Methodology

This resource uses the SPC Tornado database to evaluate the historical occurrences of tornadoes in proximity to a given point. It then computes statistics regarding those tornadoes, including strike probabilities for various time periods. These statistics and probabilities may be useful to public and private entities in developing tornado-risk mitigation plans.

The probability values shown on this resource assume uniform distribution of tornadoes in space and time. A tornado is included in the calculation if the starting, mid-point, or ending point of the tornado is within the search radius.

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"%POINT" Probability
This represents the annual probability of a tornado striking any single point within the search area. The value is calculated by comparing the mean area affected by tornadoes each year with the total circular area of the search.

Formula: %POINT = At / Ac / Yn

Where: At = The total area affected by tornadoes for the period of interest. Ac = The total area of the search circular area. Yn = Number of years in the period

Example: (May tornadoes for this search) At = 9 km2 Ac = 804.24704 km2 Yn = 70 years %POINT = 0.0112 / 70 = 0.000160 ( 0.016%)

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"%25MI" Probability
This represents the mean daily probability of at least one tornado within 25 miles of any point within the search area. The value is calculated by determining the mean number of tornadoes that occur each day within the total search area, and multiplying by the ratio of the area of a 25mi radius circle to the total domain area.

Formula: %25MI = [(Nt / Yn) * (A25 / Ac)] / Dn

Where: Nt = The total number of tornadoes within the search area for the period of interest. Yn = Number of years in the period A25 = The area of a 25 mile radius circle. Ac = The total area of the search circular area. Dn = Number of days in period.

Example: (May tornadoes for this search) Nt = 7 km2 Yn = 70 years A25 = 5084 km2 Ac = 804.24704 km2 Dn = 31 days %25MI = [(7 / 70) * (5084 / 804)] / 31 = 0.020392 (2.039%)

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Comments

* Due to the nature of the search process, tornadoes with long tracks can extend well outside of the original search area. These tornadoes can significantly affect the strike probability calculations - usually resulting in an over-estimation of tornado risk.

* Geographical areas that have significant bodies of water or mountains may have un-representative strike probabilities. This is due to the lack or tornadoes and/or reports of tornadoes in these areas. For instance, if a coastal point is used, only part of the search area will have the potential for tornado reports, while the remainder of the area is offshore. This can result in an under-estimation of tornado risk for land areas.