Twisted science: Why tornado forecasting is tough

March 7, 2011 - 0:0

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.

Beginning this year, a major upgrade to the nation's radar system could help with tornado spotting, which isn't easy to do, as evidenced by the 75-percent false alarm rate for tornado warnings. ""It's easier to predict a large outbreak of thunderstorms than how many tornadoes there might be,"" said Bob Henson, a meteorologist with the University Corporation of Atmospheric Research in Boulder, Colo.
Last year saw 1,280 tornadoes, about what is expected for a given year in the United States, said Greg Carbin, the warning coordination meteorologist with the Storm Prediction Center in Norman, Okla. The 2011 season is off to a slow start for tornadoes, Carbin said, with only about half as many tornado reports as usual for this time of the year.
The early storms are tied to an unusually prolonged Arctic outbreak, said climatologist Bill Patzert of NASA's Jet Propulsion Laboratory in Pasadena, Calif.
""Hopefully this pattern that's going on right now will not be a preview of coming attractions,"" Patzert told OurAmazingPlanet.
-------Tornado season
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.  Most of the biggest and baddest tornadoes spin off supercell storms, which are up to 50,000 feet (15,000 meters) tall and can last for hours. Supercells need three main ingredients to form: energy, rotation and a cap. The United States has plenty of all three and once they mix, a supercell storm can spit out a tornado in minutes.
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.
""It's easy enough to put a thunderstorm together. It's very difficult to get that thunderstorm to produce a significant tornado,"" Carbin said.
Only a fraction of supercell storms create tornadoes, and meteorologists are not exactly sure why. Meteorologists can't even say when during a supercell's lifetime a tornado will form. This is not what the good folks in Tornado Alley want to hear.
Most of the Earth's tornadoes touch down in the hotbed known as Tornado Alley, bordered by the Dakotas to the north, the Gulf Coast to the south, the Rocky Mountains to the west and the Appalachian Mountains to the east.
Southeast of Tornado Alley is Dixie Alley, home to the deadliest tornadoes. Dixie Alley spreads from the Lower Mississippi Valley to the Upper Tennessee Valley, including Arkansas, Mississippi, Louisiana, Alabama, Georgia and the Florida panhandle. Dixie Alley is where storm assessment teams are sifting through the damage from last week's tornadoes, which had estimated strengths up to EF2.
A tornado's strength is based on the amount of damage it causes. The Enhanced Fujita tornado damage scale runs from 0 (minor damage) to 5 (a storm that is powerful enough to  destroy a house).
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.
The problem with how scientists count and rank tornadoes is that someone has to see it or it has to hit something. The biggest tornado of all time could have roared through an open field and no one would have known it.