Understanding the Threat Posed by Tornadoes

A recent NSSL study, using data from 1921 to 1995, estimated the daily climatological probability of an F2 or greater tornado occurring near any location in the U.S. For this work developing highly accurate and accessible estimates of the long-term threat from thunderstorms, winds, and large hail as well as tornadoes, an NSSL scientist was awarded a Department of Commerce Silver Medal.

(a)
Tornado days per year
(c)
Probability of F4 or greater tornado days per thousand years
(b)
Probability of significant (F2 or greater) tornado days per 100 years
(d)
Graph shows 5-year annual cycles, with the peak period of occurance in May
  1. Probability of Any Tornado:
    The map shows the average number of days per year any tornado, no matter how strong or weak, might occur within 25 miles of a point. The highest numbers indicate where at least one tornado might occur somewhere within 25 miles as often as on 1.5 days per year.
  2. Significant Tornado (F2 or greater):
    Now we're looking at days per century. In other words, central Oklahomans can expect an F2 or greater tornado within 25 miles about every 3 years.
  3. Violent Tornado (F4 or greater):
    Now the scale is days per millennium, meaning that southcentral Oklahoma may have a violent tornado within 25 miles about once every 20 years.
  4. Annual Cycle:
    Residents of Norman, OK experience a distinct tornado season, beginning late February and peaking late May. Even though we are in the heart of tornado alley and can expect one- to one-and one-half tornado days per year, our chances on any particular day peak at only about two percent.

More information about these hazard graphs and other severe thunderstorm climatology

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