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Your top 4 questions about tornadoes

May is the peak month for twisters here in the U.S.

Don't try this at home: Researchers study tornadoes from a safe distance. Photo credit: Josh Wurman, CSWR

Don’t try this at home: Researchers study tornadoes from a safe distance. Image via Josh Wurman, CSWR

National Science Foundation (NSF) Assistant Director for Geosciences Roger Wakimoto is a tornado expert. He talks here about these deadly storms – including what new studies tell us about how, where, and when twisters form.

1. What are the peak months for tornadoes in the U.S.?

The highest average number of U.S. tornadoes per month is in May, followed by June. May is the time when the two ingredients that are required – very unstable air and strong vertical wind shear – are most common. That being said, we’re now seeing a trend of tornadoes breaking out earlier in spring, such as April or even March.

2. What U.S. states/regions have the most tornadoes, and why?

The Great Plains is where the most tornadoes occur; the region is often referred to as Tornado Alley. It’s an ideal location due to warm, humid air flowing northward from the Gulf of Mexico at low levels, and cold, dry air coming down from Canada at upper levels, producing very unstable air. Beyond the Great Plains, tornadoes occurring over the southeastern U.S. have recently attracted scientists’ interest.

3. Are there tornadoes in countries other than the U.S.?

Tornadoes are typical in the mid-latitudes, between 30 and 50 degrees north and south. Countries that experience tornadoes include Bangladesh, Japan, Australia, New Zealand, China, South Africa, Argentina and many nations in Europe.

4. What are we learning about how and when tornadoes form?

Tornadoes usually occur in association with particular types of severe storms, such as supercells and squall lines, called tornado parental storms. But not all these parental storms generate tornadoes. Tornadogenesis, as the formation of tornadoes is called, remains the “holy grail” of tornado research. Recent work suggests that the temperature of the outflow air from the parent thunderstorm could play a critical role. There is a lot we don’t yet understand, including the circumstances that produce tornado outbreaks.

A large wall cloud arcs around a rotating thunderstorm updraft. This storm was documented by the VORTEX2 field campaign on June 6, 2010 near Ogallala, Nebraska. Image via Roger Wakimoto/NSF

A large wall cloud arcs around a rotating thunderstorm updraft. This storm was documented by the VORTEX2 field campaign on June 6, 2010 near Ogallala, Nebraska. Image via Roger Wakimoto/NSF

Image of a strong tornado near Arab, Alabama, part of the outbreak on April 27, 2011. Image via Charles Whisenant/NSF

Image of a strong tornado near Arab, Alabama, part of the outbreak on April 27, 2011. Image via Charles Whisenant/NSF

An NSF-sponsored field project called TWIRL (Tornadic Winds: In situ and Radar measurements at Low-levels) is currently underway to help improve our understanding of tornadogenesis and tornado evolution. The research includes the use of three Doppler on Wheels (DOW) mobile radars, and vehicles with deployable pods for weather observations.

NSF is also working with NOAA on a project called VORTEX-SE, which focuses on studies of tornadoes that bring deadly threats to the southeastern U.S. From these efforts, researchers hope to discover critical information about which storms are most likely to produce tornadoes so forecasters can issue earlier warnings.

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Bottom line: A tornado expert answers 4 basic questions about tornadoes.

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Eleanor Imster

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