Figure 4-45 Potential Damage Impacts from a Tornado

Source: National Oceanic and Atmospheric Administration

Information from the event of record is used to calculate a magnitude and severity rating for comparison with other hazards, and to assist in assessing the overall impact of the hazard on the planning area. In some cases, the event of record represents an anticipated worst-case scenario, and in others, it is a reflection of common occurrence. The event of record for Jefferson County is the June 3, 1981 which was an F2. The damages inflicted on critical facilities and services (critical infrastructure) resulted in no loss or disruption of services. Documented deaths and injuries were considered minimal (as none were reported) and the medical response of the County was considered non-impacted. However, $2.5 million dollars of damage ($7.1 million in 2020 dollars) was reported. Based on these factors, the magnitude severity rating for tornadoes is considered limited.

Climate Change Considerations

There presently is not enough data or research to quantify the magnitude of change that climate change may have related to tornado frequency and intensity. NASA’s Earth Observatory has conducted studies which aim to understand the interaction between climate induced tornadoes. Based on these studies meteorologists are unsure why some thunderstorms generate tornadoes and others don’t, beyond knowing that they require a certain type of wind shear. Tornadoes spawn from approximately one percent of thunderstorms, usually supercell thunderstorms that are in a wind shear environment that promotes rotation. Some studies show a potential for a decrease in wind shear in mid-latitude areas. Because of uncertainty of climate induced tornadoes, future updates to the mitigation plan should include the latest research on how the tornado hazard frequency and severity could change. The level of significance of this hazard should be revisited over time.

Vulnerability Assessment

All assets located in Jefferson County can be considered at risk from tornadoes although based on historic tornado paths, the risk for communities in the eastern portion is higher compared to those in western and southern portions of the county which are more mountainous. Most structures, including the

County’s critical facilities, should be able to withstand and provide adequate protection from tornadoes rated up to EF4. Those facilities with back-up generators should be fully equipped to handle tornado events should the power go out.

General Property

General damages can be both direct and indirect. Direct damage refers to what the wind event physically destroys. Indirect damage focuses on additional costs, damages and losses from secondary hazards spawned by the event. Depending on the magnitude of the wind events as well as the size of the tornado and its path, a tornado is capable of damaging and eventually destroying almost anything. Construction practices and building codes can help maximize the resistance of the structures to damage. Mobile homes, which are most often occupied by low-income, socially vulnerable residents, are the most dangerous places during a tornado. Studies indicate that 45% of all fatalities during tornadoes occur in mobile homes, compared to 26% in traditional site-built homes (Ashley 2008).

Secondary impacts of damage caused by wind events often result from damage to infrastructure. Downed power and communications transmission lines, coupled with disruptions to transportation, create difficulties in reporting and responding to emergencies. These indirect impacts of a wind event put tremendous strain on a community. In the immediate aftermath, the focus is on emergency services.

People

Community members are the most vulnerable to tornado events. Over the past 70 years there have been no deaths reported in Jefferson County due to a tornado event. During the same time period, there have been no reported injuries from tornadoes. The availability of sheltered locations such as basements, buildings constructed using tornado-resistant materials and methods, and public storm shelters, all reduce the exposure of the population. However, there are also segments of the population that are especially exposed to the indirect impacts of damaging winds and tornadoes, particularly the loss of electrical power. These populations include the elderly or disabled, especially those with medical needs and treatments dependent on electricity. Nursing homes, community-based residential facilities, and other special needs housing facilities are also vulnerable if electrical outages are prolonged, since backup power generally operates only minimal functions for a short time.

Critical Facilities and Infrastructure

Inventory assets exposed to severe wind is dependent on the age of the building, type, construction material used, and condition of the structure. Possible losses to critical infrastructure include:

Electric power disruption

Communication disruption

Water and fuel shortages

Road closures

Damaged infrastructure components, such as sewer lift stations and treatment plants

Damage to homes, structures, and shelters

Because of the unpredictability of wind events’ strength and path, most critical infrastructure that is above ground is equally exposed to the storm’s impacts.

Economy

Tornadoes can impact exposed critical infrastructure; depending on the impact and the function, this could cause a short-term economic disruption. The most common problems associated with tornadoes and damaging winds are loss of utilities. Downed power lines can cause power outages, leaving large parts of the County isolated, and without electricity, water, and communication. Damage may also limit timely emergency response and the number of evacuation routes. Downed electrical lines following a storm can also increase the potential for lethal electrical shock and can also lead to other hazard events such as wildfires.

Historical, Cultural, and Natural Resources

Damaging winds and tornadoes can cause massive damage to the built and natural environment, uprooting trees and other debris. Historic properties listed on the National Register and the State Register throughout the county may have increased vulnerability to the wind speeds generated by a tornado.

Future Development

As the County continues to develop, the number of people and housing developments exposed to the hazard increases. Proper education on building techniques, strict adherence to building codes, and the use of sturdy building materials, basements, attached foundations, and other structural techniques may minimize the property vulnerabilities. The increased availability of accurate, real-time weather forecasting and alerts the most some protection to both residents and visitors. In some cases, the costs of future mitigation efforts, even in new future development, may outweigh the potential insurance losses; for example, Jefferson County does not generally consider shelters a cost effective mitigation effort in built environments.

Overall Hazard Significance

Historically, tornadoes in Jefferson County do not have a particularly large or frequent impact on the planning area. The geographic extent of the hazard is considered limited. The probability of future occurrences is considered likely and the magnitude/severity for the event of record is limited. In addition, the HMPC considers the hazard to have a medium overall impact rating on the County. This equates to an overall impact rating of medium.