Why the southern United States is prone to December tornadoes | Kiowa County Press


Damage in Mayfield, Ky., After a tornado swept through the area on December 11, 2021. Brett Carlsen / Getty Images

Alisa Hass, Middle Tennessee State University and Kelsey Ellis, University of Tennessee

On the night of December 10-11, 2021, an outbreak of powerful tornadoes ravaged parts of Arkansas, Mississippi, Missouri, Kentucky, Tennessee and Illinois, killing dozens of people and leaving wrecks for hundreds of kilometers. Hazard climatologists Alisa Hass and Kelsey Ellis explain the conditions that generated this event – including what could be the first “four-state tornado” in the United States – and why the Southeast is vulnerable to these disasters all year round, especially at night.

What factors came together to cause such a huge epidemic?

On December 10, a powerful storm system approached the central United States from the west. As the system brought heavy snow and slippery conditions to the colder west and northern Midwest, the south took advantage almost record heat, thanks to the warm and humid air coming from the northern Gulf of Mexico.

The storm system brought cold, dense air into the area, which interacted with the warm air, creating unstable atmospheric conditions. When hot and cold air masses collide, warm, less dense air rises to cooler levels in the atmosphere. As this warm air cools, the moisture it contains condenses into clouds and can form storms.

When this instability combines with strong wind shear – winds changing direction and speed at different heights in the atmosphere – it can create an ideal pattern for strong rotating storms to occur.

Atmospheric instability develops when the air is warm at the surface and cold at higher levels. This causes patches of warm air to rise and form clouds which can produce thunderstorms and, under certain conditions, tornadoes.

On a tornado rating scale, what was the intensity of this event?

At least 38 tornadoes were reported in six states during this outbreak, causing widespread power outages, damage and deaths. The National Weather Service rates tornadoes by damage intensity using 28 damage indicators from the Enhanced Fujita Scale, or EF. Storm and tornado assessments can take several days or more.

As of December 12, at least four EF-3 and five EF-2 tornadoes have been confirmed. The EF-2 and EF-3 tornadoes are considered strong, with wind speeds of 113-157 mph and 158-206 mph respectively.

Strong straight-line winds also occur with severe storms and can cause as much damage as a tornado. After severe storms and tornado reports, the National Weather Service conducts in-person storm damage investigations to determine whether a tornado or straight-line winds caused the reported damage and the degree of damage. Investigators will look to see if the debris is scattered in one direction, which would indicate straight-line winds, or in many different directions – the hallmark of a tornado.

Graph showing the types of damage inflicted by variable winds and speeds on the EF scale.
Expected damage from tornadoes at different levels of the Enhanced Fujita (EF) scale, using examples from a massive outbreak in 2011. National Meteorological Service

A tornado would have traveled 240 miles across four states. Why is this unusual?

Most tornadoes stay on the ground for a short period of time and travel short distances – 3 to 4 miles on average. Long and very long track tornadoes – those that travel at least 25 and 100 miles respectively – are relatively rare. They represent less than 1% of all tornadoes in the United States.

Long-trajectory tornadoes require a very specific set of ingredients that must exist over a large area. These rare tornadoes form from a single supercell storm – a storm with a rotating updraft called a mesocyclone – that can linger for hours.

Large tornadoes often stay on the ground longer than weaker tornadoes. Their tracks are particularly long in the southeast, where large cool-season tornadoes move quickly, covering more ground.

The previous record for a long track tornado dates back to 1925, when the F-5 Tri-State Tornado traveled 219 miles through Missouri, Illinois and Indiana. The “four-state tornado,” as this tornado has been dubbed, is expected to break this record. In the coming days, the National Weather Service will confirm if a tornado has been on the ground for more than 200 miles or if multiple tornadoes have resulted from the same storm. The agency issued a preliminary rating of EF-3 or higher for this event.

Why are there more nighttime and winter tornadoes in the Southeast than in other areas?

Spring is generally considered tornado season, but tornadoes can occur at any time of the year. The Southeast experiences a second peak of tornado activity in the fall and early winter, and winter tornadoes are not uncommon.

Likewise, tornadoes can occur at any time of the day. Nighttime tornado events are particularly common in the Southeast, where storm ingredients are different and more prone to nighttime tornadoes than in “Tornado Alley” on the Great Plains.

Tornado storms in the southeast are often fueled by an abundance of wind shear. They are not so dependent on the rise of warm, humid air which creates atmospheric instability – conditions that require daytime warming of the earth’s surface and are more common in the spring.

Predictions of this event were accurate and predicted a major outbreak several days in advance. The National Weather Service’s storm forecasting center in Norman, Oklahoma, and relevant National Weather Service local weather forecasting offices have issued watches, warnings and timely information on how to stay safe.

But nighttime tornadoes can be particularly deadly. More deaths tend to occur because people often do not receive warning messages when they are asleep. Storms are more difficult to spot in the dark, and people are more likely to be in more vulnerable housing, such as mobile homes, at night than during the day when they are at work in sturdier buildings. .

Having several reliable methods of receiving warnings at night is essential, as the power can be cut off and cell phone service can fail in severe weather. Unfortunately, during the event from December 10 to 11, some people who went to shelters were killed when tornadoes hit the building they were in. during less devastating events.

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The conversation

Alisa Hass, Assistant Professor of Geography, Middle Tennessee State University and Kelsey Ellis, associate professor of geography, University of Tennessee

This article is republished from The Conversation under a Creative Commons license. Read the original article.


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