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Supercells: tornadoes that last

By Curtis Krueger and Graham Brink
Published February 3, 2007


They call it a supercell - an especially deadly type of tornado that rarely strikes the state.

The one that hit Florida early Friday, killing at least 19 people and destroying hundreds of homes, was the most powerful tornado to hit the state in nearly a decade.

Supercells last longer than other kinds of tornadoes, which is why this twister stayed powerful enough to rip across three Florida counties.

It reached 140 to 150 mph, rated an EF-3 on the National Weather Service tornado scale, and was strong enough to splinter homes and toss semitrailer trucks.

Some unusual weather patterns that may be related to El Nino helped create the explosive combination of wind, moisture and low pressure that spawned the storm.

"You're seeing the power of creation right in front of you," said Tony Reynes, a National Weather Service meteorologist in Ruskin.

Teams from the National Weather Service toured the storm-damaged area Friday to assess its severity, and that effort is continuing.

But Shawn Bennett, meteorologist in charge of the National Weather Service in Ruskin, said it was the fiercest tornado since 1998. That year a group of seven tornadoes struck in the middle of the night in Central Florida, killing 42 people.

Barry Goldsmith, also a National Weather Service meteorologist, said the story of this tornado began with high winds in the upper atmosphere. The "disturbance" of winds churned so swiftly that it created an area of low pressure below it, near the surface of the Gulf of Mexico, south of the Mississippi-Alabama border.

Wide bands of wind began swirling around this low-pressure center, in the same counterclockwise motion you see when watching satellite images of a hurricane. These bands scooped up warm, moist air from the tropics and shoved it toward Florida.

Warm air rises, so this action created a powerful updraft, an engine for starting the tornado.

Meanwhile, winds rushed across Florida toward the low-pressure zone, smashing into the twirling center and pushing the winds higher by sheer force. More fuel for the engine.

The storm crossed over Citrus County and moved slightly to the northeast as it came inland.

Goldsmith said tornadoes often die shortly after the warm, moist air starts to shoot skyward. Sometimes falling rain cools off the warm air below and stops it from rising,

But in this case, high-level winds called jets streaked across the top of the storm. This action helped to spin the tornado, and also acted like a suction pump to keep the updraft going.

"Normally, on the average, the structure of that jet doesn't make it this far south," Reynes said.

El Nino effects

So what caused the high-level winds? They can zip across Florida during El Nino years, when the powerful jet stream tends to flow farther south than in other years.

Much of this winter's weather has been influenced by El Nino, which also is linked to increased tornado activity in Florida.

The strong El Nino in 1998 and a similar one in 1983 coincided with the two most active tornado seasons in Florida history, according to the National Weather Service. But researchers are reluctant to say that any single tornado was a result of El Nino.

"El Ninos of the magnitude of 1983 and 1998 increase the chances of severe weather outbreaks and stronger ... tornadoes simply by providing more opportunities for all the right ingredients to come together," stated one study.

The National Weather Service's Tropical Prediction Center announced last month that El Nino-related effects were increasingly probable over North America at least through the end of March.

Goldsmith said meteorologists tracked the potential for a storm for a couple of days, seeing that a "disturbance" of winds in the upper atmosphere was traveling toward Florida, where there was a mass of warm air nearby.

"Two days ago we kind of looked at this and said, 'Hmm, this looks interesting.' Yesterday we looked at it and said, 'Uh-oh.' "