Twisted Science: Why Tornado Forecasting Is Tough

Storm chasing scientists ride toward a tornado in Wyoming, just across the border from Nebraska, during the VORTEX2 mission in 2009.

(Image credit: UCAR)

The average tornado warning sounds 13 minutes before touchdown.

The warning time is so short that residents of towns in Tornado Alley are told not to flee their homes when they hear a siren. That would be too dangerous, especially during a tornado that strikes in the dead of night. Hunker down and hope for the best, people are often told.

Tornado warnings are so short because tornadoes are almost impossible to forecast until it's too late. Unlike hurricane season, there is no forecast for tornado season — and no straightforward climate change link. But the same atmospheric players that fueled this year's wild winter of bitter cold and tremendous snowfalls could create dangerous tornadoes this spring and summer, scientists said, which could cast the light on the limits of tornado prediction.

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Tornado season typically starts in March and hits its stride from May to June, though it's possible for tornadoes to pop up during any time of the year (November is often called the second tornado season). During the spring and early summer, thousands of tornadoes will strike the United States. And tornadoes can strike anywhere at any time, day or night — a tornado hit New York City at night in September last year.

The energy comes from the collision of warm, moist air from the Gulf of Mexico and the cool, dry Arctic air. The warm air rises and hits wind shear, a layer of the atmosphere where the winds change direction over a short height, and creates the rotation — think of a pinwheel with air pushing in opposite directions on the top and bottom. Below the pinwheel, a cap of warm air bottles up surface heat below about 10,000 feet (3,000 m).

Heat builds until it punches through the cap, triggering a thunderstorm. With enough air shooting up and down, the pinwheel is knocked on its side, creating a huge rotating mass of clouds called a mesocyclone — the hallmark of a tornado-spawning supercell storm. But there are a number of flies in the tornado-generating ointment, Carbin said — and because of that, a tornado is actually a rare event.

Tornadoes are also rated based on their wind speeds. An average tornado has maximum wind speeds of about 112 mph (180 kph) or less, measures around 250 feet (76 m) in width and travels approximately one mile before unraveling. Some chart toppers have had 300 mph (480 kph) winds— almost twice that of 1992's devastating Hurricane Andrew. The hurricane was a Category 5 storm, the highest hurricane rating.

This year, meteorologists are rolling out the latest in weather radar technology to help see the unseen. Say goodbye to Doppler radar and say hello to Dual Pol, or Dual Polarization radar. These radars can see debris such as grass and leaves circulating within a tornado, even when the twister is cloaked by a rain cloud. A Dual Pol radar will fire up in Oklahoma this week and five more will be tested this summer. Over the next two years, all 169 of the National Weather Service's Doppler radars will be replaced.

The Arctic Oscillation tends to be weekly to monthly, so there's really no way to create a tornado forecast based on it. Numerical models are becoming more advanced, but they take months to calculate the probabilities of a tornado forming, so that's not practical either, said Andrew Taylor, a meteorologist with the National Weather Service. So it's watch and wait for those hoping to avoid tornadoes.

Brett Israel was a staff writer for Live Science with a focus on environmental issues. He holds a bachelor’s degree in biochemistry and molecular biology from The University of Georgia, a master’s degree in journalism from New York University, and has studied doctorate-level biochemistry at Emory University.