Penn State NASA

Severe Storms (not including Hurricanes)


Severe Storms (not including Hurricanes)

The night of December 10 and early morning of December 11, 2021 saw devastating tornadoes tear across the central US from Arkansas to Kentucky. Up to 71 different tornadoes have been confirmed rated up to EF-4 with wind speeds up to 190 miles per hour.  The tornadoes caused massive damage totaling $3.9 billion, destroying whole communities, causing 88 fatalities and over 600 injured. 71 tornadoes have been confirmed. The worst damage was in the town of Mayfield, Kentucky, which was leveled by a strong EF-4 tornado. Much of the center of the town was irreparably damaged with houses and stores receiving devastating damage, 22 people died in that town alone. The tornado forecasts were generally highly accurate that night, but sadly, warnings weren’t always acted upon. What was particularly significant about the outbreak is how late in the season they occurred. Tornadoes in the central US are frequent in spring, when warm and cold air-masses collide over the region.  It is very unusual for these strong storms to occur in the winter.

Aerial view of destroyed buildings and landscape after Mayfield, KY tornado.
Aerial view of EF4 damage in Mayfield Kentucky on December 12, 2021

By studying the network of weather stations across the US, researchers have found a couple of interesting results — the biggest storms recorded in a given year not including tornadoes are increasing in strength, and the frequency of extreme storms is also increasing. In addition the area known as "tornado alley" where touch-downs are common is shifting eastward from Texas, Oklahoma and Kansas to Arkansas, Louisiana, Mississippi, Alabama and Tennessee.

Increasing trend of larger storms particularly on the eastern half of the US with a great increase in the north east and new england.
The above figure shows the results of the trends of the biggest storms of each year (not including tornadoes) as recorded by weather stations across the US, and you can see that although the pattern is somewhat complex, the general story here is that big storms are getting bigger.

In addition, the frequency of extreme storms is also increasing. An extreme storm is one where the rate of precipitation exceeds by a certain amount the long-term mean rate of precipitation for a given site (so an extreme storm in a wet region like the northeast US has a much higher precipitation rate than an extreme storm for a drier region). So, you might be asking yourselves whether these results translate into more frequent, massive tornadoes. This is a somewhat controversial topic. The consensus answer is that with a warmer atmosphere, tornadoes will definitely become more powerful (just like hurricanes), but the word is still out whether they will become more frequent. What is obvious from the December 2021 tornadoes is that these powerful weather systems will occur throughout the year in the future.

Increasing storm frequency storms. Greater increase east of the rockies. Large increase in north east & significant increase in midwest
The figure above shows the results of the trends in the frequency of extreme storms as recorded by weather stations across the US, and you can see that the overwhelming pattern is one of more frequent extreme events.

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