Jefferson County - East-West Gateway Coordinating Council

A Regional Overview All-Hazard Mitigation Plan 67
Seasonal Pattern
Tornadoes occur mostly during the spring and summer; the tornado season comes early in
the south and later in the north because spring comes later in the year as one moves
northward. Tornadoes and storms usually occur during the late afternoon and early
evening, but they have been known to occur in every state in the United States, on any day
of the year, and at any hour.
Based on Table J38, in Jefferson County, most of the storms occurred in the month of April
with 8 tornadoes. May had a total of 7 tornadoes, both June and July had 2, and
September, October, November and December had 1 each per month.
Speed of Onset And/Or Existing Warning Systems
Tornadoes and other severe windstorms can occur instantly. The National Oceanic and
Atmospheric Agency and other agencies (National Weather Service) have prioritized the
research and understanding of the development of these types of storms in order to
protect citizens and their property. As a result of this research, Doppler Radar was
developed. Doppler Radar research was started in the 1950s by the Weather Radar
Laboratory. At about the same time, research was beginning on severe storms through the
National Severe Storms Project. In late 1963 the NSSL was formed to continue and
enhance these two efforts. By the 1970's it was clear that Doppler Radar would greatly
benefit the National Weather Service and could help to provide much-improved severe
thunderstorm and tornado warnings.
The new Radar, or NEXRAD for Next Generation Radar (officially WSR-88D), provides
forecasters with a detailed look at storms through reflectivity and velocity displays.
Reflectivity indicates rainfall or precipitation intensity and velocity displays the speed and
direction of the winds within the storm.
Through the Doppler Effect, a physical phenomenon marked by a change in frequency
depending on the motion of an object toward or away from a point, the radar can give a
picture of the winds within a storm. If, within a small area, high winds toward the radar
are adjacent to high winds away from the radar, a circulation has developed and
forecasters prepare to issue a warning. With this capability, tornado warning lead times
have increased in the last 10 years from less than 5 minutes to nearly 12 minutes (NWS).
Phased Array Radar - NSSL will soon begin adapting SPY-1 radar technology for use in
spotting severe weather.
The mission of the Severe Weather Warning Applications and Technology Transfer (SWAT)
team is to develop severe weather warning applications and transfer them to users to
enhance their capability to warn of severe weather. There are two focus groups within
SWAT: