High Wind & Tornado Hazard
Stillwater Hazard Identification & Risk Assessment
Hazard Description
High Winds: Wind is the motion of air relative to the earth’s surface. Extreme windstorm events are associated with cyclones, severe thunderstorms, and accompanying phenomena such as tornadoes and downbursts. High winds can result from thunderstorms, strong cold front passages, or gradient winds between high and low pressure. Damaging winds are often called “straight-line” winds to differentiate the damage they cause from tornado damage. Downdraft winds are a small-scale column of air that rapidly sinks toward the ground, usually accompanied by precipitation as in a shower or thunderstorm. A downburst is the result of a strong downdraft associated with a thunderstorm that causes damaging winds near the ground. Damaging winds exceed 50-60 mph.
FEMA High Wind Zone Map
Tornado: According to the National Weather Service, a tornado is a violently rotating column of air, usually pendant to a cumulonimbus, with circulation reaching the ground. Tornadoes generally form from severe thunderstorms, mainly supercell thunderstorms – those that are isolated with the unimpeded inflow of moisture and enhanced by wind shear. Tornadoes may also develop along squall lines or in bands of storms associated with hurricanes. Tornadoes require moist air, instability (warm air rising), a source of lift such as a front, dryline, or heating, and wind shear (change in wind direction and speed with height). It is often difficult to separate windstorms and tornado damage when winds get above 73 mph.
Tornado Forecast Terminology
Location
Both high wind and tornado events can occur in the City of Stillwater and Stillwater Public School District.
Tornado events are usually localized; however, severe thunderstorms may result in conditions favorable to the formation of numerous or long-lived tornadoes.
The risks of these hazards are uniform over the planning area.
Extent
The Enhanced Fujita Scale or EF Scale, which became operational on February 1, 2007, is used to assign a tornado a 'rating' based on estimated wind speeds and related damage. The EF Scale was revised from the original Fujita Scale to reflect better examinations of tornado damage surveys to align wind speeds more closely with associated storm damage. The City of Stillwater and Stillwater Public Schools are located in Zone IV on the FEMA Wind Zone Map and may experience wind speeds of 250mph or a tornado with a rating of EF5 on the Enhanced Fujita Scale. According to the National Weather Service, sustained winds at 40-50mph can cause isolated wind damage. During strong thunderstorms, Stillwater may experience straight-line winds exceeding 100 mph.
Tornado/Severe Storm Risk Categories
Overall Significance Rating for Stillwater and Stillwater Public Schools: High: The criteria consistently fall in the high classifications and the event is likely/highly likely to occur with severe strength over a significant to extensive portion of the planning area
Previous Occurrences
High Wind and Tornado events have occurred in the City of Stillwater. From 2009 through 2018, the NCEI database included 87 high wind events on 43 days for Payne county. Due to the nature of high winds, it is likely that Stillwater was in some way impacted during each of those days by severe weather. Total damages from these events were estimated at $169,000, including damages within Stillwater and at Stillwater Airport. During the same time period, the database included 10 tornadic events on 7 days. Stillwater was specifically included in 3 of those events, and another event involved Lake Carl Blackwell Dam.
Damage from the 1990 Stillwater Tornado
Probability of Future Events
Based on previous occurrences, Stillwater should expect to experience damaging straight-line wind events on an annual basis. Looking at the history of high wind events over the last decade, it is likely that Stillwater will experience future events annually. The Tornado Risk Assessment Tool, from the Storm Prediction Center, was used to estimate the probability of a tornado within Stillwater. A historical analysis was run to determine the annual probability of a tornado striking any single point within Stillwater. The value is calculated by comparing the mean area affected by tornadoes each year with the total circular area of the search. The area for this analysis was 10 km at (36.150, -97.060). Based on this, there is a .126% chance of a tornado striking any point within A tornado is most likely to occur in the month of May.
Probability Rating for High Wind and Tornadoes: Highly Likely
NWS Tornado threat map for central Oklahoma for May 20, 2019
More Information
FEMA provides excellent literature that can provide more information about tornado preparedness. Follow the link below for more information:
Vulnerability and Risk Assessment
People
All the population of Stillwater is exposed and at risk for experiencing this hazard. Adequate warning systems are essential to public safety during high wind and tornado events. Though the purpose of outdoor warning sirens is to provide a warning for people participating in outdoor activities, some residents rely on them as their primary notification. Stillwater should continue to educate the public on alternate means of severe weather alerts.
Oklahoma State University draws in students from all over the United States, some from areas that do not experience extreme wind events. Main campus enrollment exceeded 24,000 students in 2019. Though Stillwater has a very good emergency management program, the impact of additional population in Stillwater during certain months of the year makes the community as a whole more vulnerable to tornado events. Stillwater and Oklahoma State University should continue to work together and ensure new students are made aware of sheltering options and how to appropriately respond to tornado watches and warnings.
Areas in Stillwater with a higher income disparity are more vulnerable to high wind and tornadoes than areas with a higher per capita income. Quality of housing, language barriers, and education level play a role in increased vulnerability to this hazard and the level of resilience post-event. Local government facilities should not be relied upon for shelter. The best option is for residents to install Safe Rooms in their home, which provide near-absolute protection to wind events of at least 250mph. Stillwater has done a tremendous job promoting residential storm shelters and funded hundreds of safe room rebates through a 2015 Hazard Mitigation Grant Program award. The City of Stillwater maintains an inventory of shelter locations. The number of storm shelters registered throughout Stillwater is fairly uniform. No specific area appears to have more or less registered shelters. The City of Stillwater should continue to promote safe rooms that meet or exceed the requirements of FEMA safe room standards, and the ICC-500.
The vulnerability of a school’s population to the impact of tornadoes not only depends on sheltering options but also on adequate warning and response. Tornadoes can happen suddenly, leaving only 3 or 4 minutes to find shelter. The education and awareness of staff and students is crucial to ensuring that this is enough time to get to a safe place within the school. Those who are unaware of tornado warning procedures or unable to receive the warning signals on their own (at-risk populations) will be at elevated risk to the impact of tornadoes. Stillwater Public Schools coordinates closely with Stillwater Emergency Management during severe weather events and when severe weather threatens the area.
Economy
While forecasting and early warning have decreased the number of fatalities associated with wind events, little has been done to address economic losses. High wind and tornado events will cause direct and indirect losses to the economy anytime businesses are affected and closed for a period. Expensive repairs and prolonged closures can cause businesses to close or relocate to other municipalities. This can lead to a loss of tax revenue and tax base for the City, as well as decreased job opportunities for City residents. The Stillwater Chamber of Commerce participated in the planning process. As part of the strategy for this plan update the Chamber emphasized placing a priority on preparing businesses in Stillwater for future disaster events.
Equipment and personal property damaged during an event also takes time and money to replace. Even if it is covered by insurance, loss of equipment may result in a loss of efficiency. Response to an event can also drain department budgets.
Built Environment
Existing Structures The residential building stock in Stillwater is diverse in the type of construction, age, and size of the building. Mobile homes or manufactured homes are the most vulnerable to high wind and tornado events.
Based on Census numbers from 2017, approximately 3.4% of housing was mobile homes. The federal government established standards in 2007 requiring all new manufactured homes to meet the requirement for installation and anchoring in accordance with its structural design and windstorm standards. Even anchored mobile homes can be severely damaged when winds gust over 80 mph.
Damages to residential properties depend on the tornado’s wind-speed and the level of wind resistance the property has been constructed to withstand. Houses with crawl spaces are more susceptible to lift. The manner in which foundations and roofs are constructed can affect a structure’s ability to withstand wind pressure.
Homes constructed to be more wind resistant, meeting high wind design requirements, such as the Insurance Institute for Business and Home Safety’s (IBHS) fortified home construction recommendations, are less vulnerable to tornado damage. Homes constructed to this structural capacity can withstand winds up to 130 mph, which is 95% of tornadoes.
Older homes are especially vulnerable to tornado events. About 14.9% of residential structures in the City of Stillwater were built before 1959. These older homes in the jurisdiction are generally more vulnerable to tornado damage than more recently built homes constructed to higher standards.
Stillwater Public Schools: Highland Park and Will Rogers, were designed with pupil safety in mind. Each school has integrated reinforced concrete ‘tube’ hallways with drop down steel doors to seal these spaces. There five such areas at the Highland Park building and two larger areas on the first floor at the Will Rogers building. The District took the design elements of a FEMA safe room and incorporated them into a school building with room for 1200 inhabitants as per the following:
- Specified hallway spaces with 8” reinforced concrete walls
- 12” reinforced concrete roof deck
- 30” reinforced concrete foundation
- Hurricane resistant steel doors on each of these spaces (built to withstand hurricane wind forces over 200 mph)
The only structural difference between these spaces and safe rooms constructed to meet specifications outlined in FEMA Publication 361, Second Edition, 2008, is the number of classroom doors entering the spaces. This was done to combine safety with functionality. At the other existing schools, reinforced areas of refuge were not incorporated into their designs (safe rooms were not part of the design vernacular until the last 20 years or so). Soon after the May 2013 Moore Tornado, Stillwater Public Schools began a review of all existing school spaces with the Architects Contracted with the Stillwater Public School System to establish what would be the safest spaces to shelter students during severe weather. While these spaces for shelter were established many years ago, it is prudent to have a new set of eyes review these decisions. The engineers indicated that current schools are still some of the most structurally sound buildings in their neighborhoods.
Infrastructure: Tornadoes and high winds in Stillwater can cause significant damage to infrastructure. Stillwater should be prepared to face the loss of power, and damage to critical infrastructure (e.g., storage tanks, hydrants, residential plumbing fixtures, distribution system) due to hail, wind, debris and flash flooding, resulting in loss of service and/or reduced pressure throughout the system. Restricted access to the facility due to debris and damaged roads is likely. Loss of power and communication lines will require alternate methods of communication until cellular service or landlines can be restored. Substations could be damaged or destroyed by storms, disrupting power distribution. Power disruptions could impact lift stations, traffic control devices, and water distribution systems.
Critical Facilities: It is impossible to predict the geographical area of impact of high wind and tornadoes. All critical facilities in Stillwater are exposed to this hazard. Stillwater should consider the purchase and installation of generators at essential critical facilities. Stillwater and critical facility operators should also consider saferooms to protect personnel.
The airport’s operations are susceptible to being disrupted by storm events. Similarly, hospitals and assisted living facilities have increased vulnerability to damages or power outages that result from high winds or tornadoes due to the populations they house. The airport intends to install safe rooms in the new airport terminal to protect employees and patrons.
Cultural Resources: Loss of structures listed on the National Historic Register would be devastating for Stillwater. All are vulnerable to high wind and tornadoes. Structural mitigation measures should maintain the historical integrity of National Register eligible or listed properties. For example, impact resistant glass systems in windows and doors should match the period and style of a historic structure.
Future Development: Future development is assumed to be less vulnerable to high wind events because of the higher building standards in place. Stillwater and Stillwater Public Schools regulate development to the 2015 IBC. There are some changes that communities can make to partially mitigate the impacts resulting from strong winds. Building codes for new structures can be strengthened, requiring increased rebar in foundations, enhanced nailing patterns for wall sheathing, and the use of Simpson Strong Ties and Straps. Building codes can also be strengthened to require the use of anchors and tie-downs of mobile homes. Additionally, individuals can choose to build to an optional Code Plus Standard, such as Fortified for Safer Living. Saferooms can be installed in new structures as well as made to adapt to existing structures. In-ground saferooms can be installed in existing structures for as little as $2,500. New development will be exposed to high winds and tornadoes just as existing development.
Natural Environment: The effects of damaging wind from high wind events or tornadoes on the natural environment are not always obvious or immediately apparent. Debris from damaged or destroyed homes can result in asbestos being deposited. Hazardous household waste, such as cleaning and automotive products, becomes an issue to animals and plants in the area, and can also contaminate water and soil. Household and industrial waste can spread into animal habitats, stormwater, rivers, and lakes. Stillwater should address these issues with the Department of Environmental Quality during the recovery period, and plan for proper disposal of debris in advance of wind or tornado events.
