Tornadoes wrecked deadly havoc on numerous US states over the weekend and the worst may be yet to come, as twister season continues for another few months.
Powerful storms brought tornadoes to Nebraska and Iowa on Friday and Saturday, and twisters tore down towns in Oklahoma on Sunday.
A tornado is a ‘narrow, violently rotating column of air that extends from a thunderstorm to the ground’ that ‘forms a condensation funnel made up of water droplets, dust and debris’, according to the National Oceanic and Atmospheric Administration’s (NOAA) National Severe Storms Laboratory (NSSL).
The US is hit by about 1,200 tornadoes annually and first began keeping a record of them in 1950.
This year from April 26 to 28, at least 72 twisters hit the US from Nebraska to Texas.
On Friday, a tornado destroyed homes in Omaha and left 10,000 Nebraskans without power. One of the hardest hit places was Elkorn in the western area of the city, where hundreds of residences were ripped apart. Resident Kim Woods said ‘the whole neighborhood just to the north of us is pretty flattened’.
Twisters descended on Oklahoma on Saturday into Sunday and killed at least four people including a child. Among the communities that were struck were Holdenville, Marietta and Sulpher, which seemed to be battered by several tornadoes overnight.
How are tornadoes categorized?
Tornadoes are categorized by strength based on the damage they inflict, which in turn helps determine their wind speed. The National Weather Service implemented the Enhanced Fujita Scale in 2007 to rate them.
The EF-Scale classifies tornadoes into three broad categories based on wind speeds and damage inflicted. EF0 and EF1 are ‘weak’ with wind speeds of 65mph to 110mph. EF2 and EF3 are ‘strong’ with winds speeds of 111mph to 165mph. EF4 and EF5 are ‘violent’ with wind speeds of 166mph to 200mph.
Where in the US is there tornado risk?
A map of tornado probabilities created by the NSSL based on data from a 30-year period of severe weather reports from 1982 to 2011 shows eight gradients of tornado risk, from dark to light.
The darkest shade of brown, representing the highest risk, is in the center of Oklahoma. A medium shade of brown for the second highest risk surrounds the dark brown and covers much of Oklahoma as well as a section of northern Texas and a sliver of Kansas.
A dark red representing the third highest risk touches parts of Oklahoma, Texas, Kansas, Nebraska, Iowa, Missouri, Arkansas and the northwestern tip of Louisiana.
The heat map spans out to other parts of the Southwest, Mideast, with lower risk extending to the Southeast and Northeast. The West is largely untouched.
What is Tornado Alley?
‘Tornado Alley’ is a ‘nickname invented by the media to refer to a broad area of relatively high tornado occurrence in the central United States’, according to the NSSL.
Such maps may differ because twister occurrence can be measured in different ways, including all tornadoes, only violent ones, by counties, and within certain time frames.
According to the NSSL, the term tornado alley can also be misleading.
‘The US tornado threat shifts from the Southeast in the cooler months of the year, toward the southern and central Plains in May and June, and the northern Plains and Midwest during early summer,’ stated the agency.
‘Tornadoes can occur and have been reported in all fifty states!’
When is tornado season?
The peak tornado season in the southern Plains, which include Oklahoma, Kansas and Texas, runs from May into early June.
In the northern Plains and upper Midwest including North Dakota, South Dakota, Nebraska, Iowa and Minnesota, the season is in June and July.
Meanwhile on the Gulf Coast, tornadoes occur earlier in the spring.
The NSSL cautioned that tornadoes can happen any time of the year and in the day or night, but most commonly from 4pm to 9pm.
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