SECTION 4: RISK ASSESSMENT
PROBABILITY OF FUTURE OCCURRENCE
Probability of future occurrence was calculated based on past occurrences and was assumed to be uniform across the county. In a thirty-five-year span between 1988 and 2023, Wake County experienced 21 separate tornado incidents over 18 separate days. This correlates to a 60 percent annual probability that the county will experience a tornado somewhere in its boundaries. Only four of these past tornado events was a magnitude EF2 or greater; therefore, the annual probability of a significant tornado event is approximately 11 percent.
Probability: 3 – Likely CLIMATE CHANGE
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. However, evidence suggests that the new variations to the natural climate have the potential to alter the environment necessary for tornado formation. NOAA states that “tornado activity is dependent upon two things: the strength of atmospheric instability, which promotes rising air and thunderstorm formation; and vertical wind shear, which provides the necessary rotation for tornadic thunderstorms”. Studies show that human -caused climate change is expected to increase atmospheric instability with rising temperatures and humidity that may cause an increase of days where weather conditions are suitable for tornado formation. Additionally, the National Climate Assessment Fifth Addition states that “while the average annual number of tornadoes appears to have remained relatively constant, there is evidence that tornado outbreaks have become more frequent, that tornado power has increased, that tornado activity is increasing in the fall, and that “Tornado Alley” has shifted eastward. ” Most of North Carolina, including Wake County, is seeing an upward trend in tornado frequency. Because of uncertainty with the influence of climate change on 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 PEOPLE People and populations exposed to the elements are most vulnerable to tornados. 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. According to the 2022 American Community Survey (ACS), 12,462 occupied housing units (2.7%) in Wake County are classified as “mobile homes.” Based on an estimated average of 2. 75 persons per household from the 2022 ACS, there are approximately 34,270 people in Wake County living in mobile homes. Since 1950, the NCEI records four fatalities and 70 injuries attributed to tornadoes in Wake County; these fatalities and injuries were the result of tornadoes rated as low as EF0, illustrating the destructive power of tornadoes and the dangers they pose to exposed populations without proper shelter. PROPERTY General damages to property are both direct (what the tornado physically destroys) and indirect, which focuses on additional costs, damages and losses attributed to secondary hazards spawned by the tornado, or due to the damages caused by the tornado. Depending on 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.
Wake County, NC Multi-Jurisdictional Hazard Mitigation Plan
WSP June 2024 Page 221
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