Tornado climatology

Average annual tornado reports in the United States

The United States averaged 1,274 tornadoes per year from 2001 to 2011. April 2011 saw the most tornadoes recorded to date for any month in the US National Weather Service's history: 875.[lower-alpha 4][10] It has more tornadoes yearly than any other country, and reports more violent F4 and F5 tornadoes than anywhere else.[11]

Tornadoes are common in many states but are most common to the west of the Appalachian Mountains and to the east of the Rockies. The Atlantic seaboard states: North Carolina, South Carolina, Georgia and Virginia, are also vulnerable, as well as Florida, though most Florida tornadoes are relatively weak, usually EF0 or EF1. The southern states are also highly impacted, in terms of casualties. More tornadoes occur in Texas than in any other US state,[lower-alpha 5] but the state that has the highest number of tornadoes per area is Florida. Tornadoes often occur along the edges of the hurricanes that strike the state. The state with the highest number of strong tornadoes per area is Oklahoma, while the neighboring state of Kansas records the most EF4 and EF5 tornadoes in the country.

Tornadoes can occur west of the continental divide, but they are infrequent and usually relatively weak and short-lived. Recently, tornadoes have struck along the Pacific coast towns of Lincoln City, Oregon (1996); Sunnyvale, California (1998); Port Orchard, Washington (2018); and downtown Salt Lake City, Utah (1999).[lower-alpha 6] The California Central Valley is an area of some frequency for tornadoes, albeit of very weak intensity.[12]

The deadliest U.S. tornado recorded is the Tri-State tornado of March 18, 1925 that swept across southeastern Missouri, southern Illinois, and southern Indiana, killing 695 people. The biggest tornado outbreak on record—with 353 tornadoes for just 3+1⁄2 days (including four EF5 and eleven EF4 tornadoes)—occurred starting on 25 April 2011 and intensifying on April, 26, and 27 (a record-breaking day), before ending on 28 April 2011, now referred to as the 2011 Super Outbreak.[lower-alpha 7]

Canada annually experiences numerous tornadoes, although fewer than the United States. On average 62 are reported per year, but this number is expected to in fact be higher due to undetected tornadoes in large expanses of underpopulated areas.[13] NOAA records a higher average of 100 per year in Canada.[14] These storms cause tens of millions of dollars in damage. Most are weak F0 or F1 in intensity, but there are on average a few F2 or stronger that touch down each season.

For example, the tornado frequency of Southwestern Ontario is about half that of the most tornado-prone areas of the central US plains. The last multiple tornado-related deaths in Canada were caused by a tornado in Ear Falls, Ontario on 9 July 2009, where 3 died, and the last killer tornado was on 3 August 2018 in Alonsa, Manitoba.[15] The two deadliest tornadoes on Canadian soil were the Regina Cyclone of 30 June 1912 (28 fatalities) and the Edmonton Tornado of 31 July 1987 (27 fatalities). Both of these storms were rated an F4 on the Fujita scale. The city of Windsor was struck by strong tornadoes four times over 61 years (1946, 1953, 1974, 1997) ranging in strength from an F2 to F4. Canada's first official F5 tornado struck Elie, Manitoba on 22 June 2007.[16] Tornadoes are most frequent in the provinces of Alberta, Saskatchewan, Manitoba, and Ontario.

The UK has the most annual tornadoes per land area per year, 0.14 per 1000 km² (although these tornadoes are generally weak), and other European countries have a similar number of tornadoes per area.[11] One notable tornado of recent years was the tornado that struck Birmingham, United Kingdom, in July 2005. A row of houses was destroyed, but no one was killed.

Europe as a whole has about 700 tornadoes per year, most of them being waterspouts.[17] They are most common from June to August, especially in the inland, and rarest from January to March. Strong and violent tornadoes (F3–F5) do occur, especially in some of the interior areas and in the south, but are not as common as in parts of the US. As in the US, tornadoes are far from evenly distributed. Since 1900, deadly tornadoes have occurred in Austria, Belgium, Cyprus, Czech Republic,[18][19][20] Finland, France,[21][22] Germany,[23] Italy,[24][25] Lithuania, Malta, the Netherlands, Poland, Ukraine, Portugal, Romania, Russia, and the UK. The 1984 Ivanovo–Yaroslavl outbreak, with more than 400 fatalities and 213 injured, was the 20th century's deadliest tornado outbreak in Europe. It included at least two tornadoes with a rating of F4.[lower-alpha 8]

Bangladesh and the eastern parts of India are very exposed to destructive tornadoes. Bangladesh, the Philippines, and Japan have the highest number of reported tornadoes in Asia. The single deadliest tornado ever recorded struck the Manikganj District of Bangladesh on 26 April 1989, killing an estimated 1,300 people, injuring 12,000, and leaving approximately 80,000 people homeless.[27] Five other recorded tornadic events have killed more than 500 people in Bangladesh, most recently on 13 May 1996 when a tornado swept through the Jamalpur and Tangail districts, killing more than 600.[27]

China occasionally experiences destructive tornadoes. Storms producing multiple tornadoes and hail struck a densely populated area of farms and factories near the city of Yancheng in Jiangsu province, about 800 kilometers (500 miles) south of Beijing, China.[28][29] Throughout China, an estimated 100 tornadoes may occur per year with a few exceeding F4 in intensity, with activity most prevalent in eastern regions.[30] During the period of 1948 until 2013, 4763 tornadoes were confirmed in China.[31]

South America has its own tornado alley, comprising areas in central and northern Argentina, southern and southeast Brazil, Uruguay, and part of Paraguay. It is considered the second-highest frequency tornado region in the world. Argentina has areas with high tornadic activity, and the strongest tornadoes in the southern hemisphere, such as the F5 in San Justo, Argentina, 105 km (65 mi) north of the city of Santa Fe, with winds that exceeded 400 kilometres per hour (250 mph). In Brazil, one of the most remarkable events occurred on 24 May 2005 when an F3(EF3) multiple-vortex tornado[32] struck the industrial district of the city of Indaiatuba in the state of São Paulo.

Tornadoes do occur in extreme southern Africa (including the countries of South Africa, Lesotho, and Eswatini). In October 2011, two people were killed and nearly 200 were injured after a tornado formed, near Ficksburg in the Free State; more than 1,000 shacks and houses were flattened.[33] There is also the seasonal incidence of tornadoes in the coast of western Africa. These occur during the onset of the rainy season when tumultuous winds accompanied by sheets of rain as well as spectacular thunder and lightning batter the coast.[34] The tornado generating storms were often welcomed by colonial settlers in the region since they dissipated extreme heat and humidity during the last days of the dry season. Tornadoes are often embedded in the African squall lines,[35][36] but they damage crops, and diminish any beneficial effect of its rains.

Australia has about 16 tornadoes per year – excluding waterspouts, which are common.[37] In New Zealand, a tornado hit the northern suburbs of Auckland on 3 May 2011, killing one and injuring at least 16 people.

Tornadoes are most common in spring and least common in winter.[38] The seasonal transition during autumn and spring promotes the development of extratropical cyclones and frontal systems that support strong convective storms. Tornadoes are common in landfalling tropical cyclones, where they are focused in the right poleward section of the cyclone. Tornadoes can be spawned as a result of eyewall mesovortices, which persist until landfall.[39]

The time of year is a major factor in the intensity and frequency of tornadoes. On average, in the United States as a whole, the month with the most tornadoes is in May, followed by the months June, April, and July. There is no "tornado season" though, as tornadoes, including violent tornadoes and major outbreaks, can and do occur anywhere at any time of year if favorable conditions develop. Major tornado outbreaks have occurred every month of the year.[40]

The reason for the peak period for tornado formation in North America being skewed toward spring has much to do with temperature patterns in the U.S. Tornadoes often form when cool, polar air traveling southeastward from the Rocky Mountains overrides warm, moist, unstable Gulf of Mexico air in the eastern states. Tornadoes tend to be commonly found in front of a cold front, along with heavy rains, hail, and damaging winds. Since both warm and cold major weather patterns collide in Spring, the conflict between the two air masses tends to the formation of tornado producing storms and supercells. As the weather warms across the country, the occurrence of tornadoes spreads northward. There is a second active tornado season of the year that usually appears in late September to mid-November. Autumn, like spring, is a time of the year when warm weather frequently alternates with cold weather, especially in the Midwest; but the season is not as active as it is during the springtime, and tornado frequencies are higher along the Atlantic coastal plain. On average, there are around 294 tornadoes throughout the United States during May, and as many as 543 tornadoes have been reported in May alone (in 2003). The months with the fewest tornadoes are usually December and January, although major tornado outbreaks can and sometimes do occur even in those months. In general, in the Midwestern and Plains states, springtime (especially the month of May) is the most active season for tornadoes, while in the far northern states (like Minnesota and Wisconsin), the peak tornado season is usually in the summer months (June and July). In the colder late autumn and winter months (from early December to late February), tornado activity is generally limited to the southern states, where warm Gulf of Mexico air can penetrate.

July is the peak month in Austria, Finland, and Germany.[41]

Tornado occurrence is highly dependent on the time of day because of solar heating.[42] Austria, Finland, Germany, and the United States'[43] peak hour of occurrence is 5 pm, with roughly half of all tornado occurrence between 3 p.m. and 7 p.m. local time.[44][45][46][47] Due to this being the time of peak atmospheric heating, and thus the maximum available energy for storms; some researchers, including Howard B. Bluestein of the University of Oklahoma, have referred to this phenomenon as "five o'clock magic." Despite this, there have been several morning tornadoes reported, such as evidenced by the Gainesville Tornado of 1936 (one of the deadliest tornadoes in history) that occurred at 8:30 a.m. local time.[38]