HAZARD IDENTIFICATION

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1 6.5.3 Tornado Hazard Identification Description of the Tornado Hazard A tornado is a violent windstorm characterized by a twisting, funnel-shaped cloud extending to the ground. Tornadoes are most often generated by thunderstorm activity (but sometimes result from hurricanes and other coastal storms) when cool, dry air intersects and overrides a layer of warm, moist air forcing the warm air to rise rapidly. The damage caused by a tornado is a result of the high wind velocity and wind-blown debris, also accompanied by lightning or large hail. According to the National Weather Service, tornado wind speeds normally range from 40 to more than 300 miles per hour. The most violent tornadoes have rotating winds of 250 miles per hour or more and are capable of causing extreme destruction and turning normally harmless objects into deadly missiles. Each year, an average of over 800 tornadoes is reported nationwide, resulting in an average of 80 deaths and 1,500 injuries (NOAA, 2002). They are more likely to occur during the spring and early summer months of March through June and can occur at any time of day, but are likely to form in the late afternoon and early evening. Most tornadoes are a few dozen yards wide and touchdown briefly, but even small short-lived tornadoes can inflict tremendous damage. Highly destructive tornadoes may carve out a path over a mile wide and several miles long. Severity of the Tornado Hazard The destruction caused by tornadoes ranges from light to inconceivable depending on the intensity, size, and duration of the storm. Typically, tornadoes cause the greatest damages to structures of light construction such as residential homes (particularly mobile homes), and tend to remain localized in impact. The Fujita Scale for Tornadoes (Table ) was developed to measure tornado strength and associated damages. F-Scale Number F0 F1 F2 F3 F4 Intensity Phrase Gale tornado Moderate tornado Significant tornado Severe tornado Devastating tornado Table Fujita Scale for Tornadoes (Pre-2007) Wind Speed MPH MPH MPH MPH MPH Type of Damage Done Some damage to chimneys; breaks branches off trees; pushes over shallow-rooted trees; damages to sign boards. The lower limit is the beginning of hurricane wind speed; peels surface off roofs; mobile homes pushed off foundations or overturned; moving autos pushed off the roads; attached garages may be destroyed. Considerable damage. Roofs torn off frame houses; mobile homes demolished; boxcars pushed over; large trees snapped or uprooted; light object missiles generated. Roof and some walls torn off well-constructed houses; trains overturned; most trees in forest uprooted. Well-constructed houses leveled; structures with weak foundations blown off some distance; cars thrown and SECTION 6: PAGE 64

2 F5 F6 Incredible tornado Inconceivable tornado Source: The Tornado Project, MPH MPH large missiles generated. Strong frame houses lifted off foundations and carried considerable distances to disintegrate; automobile sized missiles fly through the air in excess of 100 meters; trees debarked; steel re-enforced concrete structures badly damaged. These winds are very unlikely. The small area of damage they might produce would probably not be recognizable along with the mess produced by F4 and F5 wind that would surround the F6 winds. Missiles, such as cars and refrigerators would do serious secondary damage that could not be directly identified as F6 damage. If this level is ever achieved, evidence for it might only be found in some manner of ground swirl pattern, for it may never be identifiable through engineering studies. In February 2007, use of the Fujita Scale was discontinued. In its place, the Enhanced Fujita Scale was put into use. The Enhanced Fujita Scale retains the same basic design as its predecessor, but reflects a more refined assessment of tornado damage surveys, standardization and damage consideration to a wider range of structure types. The new scale takes into account how most structures are designed, and is thought to be a much more accurate representation of the surface wind speeds in the most violent tornadoes. It is important to note the date a tornado occurred, as tornadoes which occurred prior to February 2007 are classified by the old scale and will not be converted to the Enhanced Fujita Scale. Table Enhanced Fujita Scale for Tornadoes (Post-2007) F-Scale Number Wind Speed Type of Damage Done EF EF EF EF EF EF5 >200 Light damage. Peels surface off some roofs; some damage to gutters or siding; branches broken off trees; shallow-rooted trees pushed over. Moderate damage. Roofs severely stripped; mobile homes overturned or badly 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; SECTION 6: PAGE 65

3 automobile-sized missiles fly through the air in excess of 100 m (109 yd); high-rise buildings have significant structural deformation; incredible phenomena will occur. Source: Storm Prediction Center The entire planning area is subject to experiencing the entire range of the tornado hazard. Impact to People and Property from the Tornado Hazard Due to their spontaneous development and unpredictable paths, as well as their violently rotating winds, tornadoes have the potential to catastrophically impact people and property, to produce widespread damage, and to cause both injury and loss of life. Even smaller, less intent tornadoes can wreak havoc and total destruction on the people and property in their paths. Especially at risk are those buildings that are constructed to older or lesser standards, such as mobile homes. These populations are particularly vulnerable, as even smaller tornadoes such as EF1 can devastate both the structure and the inhabitants. Occurrences of the Tornado Hazard According to the NOAA Storm Prediction Center (SPC), the highest concentration of tornadoes in the United States has been in Oklahoma, Texas, Kansas and Florida respectively. Although the Great Plains region of the Central United States does favor the development of the largest and most dangerous tornadoes (earning the designation of tornado alley ), Florida experiences the greatest number of tornadoes per square mile of all U.S. states (SPC, 2002). Figure shows tornado activity in the United States based on the number of recorded tornadoes per 1,000 square miles. As this figure shows, the planning area experiences 1-5 recorded tornadoes per 1,000 square miles, on average. SECTION 6: PAGE 66

4 Figure Tornado Activity in the United States Source: American Society of Civil Engineers. The tornadoes associated with tropical cyclones are most frequent in September and October when the incidence of tropical storm systems is greatest. This type of tornado usually occurs around the perimeter of the storm, and most often to the right and ahead of the storm path or the storm center as it comes ashore. These tornadoes commonly occur as part of large outbreaks and generally move in an easterly direction. Figure shows how the frequency and strength of extreme windstorms vary across the United States. The figure was produced by the Federal Emergency Management Agency and is based on 40 years of tornado history and over 100 years of hurricane history. Zone IV, the darkest area on the illustration, has experienced both the greatest number of tornadoes and the strongest tornadoes. As demonstrated in the figure, the planning area is located in Zone III, which is a 200 MPH design wind-speed zone, and is just outside of a designated Hurricane- Susceptible Region. SECTION 6: PAGE 67

5 Figure Wind Zones in the United States Source: Federal Emergency Management Agency. When compared with other states, Texas ranks first in the number of tornado events, first in tornado deaths, first in tornado injuries, and first in damages. These rankings are based upon data collected for all states and territories for tornado events between 1950 and 1994 (SPC, 2003). On July 24, 2008, at approximately 10:00am, a small tornado formed from the remnants of Hurricane Dolly and touched down in San Antonio, causing damage to numerous homes and businesses south of downtown. At least two other tornados touched in different parts of San Antonio. 311 customer service department call takers took in 34 calls from residents reporting damages to their homes and requesting assistance with debris removal. One home sustained major damage from a large tree being uprooted and lifting the foundation of an addition to the home. Gas and electricity were turned off, and the residents sought shelter elsewhere. Map shows the recorded tornadoes in the planning area, from SECTION 6: PAGE 68

6 Map Historical Occurrences of Tornadoes in the Planning Area According to the National Climatic Data Center, the geographic area of the Alamo Area Council of Governments experienced 243 tornado events from 1950 through 2009 (Table ). These events caused 11 deaths and a total of approximately $85,976,000 in property damage (NCDC, 2010). SECTION 6: PAGE 69

7 County Table Tornado Activity in the AACOG Region ( ) # of Recorded Tornadoes Total Property Damage Recorded Deaths Atascosa 15 $308,000 0 Bandera 8 $53,000 0 Bexar 62 $60,211,000 3 Comal 13 $600,000 0 Frio 16 $90,000 5 Gillespie 15 $2,260,000 0 Guadalupe 21 $513,000 0 Karnes 24 $483,000 0 Kerr 11 $360,000 0 Medina 22 $20,993,000 3 Wilson 14 $105,000 0 TOTAL 221 $85,976, Source: National Climatic Data Center. The following section describes some notable tornado events that have occurred in the planning area since Note that this list is not exhaustive, but is intended to be illustrative only. 10/12/ Gillespie County A tornado in Stonewall, rated F3 on the Fujita Scale. The worst damage was to trees and homes between RR1623 and SH290, across the northwest and north central part of Stonewall. Ninety homes sustained damage with six destroyed. In addition, seven mobile homes were destroyed and 20 damaged. Damage was also reported to 22 buildings. Approximately $1 million in damage was recorded and five persons were injured as a result of the tornado. 10/12/ Medina County The tornado nearly 500 yards in width was rated F2 because of the level of damage from the airport to the armory. Almost 150 homes in Hondo and almost 50 more outside the city were damaged, with nearly 100 mobile homes damaged. At least 10 houses and 30 mobile homes were destroyed. Losses to homes and mobile homes were estimated at $1.5 million dollars. In addition, nearly two-dozen aircraft were damaged and destroyed at the Hondo Airport. Commercial losses totaled nearly $18 million dollars. At least 25 persons were injured. 03/19/ Bexar County A series of six tornadoes struck the area. Tornado number five, strongest of the six and rated as an EF2 tornado on the Fujita Scale, produced a hit-and-miss damage path. It completely destroyed four mobile homes and damaged several others, causing $3.5 million in property damage and 30 injuries. 07/21/07 Guadalupe County City of Seguin An EF2 tornado touched down in the city, resulting in more than $50,000 in property damage. SECTION 6: PAGE 70

8 Probability of Future Occurrences of the Tornado Hazard Tornado probability has not been mapped because it can be assumed that based on previous occurrences, the entire region is likely to experience a tornado and all areas are equally vulnerable. Considering historical occurrences, the probability of future occurrences of the tornado hazard in the planning area is high. Location and Extent of the Tornado Hazard Maps through show the recorded tornado occurrences for each participating county, as recorded by NCDC, from Note that these maps only show the tornadoes recorded by NCDC, and therefore do not necessarily account for every tornado that has occurred in the stated timeframe. Where available, track information is included in the map. SECTION 6: PAGE 71

9 Map Atascosa County Historic Tornado Occurrences SECTION 6: PAGE 72

10 Map Bandera County Historic Tornado Occurrences SECTION 6: PAGE 73

11 Map Bexar County Historic Tornado Occurrences SECTION 6: PAGE 74

12 Map Comal County Historic Tornado Occurrences SECTION 6: PAGE 75

13 Map Frio County Historic Tornado Occurrences SECTION 6: PAGE 76

14 Map Gillespie County Historic Tornado Occurrences SECTION 6: PAGE 77

15 Map Guadalupe County Historic Tornado Occurrences SECTION 6: PAGE 78

16 Map Karnes County Historic Tornado Occurrences SECTION 6: PAGE 79

17 Map Kerr County Historic Tornado Occurrences SECTION 6: PAGE 80

18 Map Medina County Historic Tornado Occurrences SECTION 6: PAGE 81

19 Map Wilson County Historic Tornado Occurrences SECTION 6: PAGE 82

20 Risk Assessment This subsection of the Plan Updates provides estimates of future tornado losses, i.e. risk. Each of the loss calculations is based on best available data, but they must be considered estimates because highly detailed engineering was not performed as part of this planning process. Methodology and Limitations After discussion and review of the best available data regarding this hazard, and in consideration of the potential impacts of this hazard to the planning area, the EMC determined that the risk assessment should be limited to a qualitative analysis. At the time of this update, insufficient data exists to quantify the planning area s risks from and exposure to this hazard. Qualitative Each participating jurisdiction was asked to provide a qualitative risk assessment ranking regarding the tornado hazard. (For definitions of these rankings, please see Table 6.4-1, earlier in this section.) The results of this assessment are presented in the table below. Jurisdictions Ranking Hazard as Low City of Selma Comal County City of Garden Ridge City of Bulverde City of New Braunfels City of Schertz City of La Vernia Qualitative Risk Assessment Results Tornado Jurisdictions Ranking Hazard as Moderate City of Alamo Heights City of Balcones Heights City of Converse City of Helotes City of San Antonio City of St. Hedwig City of Somerset City of Terrell Hills City of Windcrest Frio County Guadalupe County City of New Berlin City of Seguin City of Karnes City of Kenedy City of Runge City of Falls City Medina County City of Castroville City of Devine City of Hondo City of Natalia San Antonio River Authority Jurisdictions Ranking Hazard as High Atascosa County City of Charlotte City of Christine City of Jourdanton City of Lytle City of Pleasanton City of Poteet Bandera County City of Bandera Bexar County City of Kirby City of Leon Valley City of Live Oak City of Universal City Kerr County City of Ingram City of Kerrville Wilson County City of Stockdale City of Von Ormy City of Dilley City of Pearsall Gillespie County City of Fredericksburg Karnes County Risk Assessment Conclusions The majority of the participating jurisdictions ranked the tornado hazard as a high or moderate impact. It is worth noting that all buildings and facilities can be considered at risk from this hazard. SECTION 6: PAGE 83