United States Nuclear Regulatory Commission - Protecting People and the Environment

Tornado Climatology of the Contiguous United States (NUREG/CR-4461, Revision 2; PNNL-15112, Revision 2)

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Publication Information

Manuscript Completed: December 2006
Date Published:
February 2007

Prepared by:
J.V. Ramsdell, Jr.,
J.P. Rishel

Pacific Northwest National Laboratory
P.O. Box 999
Richland, Washington 99352
Operated by Battelle Memorial Institute

A.J. Buslik, NRC Project Manager

Prepared for:
Division of Risk Assessment and Special Projects
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

NRC Job Code N6337

Availability Notice


Characteristics of tornadoes reported in the contiguous United States from January 1950 through August 2003 have been used to determine tornado strike probabilities and maximum wind speeds for use in nuclear power plant design. Maps have been prepared that show the distribution of tornado events and design wind speeds. Appendices contain the number of tornadoes and estimates of strike probabilities and maximum wind speeds by 1°, 2°, and 4° latitude and longitude boxes.

The methods used in this analysis are identical to those used in the analysis leading to publication of the tornado climatology in NUREG/CR-4461, Rev. 1. The primary difference between the climatology presented here and the climatology in Rev. 1 is that wind speed estimates in this climatology are based on the Enhanced Fujita Scale that correlates wind speeds with damage caused by tornadoes. These wind speeds are significantly lower than the wind speeds in the original Fujita Scale. A second difference is that the wind speeds in the Enhanced Fujita Scale are nominally 3-second average wind speeds, whereas the wind speeds in the original Fujita Scale were nominally the fastest quarter-mile wind speeds. The National Weather Service intends to implement the Enhanced Fujita Scale by February 2007.

The results of this analysis indicate that a maximum wind speed of about 230 mph is appropriate for tornadoes with a best estimate probability of 10-7 per year for the central portion of the United States; a maximum wind speed of 200 mph is appropriate for a region of the United States along the east coast, and the western great plains; and a maximum wind speed of 160 mph is appropriate for the western United States. Corresponding wind speeds in NUREG/CR-4461, Rev. 1, were 300 mph, 260 mph, and 200 mph, respectively.

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