Tornado Swarms Increasing In U.S.: NOAA Study Tracks Deadly Storms

According to a study published by government meteorologists in the journal Science on Thursday, tornadoes are increasingly coming in swarms, meaning on the days a tornado hits, it is far more likely that several more — possibly dozens — will occur that same day.

Researchers looked at tornado activity over the past 60 years and found that although the number of the deadly storms occurring each year had remained relatively steady, since the mid 1990s they have been more likely to come in swarms over fewer days per year rather than as isolated tornadoes spread throughout the season.

A senior research scientist with the International Research Institute for Climate and Society at Columbia University told Scientific American that the study is the latest in a series of recent findings suggesting that tornado variability has been on the rise.

The average number of days with at least 20 tornadoes has more than doubled in the past 40 years, according to Reuters, with many years in the past decade having five or more days, compared to less than less than one day per year in the 1970s.

Climate change has been cited by some experts as the cause of recent severe tornado outbreaks, such as this swarm reported by The Inquisitr which swept through Oklahoma and Arkansas earlier this year, destroying homes and killing several people. However, the National Oceanic and Atmospheric Administration (NOAA) scientists admit that they actually aren’t sure what is causing the change in severe weather patterns.

“Whether or not climate change has had an impact on the occurrence of tornadoes in the United States has become a question of high public and scientific interest, but changes in how tornadoes are reported have made it difficult to answer it convincingly. We show that, excluding the weakest tornadoes, the mean annual number of tornadoes has remained relatively constant, but their variability of occurrence has increased since the 1970s. This is due to a decrease in the number of days per year with tornadoes combined with an increase in days with many tornadoes, leading to greater variability on annual and monthly time scales and changes in the timing of the start of the tornado season.”

The tendency for tornadoes to come in swarms means that there is a potential for shorter periods of times, such as weeks or months, to have extremely high numbers of destructive storms.

One extreme example of this is the tornado swarm that occurred in April 2011, when 175 confirmed twisters cut a swath across the south, causing billions of dollars in damages and hundreds of deaths in only two days. According to MSN, that single month in 2011 saw 751 tornadoes in the U.S. — almost twice as many as the mean annual average of 495.

Do you think climate change is causing the shift in tornado patterns? Or is it just a natural weather cycle?

[Image via Tamu Times]