Jefferson County - East-West Gateway Coordinating Council

A Regional Overview All-Hazard Mitigation Plan 55
F-Scale (Fujita 1981), as listed in Storm Data. This study follows the accepted nomenclature
that F2 and F3 tornadoes are strong and F4 are violent.
Ostby (1993) found that the occurrence of weak tornadoes (F0-F1) has shown a dramatic
increase since 1980, while the number of strong and violent tornadoes have either
remained steady or decreased. Reasons for this include improved verification efforts by
local NWS offices and the marked increase in storm chasing. Since strong and
violent tornadoes produce a more stable long-term data set, these categories were the
main focus of this study.
Description of Hazard
A tornado is a vortex of rapidly rotating air that must be in contact with the ground. This
means that to be a tornado, the swirling winds must be at the surface, capable of doing
damage. If there is debris (dust and other objects swirling in the winds), it is definitely a
tornado, even if there is no visible funnel cloud. If there is no debris with a funnel cloud,
then it might be a tornado but one cannot be certain that it is (or is not). A tornado can
move over a surface with few objects to be picked up and swirled about, or one may not
be able to see all the way to the surface beneath a funnel cloud because of intervening
hills, trees, or buildings. All funnel clouds should be treated as if they are tornadoes, unless
one can be certain that they will not touch down. See Figure J32 below.
FIGURE J32 VIEW OF TORNADIC THUNDERSTORM
Source: NOAA
When storms influence a large area, the chances for significant hazards increase. The
majority of windstorms in a convective system are of marginal severity, with only isolated
events reaching high intensity. The most threatening situation would be for a very intense
convective wind event that also affected a large area. It appears that a few times each year
in North America, extreme convective wind events of this sort do occur. To date, no such
storm has struck a major city during a vulnerable time (e.g., the morning or evening rush
hours). However, it is only a matter of time until this sort of unfortunate concatenation
actually occurs. Given that the area affected can approach that of a tropical cyclone's
damage swath, and certainly far exceeds that affected during a tornado outbreak (while