SECTION 4: RISK ASSESSMENT
Tornados can occur anywhere in the County. Tornadoes typically impact a small area, but damage may be extensive. Tornado locations are completely random, meaning risk to tornado isn’t increased in one area of the county versus another. All of Wake County is uniformly exposed to this hazard. Extent Prior to February 1, 2007, tornado intensity was measured by the Fujita (F) scale. This scale was revised and is now the Enhanced Fujita (EF) scale. Both scales are sets of wind estimates (not measurements) based on damage. The new scale provides more damage indicators (28) and associated degrees of damage, allowing for more detailed analysis, better correlation between damage and wind speed. It is also more precise because it takes into account the materials affected and the construction of structures damaged by a tornado. Table 4.5 shows the wind speeds associated with the enhanced Fujita scale ratings and the damage that could result at different levels of intensity. Table 4.72 – Enhanced Fujita Scale EF Number 3 Second Gust (mph) Damage
Light damage. Peels surface off some roofs; some damage to gutters or siding; branches broken off trees; shallow-rooted trees pushed over.
0
65-85
Moderate damage.
Roofs severely stripped; mobile homes overturned or badly
1
96-110
damaged; loss of exterior doors; windows and other glass broken.
Considerable damage. Roofs torn off well-constructed houses; foundations of frame homes shifted; mobile homes completely destroyed; large trees snapped or uprooted; light-object missiles generated; cars lifted off ground. Severe damage. Entire stories of well-constructed houses destroyed; severe damage to large buildings such as shopping malls; trains overturned; trees debarked; heavy cars lifted off the ground and thrown; structures with weak foundations blown away some distance. Devastating damage. Well-constructed houses and whole frame houses completely leveled; cars thrown and small missiles generated. Incredible damage. Strong frame houses leveled off foundations and swept away; automobile-sized missiles fly through the air in excess of 100 m; high-rise buildings have significant structural deformation; incredible phenomena will occur.
2
111-135
3
136-165
4
166-200
5
Over 200
The most intense tornado to pass through Wake County was an F4 in 1988; this tornado also had the longest path (83 miles) and resulted in the most injuries (154 people). An F2 tornado in 1981 had the widest observed path in the county at 800 yards. An F3 tornado in 2011 resulted in the most fatalities, killing six people. An EF3 tornado in 2011 caused $115 million in recordable property damage. Impact: 3 – Critical Spatial Extent: 2 – Small Historical Occurrences NCEI storm reports were reviewed from 1988 through 2017 to assess whether recent trends varied from the longer historical record. According to NCEI, Wake County experienced 19 tornado incidents between 1988 and 2017, causing 6 fatalities, 201 injuries, $369 million in property damage and $25,000 in crop damage. Table 4.72 shows historical tornadoes in Wake County during this time period.
Wake County Multi-Jurisdictional Hazard Mitigation Plan 2019
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