Jefferson County - East-West Gateway Coordinating Council

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Jefferson County – Section 2
• National Weather Service Focus Group Staff Listing (SWAT-NWS)
• Federal Aviation Administration Focus Group Staff Listing (SWAT-FAA)
Map of Hazards
Refer to Figure J45 (located in the back of the Technical Appendix) that depicts those areas
in Jefferson County that are susceptible to severe windstorms.
Statement of Probable Future Severity
Based on the previous twenty-three events in the Jefferson County, the future probable
severity is shown below.
Future Probable Severity By F-Scale
F0 Negligible
F1 Limited
F2 Limited
F3 Critical
F4 Catastrophic
F5 Catastrophic
Statement of Probable Risk
The risk of tornadoes in Jefferson County is moderate with twenty-three tornadoes.
Surrounding counties such as St. Louis County, St. Charles and Jefferson County have
greater numbers: 28, 35 and 23, respectively. By nature, tornadoes strike randomly. Based
in information from the National Oceanic and Atmospheric Administration, tornadoes
occur between five to seven times per 10,000 square miles per year and downbursts occur
between 14 to 17 times per 10,000 square miles per year within the EWG planning region.
There are between 40 to 60 thunderstorm days per year and approximately five to eight
annual events of hail per 10,000 square miles within the EWG planning region. These
figures are different from the Federal Emergency Management agency (FEMA) data. FEMA
indicates that there are between six to ten tornadoes per 1,000 square miles in the EWG
planning region. Refer to Table J40 below for risk.
TABLE J40 TORNADO RISK
F# Events Risk Probable Risk of
Occurrence
By F-Scale
F0 22% F0 Likely
F1 48% F1 Likely
F2 17% F2 Likely
F3 9% F3 Possible
F4 4% F4 Possible
F5 0 F5 Unlikely