NOAA’S NATIONAL WEATHER SERVICE IMPROVES TORNADO RATING SYSTEM

by Phil Hysell

 

NOAA’s National Weather Service has implemented the Enhanced Fujita (EF) Scale to rate tornadoes to replace the original Fujita (F) Scale. The EF Scale will continue to rate tornadoes on a scale from zero to five, but ranges in wind speed will be more accurate with the improved rating scale.

 

"The EF Scale takes into account additional variables which will provide a more accurate indication of tornado strength,” said retired Air Force Brig. Gen. David L. Johnson, director of NOAA's National Weather Service. “The EF Scale will provide more detailed guidelines that will allow the National Weather Service to more accurately rate tornadoes that strike in the United States.”

 

The original F Scale was developed in 1971 by Dr. T. Theodore Fujita to rate tornadoes and estimate associated wind speed based on the damage they cause. The new EF Scale refines and improves the original scale. It was developed by the Texas Tech University Wind Science and Engineering (WISE) Research Center, along with a forum of wind engineers, universities, private companies, government organizations, private sector meteorologists and NOAA meteorologists from across the country.

 

Limitations of the original F Scale may have led to inconsistent ratings, including possible overestimates of associated wind speeds. The EF Scale incorporates more damage indicators and degrees of damage than the original F Scale, allowing more detailed analysis and better correlation between damage and wind speed. The original F Scale historical data base will not change. An F5 tornado rated years ago is still an F5, but the wind speed associated with the tornado may have been somewhat less than previously estimated. A correlation between the original F Scale and the EF Scale has been developed. This makes it possible to express ratings in terms of one scale to the other, preserving the historical database.

 

 

Original F scale                                             Enhanced F (EF) scale

 

Rating       MPH                                                 Rating             MPH

     0            40-72                                                     0                  65-85

     1          73-112                                                    1                  86-110

     2          113-157                                                  2                110-135

     3          158-207                                                  3                136-165

     4          208-260                                                  4                 166-200

     5          261-318                                                  5                   > 200

 

With the EF Scale, an F3 tornado will have estimated wind speeds between 136 and 165 mph, whereas with the original F Scale, an F3 tornado has winds estimated between 162-209 mph. The wind speeds necessary to cause “F3” damage are not as high as once thought and this may have led to an overestimation of some tornado wind speeds. There is still some uncertainty as to the upper limits of the strongest tornadoes so F5 ratings do not have a wind speed range. Wind speed estimations for F5 tornadoes will be left open ended and assigned wind speeds greater than 200 miles an hour.

The NWS is the only federal agency with authority to provide “official” tornado EF Scale ratings. The goal is assign an EF Scale category based on the highest wind speed that occurred within the damage path.

 

For more information on the EF scale, visit: http://www.spc.noaa.gov/efscale.