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Tornadoes in the United States

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Intense tornado activity in the United States. The darker-colored areas denote the area commonly referred to as Tornado Alley.
A tornado strikes Central Oklahoma. This was part of the 1999 Oklahoma tornado outbreak on May 3, 1999.

The United States receives a large amount of severe weather throughout a year, and is the country most hit by tornadoes in the world.[1] The United States has many more tornadoes than any other country, seeing about four times the activity estimated in all of Europe. Many of these form in an area of the central United States known as Tornado Alley.[2] This area extends into Canada, particularly Ontario and the Prairie Provinces; however, activity in Canada is less than that of the US. The Netherlands has the highest average number of tornadoes per square mile in the world (at least 20 recorded per year).[3] The United Kingdom also has a very high tornado density (more than 33 tornadoes reported annually)[4], but most are small and result in minor damage. In a single year, the United States averages over 1,200 tornadoes.[5] The United States is also the country hit by the most violent tornadoes, rated EF4 and EF5 on the Enhanced Fujita Scale.[5]

The United States is hit the most by tornadoes in an area named "Tornado Alley."[6] Tornado Alley is an area in the Southern United States that receives the most tornadoes of anywhere in the world.[7] This area stretches from Extreme Southern Canada to the Texas Panhandle, and includes the states of Nebraska, Kansas, Oklahoma, Texas, and South Dakota.[8] In the central parts of Tornado Alley, there is the most activity, with about five tornadoes per latitude-longitude radius (or a circle about 50 miles wide) per year.[9]

Tornadoes are most common in spring and least common in winter in the United States.[10] Since autumn and spring are transitional periods (warm to cool and vice versa) there are more chances of cooler air meeting with warmer air, resulting in thunderstorms. Tornadoes can also be caused by landfalling tropical cyclones, which tend to occur in the late summer and autumn. But favorable conditions can occur at any time of the year.[11] They can also occur at mostly any time of day. In the U.S., tornadoes mostly occur during the late afternoon and early evening hours, from about 4:00 P.M. to 7:00 P.M.[12] This is because the temperature is at its highest point for the day, causing atmospheric conditions to become unstable, and likely to form a tornado.[13]

Seasonal activity

Template:US tornadoes by month

Tornadoes are usually considered to occur in the United States between the months of March through August.[14] This is not true, however. Tornadoes can strike at any time of the year in the U.S.[14] Although the months that most tornadoes strike (or "tornado season") is March through August, tornadoes have been documented in the United States during every month of the year.[14] 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.[14] Another notable non-season tornado was on December 9, 1991, 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.[15]

During the winter months of the year, tornadoes have been known to hit the Southern United States and Southeastern United States the most, but have hit other areas as well.[14] One notable recent example of a winter tornado outbreak was the 2008 Super Tuesday tornado outbreak on February 5 and February 6, 2008. 84 tornadoes occurred over the course of the outbreak. The storm system produced several destructive tornadoes in heavily populated areas, most notably in the Memphis metropolitan area, in Jackson, Tennessee, and the northeastern end of the Nashville metropolitan area. At least 57 people were killed across four states and 18 counties, with hundreds of others injured.[16] The outbreak is the deadliest of the modern NEXRAD doppler radar era (fully implemented in 1997.) As well as the largest single outbreak since 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.[17]

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 of Mexico.[7] This is due to air moving southward reaching its southern limit of expansion, and stops over the Gulf Coast.[7] As spring comes, hot air progressively moves back into the Gulf Coast, causing the mass of colder air to be pushed forward and out of the Gulf States and into the Southeastern states, where the frequency for tornadoes is the most during the month of April.[7]

As spring passes and summer begins, the mass of air moves northwest into the Great Plains and Midwestern states.[7] During the months of May and June, tornado activity is as its peak in the southern Great Plains. The air mass then moves northward into the Northern Great Plains and the Great Lakes area, causing a tornado activity peak in these areas during the summer months.[7] During the late summer and early fall months, tornado activity in the United States tapers off.[7] This is due to the temperature at the boundary of the hot air mass and the cool air mass is different, and an extension of the Bermuda High sitting over parts of the United States.[7] Although there may be some thunderstorms, they will not become severe enough to spawn tornadoes often.[7]

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.[18] Tornadoes are most likely to be formed in the right-front quadrant of the hurricane, but can also be formed in rain bands that are associated with the storm.[18] This is caused by the large amount of vertical wind shear to the right of the storm.[19] Tornadoes are also spawned from U.S. hurricanes to 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.[20]

Regional activity

Tornadoes have been documented in every U.S. state (not including the non-state territories of Guam, the Virgin Islands, American Samoa, and Puerto Rico) at least once since 1950, although some regions and states are hit by tornadoes far more than others.[21] For example, the average amount of tornadoes to hit the states of Alaska, Rhode Island, and Vermont is less than one, while the state of Oklahoma receives an average of 52 tornadoes per year, and the state of Texas is hit with 126 tornadoes in an average year.[22] The state which has the highest number of tornadoes per unit area is Florida, although most of the tornadoes in Florida are weak tornadoes of EF0 or EF1 intensity.[23] A number of Florida's tornadoes occur along the edge of hurricanes that strike the state.[24] The state with the highest number of strong tornadoes per unit area is Oklahoma.[25] The neighboring state of Kansas is another particularly notorious tornado state, holding the record the most F4 and F5 tornadoes in the country.[26] States such as Oklahoma and Kansas have much lower population densities than Florida so, tornadoes may go unreported.[27]

Midwest

The Midwestern states are very prone to tornado activity, as it is part of "Tornado Alley."[28] States included in the area that are hit by tornadoes the most are Kansas, Nebraska, Iowa, and South Dakota.[28] The Midwestern States are often hit by tornadoes during the late spring and early summer months, especially the Months of May and June.[7] This is due to the aforementioned air mass moving northward into the Midwest and combining with another air mass that mover southward from Canada.[7] This creates unstable air, creating the potential for storms to strike the most during these months.[7] The frequency begins to decrease in the middle of the summer, as the air mass mover northward into the Great Lakes states.[7]

A Doppler On Wheels unit observing a tornado near Attica, Kansas.

A very notable recent storm that his hit this area was the Greensburg tornado, in the May 2007 Tornado Outbreak. The tornado apparently strengthened as it neared Greensburg, and at 9:38 pm CDT (0238 UTC), storm chasers reported that it had grown to over 1⁄2 mile (0.8 km) in diameter.[29] Several satellite tornadoes were observed as the very large wedge approached the town of Greensburg. It was an estimated 1.7 miles (2.7 km) in diameter and was later confirmed to have been an EF5 on 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 (330 km/h).[30]

Nebraska is fifth overall for sheer numbers of tornadoes, whilst Indiana has had 88 violent tornado reports from the 1950–2006 period, more than any state except Oklahoma.[21] Iowa reported 83, almost as many as Texas.[21] The deadliest tornado in US history, the Tri-State Tornado, struck Missouri, Illinois, and Indiana in March 1925.[31] St. Louis, Missouri and neighboring East St. Louis, Illinois have been hit more than once by violent tornadoes, the most notorious of which was the St. Louis Tornado of May 1896.[32] The New Richmond Tornado of May 1899 and the Flint, Michigan tornado of June 1953 also rank amongst the deadliest tornadoes in US history.[33] 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).[21]

South

The Southern United States has suffered more tornado fatalities than any other part of the country.[34] Some areas experience repeated damaging tornado events, such as the Tennessee Valley in northern Alabama. The state of Alabama has reported more F5 tornadoes than any state except for Kansas.[21] 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).[21] Fourth is Arkansas (336) and fifth is Tennessee with 271 fatalities.[21]

Tornado disasters to affect the southern USA include the Great Natchez Tornado of May, 1840 - second deadliest on record in the US, behind only the Tri-State Tornado.[33] Other outbreaks included the Tupelo-Gainesville tornado outbreak of April 1936, the 1908 Southeast tornado outbreak of April 1908 and the Candlestick Park Tornado of 1966. Alabama and Kentucky were very badly affected by the Super Outbreak of 1974. The 2008 Super Tuesday tornado outbreak was the deadliest tornado outbreak in the USA in 23 years.[33] 58 people lost their lives with Tennessee, Arkansas, and Kentucky particularly badly affected.

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 Arkansas (1,644), Louisiana (1,608), Alabama (1,579), Georgia (1,324), North Carolina (1,042), Tennessee (892), South Carolina (819), Kentucky (710) and Virginia (565).[21] West Virginia, by contrast, is one of the least vulnerable states of all with just 120 tornadoes reported over the period.[21]

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 town - not far west of Boston - on 9 June 1953 and killed 90 people.[35] The tornado was generated by the same storm system that delivered a devastating tornado to the town of Flint, Michigan.[35]

Areas further south - notably Maryland - and areas to the west of the Appalachians are more vulnerable to tornadoes. NCDC figures for the period 1950 to 2006 show that Pennsylvania reported 697 tornadoes, followed by New York State (358), Maryland (269), Massachusetts (153), New Jersey (144), Maine (101), New Hampshire (86), Connecticut (82), Delaware (58), Vermont (37), Rhode Island (9), and the District of Columbia (1).[21] The worst outbreak in this region took place in Pennsylvania on May 31, 1985.[36]

Florida is one of the most tornado prone states, with only Texas, Kansas and Oklahoma reporting more storms.[21] During the period 1950 to 2006, Florida reported 2,884 tornadoes according to NCDC figures.[21] However, Florida tornadoes are usually weak in comparison with those that strike the Plains and the Southern states - there have been only four reports of F4 strength tornadoes since 1950 and none of F5 strength.[22] Florida experiences more thunderstorms than any other state but fewer supercell storms.[22] Florida tornadoes are more often spawned by the frequent ordinary thunderstorms which occur over the state. Hurricanes and other tropical storms can generate large numbers of tornadoes.[37] Non-supercell tornadoes are rarely as strong as supercell-generated storms.[38]

Florida's most violent tornadoes generally occur during the winter months when the state is most vulnerable to invasions of cold air which help generate such storms.[7] In recent years there have been some particularly disastrous tornado events. During the night of February 22 and 23, 1998, an F3 tornado struck Kissimmee and killed 25 people. Later that night, another F3 tornado struck Seminole and Volusia counties and killed 13 people (see Kissimmee Tornado Outbreak).[39] On February 2, 2007, an EF3 tornado struck Lake County with 21 fatalities resulting - see 2007 Central Florida Tornadoes.[40]

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.[41] The same can be said, to a lesser extent, of eastern Wyoming. Tornadoes are less frequent in mountainous areas.[42] 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).[21]

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.[21] New Mexico borders the notoriously tornado prone states of Texas and Oklahoma, hence the noticeably higher figures.[21] Arizona and New Mexico experience regular summer thunderstorms during their Monsoon season. These are sometimes tornadic but rarely produce violent tornadoes.

The Pacific Northwest is perhaps the least vulnerable region. NCDC figures for the 1950 to 2006 period show that Washington State reported just 96 tornadoes for the entire period and that Oregon reported 91.[21] However, Portland, Oregon and neighboring Vancouver, Washington were hit by a deadly tornado, the remarkable Portland-Vancouver Tornado, on 5 April 1972. This storm was the deadliest to hit the United States that year.[43]


Intensity

A powerful tornado hits the town of Dimmitt, Texas.

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.[44] 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.[45] The state with highest number of F5 and EF5 tornadoes since data was available in 1880 is Kansas, which is located in the center of "Tornado Alley."[46] The state with the highest number of F5 and EF5 tornadoes per square mile, however, was Iowa.[46] 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.[46]

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.[47] However, these high intensity storms do account for an average of seventy percent of all tornado-related deaths in the United States each year.[48] These storms can have winds of over 200 miles per hour and stay on the ground for over an hour.[49]

The United States has seen 52 F5 and EF5 (the highest intensity and damage ranking) tornadoes since records began in 1880. 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.[50]

There was a tornado that almost added an F6 rating to the Fujita scale. On May 3, 1999, a very powerful tornado struck the town of Moore, Oklahoma.[51] This storm was a part of the 1999 Oklahoma tornado outbreak, which also brought many violent storms to the states of Kansas, Arkansas, and Tennessee.[51] In total, the storm produced 66 tornadoes over four states.[51] The near F6 storm took place near the town of Bridge Creek, Oklahoma, and was in the same county as the aforementioned Moore.[51] 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 can take readings from storms while attached to a car.[51] When the Doppler on Wheels took data from the nearby storm, winds were estimated to be in excess of 318 miles per hour, which was over the wind speed in the F5 classification during that time.[51] Although the wind speed was over the F5 maximum, the tornado was not named the first ever F6 storm.[51] 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.[51] Also, the wind speed recorded was taken from over 200 feet above ground level.[51] The National Weather Service also did a damage check, and found that the damage was that of an average F5 tornado.[51]

See also

Notes

  1. ^ (Douglas, p. 76)
  2. ^ Perkins, Sid (2002-05-11). "Tornado Alley, USA". Science News. pp. 296–298. Retrieved 2006-09-20.
  3. ^ Rincon, Paul (July 11, 2003). "UK, Holland top twister league". BBC News. Retrieved 2008-07-14. {{cite web}}: Check date values in: |date= (help); Cite has empty unknown parameter: |coauthors= (help)
  4. ^ "Tornado Facts". TORRO. Retrieved 2008-07-14. {{cite web}}: Cite has empty unknown parameter: |coauthors= (help)
  5. ^ a b "Tornadoes: Nature's Most Violent Storms". National Oceanic and Atmospheric Administration. Retrieved 2008-07-14. {{cite web}}: Cite has empty unknown parameter: |coauthors= (help)
  6. ^ (Bluestein, p. 6)
  7. ^ a b c d e f g h i j k l m n "Tornado". Britannica Online Encyclopedia. Retrieved 2008-07-14. {{cite web}}: Cite has empty unknown parameter: |coauthors= (help)
  8. ^ (Bluestein, p. 7–8)
  9. ^ (Bluestein, p. 8)
  10. ^ (Bluestein, p. 9)
  11. ^ (Douglas, p. 79)
  12. ^ (Douglas, p. 78)
  13. ^ (Douglas, p. 79)
  14. ^ a b c d e (Lyons, p. 177)
  15. ^ (Lyons, p. 177–178)
  16. ^ CNN (February 6, 2008). "Severe weather, tornadoes kill dozens across South". CNN.com. Retrieved 2008-07-14. {{cite news}}: |author= has generic name (help); Check date values in: |date= (help)
  17. ^ TVNZ (February 8, 2008). "Tornado toll rises to 57". TVNZ.co.nz. Retrieved 2008-07-14. {{cite news}}: Check date values in: |date= (help)
  18. ^ a b "Tornadoes". National Oceanic Atmospheric Administration. Retrieved 2008-07-15. {{cite web}}: Cite has empty unknown parameter: |coauthors= (help)
  19. ^ (Bluestein, p. 155)
  20. ^ (Bluestein, p. 156)
  21. ^ a b c d e f g h i j k l m n o p "Tornado Ranks by State". National Oceanic Atmospheric Administration. Retrieved 2008-07-15. {{cite web}}: Cite has empty unknown parameter: |coauthors= (help)
  22. ^ a b c "Where tornadoes strike the most". National Oceanic Atmospheric Administration. USA Today.com. Retrieved 2008-07-15. {{cite web}}: Cite has empty unknown parameter: |coauthors= (help)
  23. ^ "U.S. Tornado Climatology". National Oceanic Atmospheric Administration. February 27, 2008. Retrieved 2008-07-15. {{cite web}}: Cite has empty unknown parameter: |coauthors= (help)
  24. ^ (Douglas, p. 114)
  25. ^ Stevens, William (May 11, 1999). "Oklahoma Tornado Offers Hints Of How a Killer Storm Is Born". The New York Times. Retrieved 2008-07-15. {{cite web}}: Cite has empty unknown parameter: |coauthors= (help)
  26. ^ "F5 and EF5 Tornadoes of the United States". National Oceanic Atmospheric Administration. Retrieved 2008-07-15. {{cite web}}: Cite has empty unknown parameter: |coauthors= (help)
  27. ^ "Population Influences on Tornado Reports in the United States" (PDF). University of Missouri. April 28, 2005. Retrieved 2008-07-15. {{cite web}}: Cite has empty unknown parameter: |coauthors= (help)
  28. ^ a b (Lyons 176)
  29. ^ Robb Lawson. "Barton and Ellsworth County Tornado". National Weather Service. Retrieved 2007-05-05.
  30. ^ CNN. "Survivors sought as more storms threaten Plains". Retrieved 2007-05-06. {{cite web}}: |author= has generic name (help)
  31. ^ Henry, Alfred (June 3, 1925). "Monthly Weather Review" (PDF). American Meteorological Society. Retrieved 2008-07-16. {{cite web}}: Cite has empty unknown parameter: |coauthors= (help)
  32. ^ "St. Louis City Tornadoes". National Oceanic Atmospheric Administration. Retrieved 2008-07-16. {{cite web}}: Cite has empty unknown parameter: |coauthors= (help)
  33. ^ a b c "The 25 Deadliest U.S. Tornadoes". National Oceanic Atmospheric Administration. Retrieved 2008-07-16.
  34. ^ "Tornado Fatality Information". National Oceanic Atmospheric Administration. Retrieved 2008-07-16.
  35. ^ a b Pombo, Jamy. "Worcester Tornado: 50 Years Later". The Boston Channel.com (WCVB). Retrieved 2008-07-03. {{cite web}}: Cite has empty unknown parameter: |coauthors= (help)
  36. ^ Dornsife, Paul. "Pennsylvania Tornado Outbreak of May, 31, 1985". Pennsylvania State University. Retrieved 2008-07-16. {{cite web}}: Cite has empty unknown parameter: |coauthors= (help)
  37. ^ (Douglas, p. 114–115)
  38. ^ (Douglas, p. 81)
  39. ^ "The Central Florida Tornado Outbreak of February 22nd & 23rd, 1998". National Oceanic Atmospheric Administration. Retrieved 2008-07-16.
  40. ^ "The Groundhog Day Tornado Outbreak". National Oceanic Atmospheric Administration. February 17 , 2008. Retrieved 2008-07-16. {{cite web}}: Check date values in: |date= (help)
  41. ^ Doesken, Nolan. "Climate of Colorado". Colorado State University. Retrieved 2008-07-17.
  42. ^ (Lyons, p. 182)
  43. ^ (Lyons, p. 186)
  44. ^ (Douglas, p. 80)
  45. ^ (Douglas, p. 84)
  46. ^ a b c (Douglas, p. 187)
  47. ^ (Douglas, p. 176)
  48. ^ (Douglas, p. 176–177)
  49. ^ (Douglas, p. 177)
  50. ^ McDonald, James R. (January 2001). "T. Theodore Fujita: His Contribution to Tornado Knowledge through Damage Documentation and the Fujita Scale". Bulletin of the American Meteorological Society. 82 (1). American Meteorological Society: 63–72. doi:10.1175/1520-0477(2001)000<0063:TTFHCT>2.3.CO;2.
  51. ^ a b c d e f g h i j (Douglas, p. 85)

Bibliography

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