley, USA". Science News. pp. 296–298. Archived from the original on 2006-08-25. Retrieved 2006-09-20.
  3. ^ a b c d e "Tornadoes: Nature's Most Violent Storms". National Oceanic and Atmospheric Administration. Retrieved 2008-07-14.
  4. ^ (Bluestein 2006, p. 6)
  5. ^ (Bluestein 2006, p. 9)
  6. ^ a b (Douglas 2007, p. 79)
  7. ^ (Douglas 2007, p. 78)
  8. ^ a b c d (Lyons 1997, pp. 177–178)
  9. ^ "Severe weather, tornadoes kill dozens across South". CNN.com. February 6, 2008. Retrieved 2008-07-14.
  10. ^ TVNZ (February 8, 2008). "Tornado toll rises to 57". TVNZ.co.nz. Archived from the original on April 11, 2008. Retrieved 2008-07-14.
  11. ^
    Britannica Online Encyclopedia
    . Retrieved 2008-07-14.
  12. ^
    National Oceanic Atmospheric Administration
    . Retrieved 2008-07-15.
  13. ^ (Bluestein 2006, p. 155)
  14. ^ (Bluestein 2006, p. 156)
  15. ^
    National Oceanic Atmospheric Administration
    . Retrieved 2008-07-15.
  16. ^
    National Oceanic Atmospheric Administration. USA Today
    .com. 2007-11-01. Retrieved 2008-07-15.
  17. National Oceanic Atmospheric Administration
    . February 27, 2008. Retrieved 2008-07-15.
  18. ^ (Douglas 2007, p. 114)
  19. ^ Stevens, William (May 11, 1999). "Oklahoma Tornado Offers Hints Of How a Killer Storm Is Born". The New York Times. Retrieved 2008-07-15.
  20. hdl:10355/9070
    .
  21. ^ a b (Lyons 1997, p. 176)
  22. ^ Robb Lawson. "Barton and Ellsworth County Tornado". National Weather Service. Retrieved 2007-05-05.
  23. ^ "Survivors sought as more storms threaten Plains". CNN. May 7, 2007. Archived from the original on 2007-05-09. Retrieved 2007-05-06.
  24. ^ Henry, Alfred (June 3, 1925). "Monthly Weather Review" (PDF). American Meteorological Society. Retrieved 2008-07-16.
  25. National Oceanic Atmospheric Administration
    . Retrieved 2008-07-16.
  26. ^
    National Oceanic Atmospheric Administration
    . Retrieved 2008-07-16.
  27. National Oceanic Atmospheric Administration
    . Retrieved 2008-07-16.
  28. ^ a b c "F5 and EF5 Tornadoes of the United States - 1950-present (SPC)". www.spc.noaa.gov.
  29. ^
    WCVB). Archived from the original
    on 2008-06-02. Retrieved 2008-07-03.
  30. ^ Dornsife, Paul. "Pennsylvania Tornado Outbreak of May 31, 1985". Pennsylvania State University. Archived from the original on July 4, 2012. Retrieved 2008-07-16.
  31. ^ (Douglas 2007, pp. 114–115)
  32. ^ (Douglas 2007, p. 81)
  33. National Oceanic Atmospheric Administration. Archived from the original
    on 2008-02-10. Retrieved 2008-07-16.
  34. National Oceanic Atmospheric Administration. February 17, 2008. Archived from the original
    on April 30, 2008. Retrieved 2008-07-16.
  35. ^ Doesken, Nolan. "Climate of Colorado". Colorado State University. Retrieved 2008-07-17.
  36. ^ (Lyons 1997, p. 182)
  37. ^ (Lyons 1997, p. 186)
  38. ^ (Douglas 2007, p. 80)
  39. ^ (Douglas 2007, p. 84)
  40. ^ a b (Douglas 2007, p. 187)
  41. ^ (Douglas 2007, p. 176)
  42. ^ (Douglas 2007, pp. 176–177)
  43. ^ (Douglas 2007, p. 177)
  44. doi:10.1175/1520-0477(2001)000<0063:TTFHCT>2.3.CO;2
    .
  45. ^ a b c d e f g h i (Douglas 2007, p. 85)
  46. ^ Statistics on the ten deadliest tornados in the US, Time. May 2011.
  47. ^
    National Oceanic Atmospheric Administration
    . Retrieved 2013-08-06.
  48. ^ a b c (Douglas 2007, p. 94)
  49. ^ a b c d (Douglas 2007, p. 95)

Bibliography

External links
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