WeatherWeather Blog



How tornadoes form, how we rate them, and what you can do to keep your family safe
Posted at 10:44 AM, Mar 27, 2024
and last updated 2024-03-27 11:44:09-04

March 25-29 is Severe Weather Awareness Week for Nebraska and Iowa. Each day, a different aspect of severe weather will be highlighted, explaining the science behind them, and how you can stay safe when severe weather threatens. This article will focus on tornadoes, how they form, how we rate them, and how to keep yourself safe during a twister.


Recall the severe weather awareness week article from yesterday about how a thunderstorm forms. There is an updraft, a column of warm rising air; and a downdraft, the rain/hail shaft. The ingredient that turns these ordinary thunderstorms into tornadic thunderstorms is wind shear. Wind shear is the change of the wind with height either in speed or direction. For a tornado, you need both speed shear and directional shear. Speed shear helps to tilt the storm so the rain falls away from the updraft, and directional shear causes the storm to rotate.

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Diagram showing tornado formation inside a thunderstorm

The rotating updraft is known as a mesocyclone. The mesocyclone then will ingest columns of rotating air near the ground which is tilted in the vertical direction. Once this rotating column of air spins fast enough, it dips to the ground and causes damage, now you have a tornado.


The way tornadoes are rated is by the enhanced Fujita scale (EF-scale for short). The Fujita scale was devised by legendary tornado researcher Ted Fujita (for more information on his life and work, click here). In 2007, his original scale was revised to account for more damage. The Fujita scale looks at the damage done by a tornado after it has already hit, and is used to estimate the windspeed of the tornado. Here is a relative breakdown of the scale:

EF-0 (65-85 mph) - "Light" Damage
This is the weakest of the categories. EF-0 tornadoes are responsible for minor damage. Examples of damage include: snapped tree branches or small trees, shingles peeled off, sheds/grain bins suffering significant damage, irrigation pivots overturned, power poles bent, and windows broken from buildings.

An example of EF-0 damage from 204th and Pacific Streets in Omaha from a tornado during the "Mothers Day Outbreak" of May 11, 2014.

EF-1 (86-110 mph) - "Moderate" Damage
EF-1 tornadoes start to do some more significant damage. Examples of damage include: garage doors being blown in, siding torn off homes, mobile homes flipped or moved, trees and power poles snapped, cars flipped, and minor roof damage to homes.

An example of an EF-1 tornado to a home in Bellevue during a tornado on June 16, 2017.

EF-2 (111-135 mph) - "Considerable" Damage
Any tornado with a rating of EF-2 or higher is classified as a "strong tornado" as the damage it can cause becomes more extreme. Examples of damage include: roofs removed from homes, interiors of homes damaged, barns destroyed, cars flipped and moved, mobile homes destroyed, and major tree damage can occur from an EF-2 tornado.

EF-2 damage to a farmstead in Dodge County from a tornado on May 12, 2023.

EF-3 (136-165 mph) - "Severe" Damage
In an EF-3 tornado, damage becomes more widespread and severe. Examples of damage include: exterior walls of homes collapsing, two-story homes losing their second floor, trees being uprooted, metal buildings sustaining heavy damage, cars being thrown far from where they originally were, and larger vehicles such as tractors being lifted. As of March 2024, the last EF-3 to impact the KMTV viewing area was on May 11, 2014, in Beaver Crossing southwest of Seward.

EF-3 damage done to a home in Beaver Crossing, NE from the "Mothers Day Tornado Outbreak" of May 11, 2014.

EF-4 (166-200 mph) - "Extreme" Damage
An EF-4 tornado is rare and is classified as a "violent tornado", and only occurs a handful of times per year in the United States. Examples of damage include: trees being debarked, cars mangled, well-constructed homes being leveled, masonry buildings suffering heavy damage, and smaller structures completely wiped away. The last EF-4 tornado in Nebraska was the Stanton/Pilger/Wakefield tornadoes of June 16, 2014. The last EF-4 in Iowa was the Winterset tornado of March 5, 2022.

EF-4 tornado damage in Pilger during the Pilger tornado of June 2014.

EF-5 (201+ mph) "Massive" Damage
The EF-5 is the rarest and most violent tornado category. Since 1950, only 67 tornadoes have been classified as (E)F-5 in the United States. The last F-5 tornado in Nebraska was on May 5, 1964, that impacted the area from Hastings to Osceola. The last EF-5 in Iowa was the Parkersburg tornado on May 25, 2008. Examples of damage include: frame homes being swept clean from their foundations, steel-reinforced concrete buildings being heavily damaged, trees being completely debarked and snapped, cars being tossed hundreds of yards away, and the ground being scoured.

As of March 2024, the United States has not seen an EF-5 tornado since the Moore, Oklahoma tornado of May 20, 2013. This image is from that tornado.


If a tornado warning is issued, there are many things to keep you and your family safe. The main thing is not to panic, despite their destruction, tornadoes are survivable (even EF-5s) if you take the precautions to protect yourself. The number 1 thing to do is get underground whether a basement or a storm shelter. If you do not have a basement, you can go to the lowest floor of a sturdy building and find an interior room. This can be a hallway, closet, or bathroom, which puts as many walls between you and the outdoors as possible. If you do either of those two things, your chance of surviving a tornado no matter the strength is quite high.

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If you do not have a basement, the lowest floor interior room is the best place to shelter.

Under any circumstances should you be in a mobile home during a tornado, even in weak tornadoes mobile homes can be tossed or destroyed. If you live in a mobile home, during a tornado warning leave and find a more sturdy shelter. If you are in a car, pull over and find the nearest sturdy structure to shelter in. If you can not do that, an absolute last resort is to lie in your car and cover your head, or find a ditch and lay in it.


Nebraska and Iowa are considered a part of "Tornado Alley" which broadly consists of the Great Plains from Texas to the Dakotas. Each state sees a couple dozen tornadoes a year, on average. Peak tornado season runs from March to July, with May and June being the most active months for tornadoes. However, as we saw back in December 2021, tornadoes can happen any time of year if the conditions are right for them.

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Average number of tornadoes in Nebraska during peak severe weather season.
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Average number of tornadoes in Iowa during peak severe weather season.

There are a lot of tornado myths that still preside in most areas. Some of these myths include tornadoes not impacting downtown areas, can not cross waterways, or do not travel over hills. These are all false, tornadoes can travel over any type of terrain.

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A few tornado myths. These are all false.