Tornado classifications modified

National Weather Service officials unveiled a newly modified system of evaluating tornado strength.

BY SIMON GRAF | ATLANTA, Ga. | February 3, 2006


National Weather Service officials unveiled a newly modified system of evaluating tornado strength that is expected to greatly assist damage assessment teams.

The National Weather Service (NWS) said it will make changes to the Fujita Scale, a system developed in 1971 to determine tornado strength. Those changes will be fully implemented by February 2007, and called the Enhanced Fujita Scale. The announcement came Thursday during a meeting of the American Meteorological Society (AMS).

"The EF Scale takes into account additional variables which will provide a more accurate indication of tornado strength," said David Johnson, NWS director.

Changes include the creation of 28 damage indicators, anything from trees to telephone poles, buildings and strip malls, to assess tornado strength. The old scale was based only on damage to a single frame house, causing assessors problems when a tornado would strike a grove of trees or tear through a mobile home park.

In addition, officials found the wind speeds were too high under the old scale. Under the definition of the strongest tornado, a category F5, winds up to 318 mph were said to be responsible for the complete obliteration of a house. But over time, engineers and weather scientists found far less speed is necessary to cause such destruction. Now, category F5 tornadoes have estimated wind speeds of at least 200 mph.

Jim St. John, a research scientist at the Georgia Institute of Technology's School of Earth and Atmospheric Sciences, said the new system will be very helpful to assessors, tornado insurance agents, architects and legal advisors.

St. John said he participated in damage assessment surveys of two tornadoes that swept through Camilla, Ga., six years ago. Both tornadoes were designated category F3.

"Assigning one number to an entire path of a tornado - I thought was not a good idea," he said. "It's generally representative, but there were places where I saw old-fashioned brick buildings turned into piles of bricks. That's more than a F3. Nothing against the Fujita Scale, but it turns out you might want something that gives you a little more flexibility, is more descriptive."

In a similar manner, the National Oceanic and Atmospheric Administration (NOAA) also unveiled a system to categorize the power of major snowstorms in the Northeast. The Northeast Snowfall Impact Scale will determine the impact of a big storm after it strikes. Its ranking system is similar to the Fujita Scale used for tornadoes.

Weather officials will compare a present snowstorm with a similar one in the past and assign the storm in one of five categories: Notable, Significant, Major, Crippling or Extreme.

"With its rankings, the scale will also give a better perspective on how these major storms affected populations in the Northeast," said Louis Uccellini, director of NOAA's National Centers for Environmental Prediction.

Also during Thursday's AMS meeting, federal weather officials have announced the return of La Nina - the periodic cooling of ocean waters in parts of the Pacific Ocean that often is associated with wet weather in the Pacific Northwest and drier conditions in the South.

La Nina is also associated with more frequent hurricanes in the Atlantic, but NOAA officials say they do not yet know whether that will be the case this season.

"It is too early to say with confidence what effects this La Nina event will have on the 2006 hurricane season," said Jim Laver, director of the agency's Climate Prediction Center.

The last La Nina happened between 2000 and 2001. NOAA officials added that the La Nina was expected to last through late spring and possibly into summer.


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