What’s with the recent wild tornadoes? Expert weighs in

In the past week, weather authorities across the U.S. reported more than 100 tornadoes across the central and southern parts of the country, destroying dozens of homes and buildings from Oklahoma to Nebraska and Iowa. In Oklahoma, where a tornado touched down in the middle of the night, severe thunderstorms and high winds resulted in the deaths of at least four people and injured more than 300 others.

As Colorado and many parts of the U.S. enter the severe weather season, CU Boulder Today spoke with Andrew Winters, assistant professor in the Department of Atmospheric and Oceanic Sciences, about how tornadoes develop, how climate change may exacerbate severe weather, and how to better prepare for these events.

What are tornadoes?

Tornadoes are rapidly spinning columns of air associated with severe thunderstorms. Typically, these tornadoes develop in areas of strong "wind shear", which is where the wind's direction and speed change rapidly moving upward in the atmosphere. Those areas of wind shear get ingested into thunderstorms, where they can facilitate rotation within a thunderstorm and eventually spawn columns of air to the ground, forming what we call tornadoes.

Are tornadoes common in the U.S.?

The U.S. is one of the most favorable environments in the world for the formation of tornadoes. They're also common in South America and regions in Europe, but the unique geography of the U.S. creates an environment that's particularly conducive to tornadoes.

The central and the southeastern states are the regions with the greatest risk of tornadoes. One reason is that these regions have a lot of rising air. The central and southeastern U.S. is where warm, moist air masses originating from the Gulf of Mexico meet dry air that originates over the Rocky Mountains. Clashes between the two create some degree of instability in the environment, producing rapid upward air motion.

We saw over 100 tornado reports across the U.S. in the past week. When does tornado season begin?

Typically, the tornado season begins around March in the southeastern United States, and the area most likely to experience severe weather will migrate westward towards the central and southern plains. Once we get to summer months, especially June, we will begin to see various severe weather events from tornadoes to strong, damaging winds to large hail at higher latitudes in the plains like Colorado.

Is it usual to have so many tornadoes?

It is somewhat surprising that we are seeing so many tornadoes. We are currently exiting a strong El Niño winter, a natural climate phenomenon typically associated with a lower likelihood of severe weather and tornado activity in the Central Plains.

During El Niño, the atmosphere can produce a strong subtropical air current. Any waves or fluctuation along the subtropical jet stream that makes the jet wavy can create an environment that facilitates the production of severe weather and tornadoes in the spring. This is what caused the tornadoes across the country over the last couple of weeks.

Are we expecting more tornadoes in the following months?

It's a little bit too early to say for sure, but if the subtropical jet stream stays relatively active, there could be a prolonged active period of tornadoes.

Here in Colorado, we typically see the largest number of severe weather events in the months of May and June. We have a greater likelihood of tornadoes, extreme hail and strong, damaging wind gusts during that period. So that's something to keep an eye on in the next few weeks.

Does climate change increase the frequency and intensity of tornadoes in the U.S.?

There are a lot of uncertainties about how climate change will impact storm-scale severe weather events such as tornadoes. Part of the reason is that most of our climate models don't have fine enough resolution to simulate a tornado.

What we can do is look at, on a large scale, how those different atmospheric ingredients that must come together to develop severe weather might change in the future climate. Some research has shown that we should expect a shift in tornado activity further east from the traditional tornado alley in the central U.S. So we might see a greater likelihood of tornado activity in the Mississippi River Valley region and the southeastern U.S.

What are some things we can do to help communities become more resilient to severe weather events like tornadoes?

We need to raise awareness for individuals, especially those who live in places that aren't traditionally susceptible to tornadoes but may become more susceptible with changes to the atmospheric circulation.

The first thing in terms of preparation is to have multiple ways to receive severe weather warnings. That means not just relying on hearing a tornado siren as your only option, but making sure you have emergency notifications activated on your phone and you follow local meteorologists or National Weather Service accounts on social media.

Think ahead of time about where you're going if a tornado happens. Sometimes with these severe weather situations, you only have a few minutes of warning time to act before that threat is on top of you. You want to go to a room in the most interior part of your house, away from windows or below ground if that's an option. If you're in a more vulnerable structure, know if there are other community shelters available.

There's a lot of emphasis right now in the weather and atmospheric science community on improving the way we communicate the potential for severe weather. People are getting their information in many different ways nowadays. Traditionally, most people would tune in to local news if they see a storm coming. Now they are getting information through their phone's alert system and social media. We also need to make sure those communication methods are accessible to diverse audiences. That means including warnings and watches in multiple languages, and making sure that there are avenues to disseminate those warnings and watches to all communities.

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