Section 10: Tornadoes

Transcription

1 Contents Section 10: Tornadoes Why Tornadoes Are a Threat Hazard Profile History of Tornadoes and Windstorms People and Property at Risk Potential Damages and Losses Why Tornadoes Are a Threat Tornadoes are unquestionably the most violent storms on the planet. A tornado is a violently rotating column of air extending between, and in contact with, a cloud and the surface of the earth. The most violent tornadoes are capable of tremendous destruction with wind speeds of 250 miles per hour or more. In extreme cases, winds may approach 300 miles per hour. Damage paths can be in excess of one mile wide and 50 miles long. The most powerful tornadoes are spawned by super-cell thunderstorms. These storms are affected by horizontal wind shears (winds moving in different directions at different altitudes) that begin to rotate the storm. This horizontal rotation can be tilted vertically by violent updrafts, and the rotation radius can shrink, forming a vertical column of very quickly swirling air. This rotating air can eventually reach the ground, forming a tornado. Creating a Disaster-Resistant Lower Colorado River Basin Page 10-1

2 Table Fujita Tornado Scale F-Scale Number Intensity Wind Speed (mph) F0 Gale tornado F1 Moderate tornado F2 Significant tornado F3 Severe tornado F4 Devastating tornado F5 Incredible tornado Type of Damage Done Some damage to chimneys; breaks branches off trees; pushes over shallow-rooted trees; damages 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 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 homes leveled; structures with weak foundations blown off some distance; cars thrown and large missiles generated. Strong frame houses lifted off foundations and carried considerable distances to disintegrate; automobile sized missiles flying through the air in excess of 100 meters; trees debarked; steel reinforced concrete badly damaged. The Lower Colorado River Basin is known for frequent severe weather and thunderstorms. Thunderstorms form when warm, moist air collides with cooler, drier air. Since these masses tend to come together during the transition from summer to winter, most thunderstorms occur during the spring and fall months. Severe thunderstorms can produce tornadoes, high winds, and hail any of which can cause extensive property damage and loss of life. Tornadoes occasionally accompany tropical storms and hurricanes that move over land. Tornadoes are the most common to the right and ahead of the path of the storm center as it comes ashore. Tornadoes vary in terms of duration, wind speed and the toll that they take, as shown in Table Page 10-2 Creating a Disaster-Resistant Lower Colorado River Basin

3 Table Variations Among Tornadoes Weak Tornadoes Strong Tornadoes Violent Tornadoes 69% of all tornadoes Less than 5% of tornado deaths Lifetime minutes Winds less than 110 mph 29% of all tornadoes Nearly 30% of all tornado deaths May last 20 minutes or longer Winds mph 2% of all tornadoes 70% of all tornado deaths Lifetime can exceed one hour Winds greater than 205 mph Creating a Disaster-Resistant Lower Colorado River Basin Page 10-3

4 Hazard Profile The impact of tornadoes can be substantial. They can cause multiple deaths, completely shut down facilities for thirty days or more, and cause more than fifty percent of affected properties to be destroyed or suffer major damage. Figure Tornado and Windstorm Hazard Profile Summary Based on historical events, it is expected that there will be tornadoes in the Lower Colorado River Basin every five years. Seasonal patterns are relevant to tornadoes. Thunderstorms form when warm, moist air collides with cooler, drier air. Since these masses tend to come together during the transition from summer to winter, most thunderstorms and resulting tornadoes occur during the spring (April, May and June) and fall (October, November, and December). Warning time for tornadoes is minimal. Page 10-4 Creating a Disaster-Resistant Lower Colorado River Basin

5 History of Tornadoes and Windstorms Historical evidence, as reflected in Figure 10-2, shows that most of the Lower Colorado River Basin is vulnerable to tornadic activity. There is no defined hazard boundary for tornadoes. Figure Historical Tornado Events (NOAA Data ) Brown Mills San Saba Lampasas Burnet Mason Llano Williamson Blanco Travis Bastrop Fayette Colorado Historical Tornadoes [Fujita Intensity Scale] FO F1 F2 F3 F4 F5 Wharton Matagorda miles Table 10-6 identifies reported tornado events in the Lower Colorado River Basin, by County. Creating a Disaster-Resistant Lower Colorado River Basin Page 10-5

6 Table History of Tornado Events in the Lower Colorado River Basin, by County Event Year Date Fujita Injuries Deaths Lower Basin COLORADO COUNTY Tornado Jul F2 0 0 Colorado Tornado Jun F2 0 0 Colorado Tornado Jul 0 0 Colorado Tornado Sep F1 0 0 Colorado Tornado Feb F2 0 1 Colorado Tornado Mar F3 0 0 Colorado Tornado May F0 0 0 Colorado Tornado Sep 0 0 Colorado Tornado Sep 0 0 Colorado Tornado Sep F2 0 0 Colorado Tornado Sep F1 0 0 Colorado Tornado Feb F2 0 0 Colorado Tornado Jun F0 0 0 Colorado Tornado Apr F0 0 0 Colorado Tornado Jan F Colorado Tornado Jan F1 0 0 Colorado Tornado May F0 0 0 Colorado Tornado Jan F0 0 0 Near Eagle Lake Tornado Mar F0 0 0 Bernardo Tornado Mar F0 0 0 Washington Tornado May F0 0 0 Sheridan Tornado Aug F0 0 0 Eagle Lake Tornado Nov F0 0 0 Borden Funnel cloud Nov 0 0 Columbus Funnel cloud Nov 0 0 Sheridan MATAGORDA COUNTY Tornado Apr F1 0 0 Matagorda Tornado May F2 0 0 Matagorda Tornado Nov F0 0 0 Matagorda Tornado Mar F1 0 0 Matagorda Tornado Sep F0 0 0 Matagorda Page 10-6 Creating a Disaster-Resistant Lower Colorado River Basin

7 Event Year Date Fujita Injuries Deaths Tornado Sep F2 0 0 Matagorda Tornado Sep F1 0 0 Matagorda Tornado Jul F0 0 0 Matagorda Tornado Sep F3 4 7 Matagorda Tornado Apr F Matagorda Tornado May F0 0 0 Matagorda Tornado Oct F1 0 0 Matagorda Tornado Mar F1 0 0 Matagorda Tornado Jun F0 0 0 Matagorda Tornado Aug F0 0 0 Matagorda Tornado Sep F1 0 0 Matagorda Tornado Apr F0 0 0 Matagorda Tornado Sep 0 0 Matagorda Tornado May F1 0 0 Matagorda Tornado Dec F1 0 0 Matagorda Tornado Jan F0 0 0 Matagorda Tornado Jan F0 0 0 Matagorda Tornado Apr F1 0 0 Matagorda Tornado May F0 0 0 Dayton Tornado Dec F0 0 0 Collegeport Tornado Dec F0 0 0 Blessing Tornado Dec F0 0 0 Markham Tornado Sep F0 5 0 Sargent Funnel cloud May 0 0 Sargent Funnel cloud May 0 0 Palacios Tornado May F0 0 0 Matagorda Tornado Sep F0 0 0 Van Vleck WHARTON COUNTY Tornado Dec F2 0 0 Wharton Tornado May F2 0 0 Wharton Tornado Aug F1 0 5 Wharton Tornado Aug F0 0 0 Wharton Tornado Jun F1 0 0 Wharton Tornado Nov F3 0 1 Wharton Tornado Jun F2 0 0 Wharton Tornado Apr F2 0 0 Wharton Tornado Sep F0 0 0 Wharton Creating a Disaster-Resistant Lower Colorado River Basin Page 10-7

8 Event Year Date Fujita Injuries Deaths Tornado Sep F1 1 4 Wharton Tornado Sep 0 0 Wharton Tornado Jun F1 0 0 Wharton Tornado Oct F2 0 0 Wharton Tornado Mar F2 0 0 Wharton Tornado Jun F1 0 0 Wharton Tornado Jul 0 0 Wharton Tornado Nov F1 0 0 Wharton Tornado Dec F1 0 1 Wharton Tornado Apr F1 0 0 Wharton Tornado Jan F1 0 0 Wharton Tornado Jan F1 0 0 Wharton Tornado Aug F0 0 0 Wharton Tornado Jan F1 0 0 Wharton Tornado May F1 0 0 Wharton Tornado Aug F0 0 0 Wharton Tornado Nov F1 0 0 Wharton Tornado Nov F1 0 0 Wharton Tornado Apr F2 0 0 Egypt Tornado Mar F1 0 0 Egypt Tornado Aug F0 0 0 El Campo Tornado Apr F0 0 0 Wharton Tornado Jun F0 0 0 El Campo Funnel cloud Oct 0 0 Mustang Funnel cloud May 0 0 Hillje Funnel cloud Sep 0 0 El Campo Funnel cloud Sep 0 0 El Campo Tornado Mar F0 0 0 Hungerford Tornado Mar F0 0 0 Wharton Tornado Sep F0 0 0 Boling Middle Basin Event Year Date Fujita Injuries Deaths BASTROP COUNTY Tornado Apr F3 0 3 Bastrop Tornado May F 0 0 Bastrop Page 10-8 Creating a Disaster-Resistant Lower Colorado River Basin

9 Event Year Date Fujita Injuries Deaths Tornado Apr F3 0 1 Bastrop Tornado Apr F 0 0 Bastrop Tornado Sep F0 0 0 Bastrop Tornado May F2 0 0 Bastrop Tornado Oct F0 0 0 Bastrop Tornado Nov F2 0 0 Bastrop Tornado Mar F1 0 0 Bastrop Tornado May F 0 0 Bastrop Tornado Apr F3 0 0 Bastrop Tornado Aug F2 0 0 Bastrop Tornado Jan F2 0 0 Bastrop Tornado Apr F0 0 0 Bastrop Tornado Apr F0 0 0 Bastrop Tornado May F0 0 0 Bastrop Tornado Oct F1 0 0 Elgin Tornado Nov F0 0 0 Rosanky Tornado Nov F0 0 0 Elgin FAYETTE COUNTY Tornado Apr F1 0 0 Fayette Tornado Apr F2 0 0 Fayette Tornado Jun F1 0 0 Fayette Tornado Oct F1 0 2 Fayette Tornado Jan F2 0 0 Fayette Tornado Sep 0 0 Fayette Tornado Sep F1 0 0 Fayette Tornado Sep F 0 0 Fayette Tornado May F1 0 0 Fayette Tornado Feb F2 0 0 Fayette Tornado Nov F1 0 0 Fayette Tornado Jul F0 0 0 Fayette Tornado Oct F2 0 1 Fayette Tornado Jun F0 0 0 Fayette Tornado Apr F1 0 0 Fayette Tornado Sep F0 0 0 Fayette Tornado Apr F0 1 0 Fayette Tornado Mar F0 0 0 La Grange Funnel cloud Jan N/A 0 0 Holman Creating a Disaster-Resistant Lower Colorado River Basin Page 10-9

10 Event Year Date Fujita Injuries Deaths Funnel cloud Jun N/A 0 0 Flatonia Tornado Jun F1 0 0 Engle Tornado Nov F0 0 0 West Pt Tornado Dec F1 0 0 Ledbetter TRAVIS COUNTY Tornado Oct F1 0 0 Travis Tornado Apr F3 0 0 Travis Tornado Oct F1 0 0 Travis Tornado Mar F2 0 0 Travis Tornado Mar F1 0 0 Travis Tornado Apr F 0 0 Travis Tornado Apr F0 0 0 Travis Tornado May F3 0 0 Travis Tornado Jul F1 0 0 Travis Tornado Apr F0 0 0 Travis Tornado May F1 0 0 Travis Tornado May F0 0 0 Travis Tornado Sep F1 0 1 Travis Tornado Jul F2 1 4 Travis Tornado Aug F0 0 0 Travis Tornado Jan F2 0 0 Travis Tornado Mar F1 0 0 Travis Tornado Mar F1 0 2 Travis Tornado May F Travis Tornado May F0 0 0 Travis Tornado May F2 0 0 Travis Tornado May F0 0 0 Travis Tornado May F1 0 0 Travis Tornado Mar F0 0 0 Travis Tornado Mar F0 0 8 Travis Tornado May F1 0 0 Travis Tornado Apr F2 0 0 Travis Tornado May F 0 0 Travis Tornado May F0 0 0 Travis Tornado 1980 Feb 29 F1 0 0 Travis Tornado Apr F3 0 3 Travis Tornado Aug F2 0 4 Travis Page Creating a Disaster-Resistant Lower Colorado River Basin

11 Event Year Date Fujita Injuries Deaths Tornado May F1 0 0 Travis Tornado Jun F1 0 0 Travis Tornado Jun F0 0 0 Travis Tornado May F1 0 0 Travis Tornado Sep F1 0 0 Lago Vista Tornado May F2 0 0 Four Pts Tornado May F4 5 1 Lakeway Tornado Aug F1 0 0 Lake Travis Funnel cloud Jul N/A 0 0 Lago Vista Tornado Mar F0 0 0 Bergstrom AFB Tornado Mar F0 0 0 Pflugerville Tornado Nov F1 0 0 Bergstrom AFB Tornado Nov F0 0 0 Pflugerville Tornado Dec F1 0 0 Near Manor WILLIAMSON COUNTY Tornado Apr F3 0 6 Williamson Tornado Jun F1 0 0 Williamson Tornado Sep F2 0 0 Williamson Tornado Apr F3 0 0 Williamson Tornado May F1 0 0 Williamson Tornado Jun F1 0 0 Williamson Tornado Mar F1 0 0 Williamson Tornado Apr F2 0 0 Williamson Tornado May F2 0 0 Williamson Tornado Sep F0 0 0 Williamson Tornado Apr F0 0 0 Williamson Tornado Apr F0 0 0 Williamson Tornado May F1 0 0 Williamson Tornado Oct F1 0 0 Williamson Tornado Oct F2 0 0 Williamson Tornado May F1 0 0 Williamson Tornado Apr F2 0 0 Williamson Tornado Jul F2 0 0 Williamson Tornado 1980 Feb 29 F1 0 0 Williamson Tornado Apr F3 1 2 Williamson Tornado Aug F0 0 0 Williamson Tornado Feb F2 0 0 Williamson Creating a Disaster-Resistant Lower Colorado River Basin Page 10-11

12 Event Year Date Fujita Injuries Deaths Tornado May F1 0 4 Williamson Tornado Apr F1 0 0 Williamson Tornado May F1 0 0 Williamson Tornado Jun F1 0 0 Williamson Tornado Sep F0 0 0 Williamson Tornado Dec F2 0 3 Williamson Tornado May F Williamson Tornado Apr F0 0 0 Williamson Tornado Jun F0 0 0 Williamson Tornado May F0 0 0 Georgetown Funnel cloud Sep N/A 0 0 Georgetown Tornado May F Jarrell Tornado May F Cedar Park Tornado Jul F1 0 0 Round Rock Funnel cloud Jul N/A 0 0 Granger Tornado Mar F2 0 0 Leander Tornado Mar F0 0 0 Georgetown Tornado Nov F0 0 0 Taylor Upper Basin BLANCO COUNTY Tornado May F3 0 2 Blanco Tornado Aug F0 0 0 Blanco Tornado Jun F0 0 0 Blanco Tornado May F1 0 0 Blanco Tornado May F1 0 0 Blanco Tornado Aug F2 0 0 Blanco Tornado Apr F1 0 0 Blanco Tornado Apr F0 0 0 E Stonewall Funnel cloud Jun 0 0 Round Mtn Tornado Nov F0 0 0 Blanco BROWN COUNTY Tornado Jun F2 0 0 Brown Tornado Jun F1 0 0 Brown Tornado Oct F2 0 1 Brown Tornado Apr F1 0 0 Brown Tornado Jun F0 0 0 Brown Page Creating a Disaster-Resistant Lower Colorado River Basin

13 Event Year Date Fujita Injuries Deaths Tornado Apr F1 0 1 Brown Tornado Apr F Brown Tornado Apr F1 0 0 Brown Tornado May F1 0 0 Brown Tornado Oct F1 0 0 Brown Tornado Jun F0 0 0 Brown Tornado Aug F1 0 0 Brown Tornado Aug F0 0 0 Brown Tornado Jun F1 0 0 Brown Tornado Jul F1 0 0 Brown Tornado Jun F1 0 0 Brown Tornado May F0 0 0 Early Tornado May F1 0 0 Lake Brownwood Tornado May F0 0 0 Grosvenor Tornado May F1 0 0 May Tornado May F0 0 0 Bangs Tornado May F1 0 0 Brownwood Tornado May F1 0 0 Owens BURNET COUNTY Tornado Jun F1 0 0 Burnet Tornado Aug F1 0 0 Burnet Tornado Sep F1 0 1 Burnet Tornado Sep 0 0 Burnet Tornado Sep 0 0 Burnet Tornado May F3 0 0 Burnet Tornado Mar F Burnet Tornado May F1 0 0 Burnet Tornado May F1 0 0 Burnet Tornado Sep F1 0 0 Burnet Tornado Oct F0 0 0 Spicewood Tornado May F3 0 1 Marble Falls Tornado Jun F1 0 0 Burnet Tornado Sep F0 0 0 Smithwick Tornado Mar F0 0 0 Marble Falls Tornado Nov F0 0 0 Burnet Creating a Disaster-Resistant Lower Colorado River Basin Page 10-13

14 Event Year Date Fujita Injuries Deaths LAMPASAS COUNTY Tornado May F1 0 0 Lampasas Tornado May F1 0 0 Lampasas Tornado Jan F1 0 0 Lampasas Tornado May F2 0 0 Lampasas Tornado May F1 0 0 Lampasas Tornado May F1 0 0 Lampasas Tornado Oct F0 0 0 Lampasas Tornado Oct F0 0 0 Lometa Tornado May F0 0 0 Kempner Tornado Feb F0 0 0 Lampasas LLANO COUNTY Tornado Aug F1 0 0 Llano Tornado May F1 0 0 Llano Tornado May 0 0 Llano Tornado Jul F1 0 0 Llano Tornado May F0 0 0 Llano Tornado Oct F1 0 0 Llano Funnel cloud Apr 0 0 Llano Tornado Mar F1 0 0 Llano Tornado Mar F0 0 0 Kingsland MASON COUNTY Tornado Jun F2 0 0 Mason Tornado Sep 0 0 Mason Tornado Sep F0 0 0 Mason Tornado May F1 0 0 Mason Tornado Mar F3 0 1 Mason Tornado May F4 1 6 Loyal Valley Tornado Apr F0 0 0 Katemcy MILLS COUNTY Tornado Mar F1 0 0 Mills Tornado Oct F1 0 0 Mills Tornado May F0 0 0 Mills Tornado Apr F1 0 0 Mills Tornado May F2 0 0 Mills Page Creating a Disaster-Resistant Lower Colorado River Basin

15 Event Year Date Fujita Injuries Deaths Tornado Apr F0 0 0 Mills Tornado Jan F1 0 0 Goldthwaite Tornado Apr F0 0 0 Goldthwaite SAN SABA COUNTY Tornado Jul F1 0 0 San Saba Tornado Jan F1 0 0 San Saba Tornado Apr F1 0 0 San Saba Tornado May F2 0 0 San Saba Tornado Jun F0 0 0 San Saba Tornado May F0 0 0 San Saba Tornado Jun F1 0 0 San Saba Tornado Apr F0 0 0 Hall Valley Tornado Jan F0 0 0 San Saba Tornado Mar F2 0 0 Richland Springs Tornado May F0 0 0 Elm Grove Tornado May F0 0 0 San Saba Source: National Climatic Data Center, National Oceanic and Atmospheric Administration. People and Property at Risk Because it cannot be predicted where a tornado will touch down, all buildings and facilities are considered to be exposed to the tornado hazard and could potentially be impacted. All the population, buildings, critical facilities, infrastructure and lifelines, and hazardous materials facilities are considered exposed to the hazard and could potentially be impacted. Table Exposure to Tornadoes, by County, in the Lower Colorado River Basin Population Critical Residential Buildings Commercial Buildings Facilities Jurisdiction (2000) Number Value ($) Number Value ($) Number Bastrop 57,733 25,225 5,217,746, ,610, Blanco 8,418 4, ,587, ,325, Brown 37,674 15,396 3,458,440, ,457, Burnet 34,147 16,142 3,468,080, ,632, Creating a Disaster-Resistant Lower Colorado River Basin Page 10-15

16 Colorado 20,390 8,668 1,913,971, ,172, Fayette 21,804 10,551 2,374,106, ,805, Lampasas 17,762 7,263 1,625,322, ,173, Llano 17,044 12,235 2,798,204, ,209, Mason 3,738 2, ,552, ,275,000 8 Matagorda 37,957 15,786 3,661,791, ,273, Mills 5,151 2, ,705, ,211, San Saba 6,186 2, ,886, ,222, Travis 812, , ,547,615,000 3,874 16,786,680, Wharton 41,188 14,626 3,480,988, ,973, Williamson 249, ,280 44,379,555, ,477,991, TOTAL 1,371, , ,408,548,000 5,674 24,505,008, Infrastructure and Lifelines Hazardous Materials Facilities Oil Pipe Gas Pipe Highway Railroad Number of Number of Jurisdiction (km) (km) (km) (km) Sites Materials Bastrop Blanco Brown Burnet Colorado Fayette Lampasas Llano Mason Matagorda 209 1, Mills San Saba Travis Wharton 162 1, Williamson TOTAL 1,247 3,271 3,113 1, Page Creating a Disaster-Resistant Lower Colorado River Basin

17 Potential Damages and Losses A review of the data on historical tornado events indicates a relative constant frequency of tornado events, with 20 to 40 tornadoes occurring every five years in the Lower Colorado River Basin. Figure Frequency of Tornadoes (NOAA Data ) Frequency Year Williamson Wharton Travis San Saba Mills Matagorda Mason Llano Lampasas Fayette Colorado Burnet Brown Blanco Bastrop On average, the Lower Colorado River Basin experiences an annualized loss of over $14.5 million from tornadoes. Table 10-5 shows potential annualized expected property losses and annualized percent loss ratios for each county in the Lower Colorado River Basin. Colorado County, followed by Mason County, experiences the highest relative risk as measured against replacement value. Creating a Disaster-Resistant Lower Colorado River Basin Page 10-17

18 Table Potential Annualized Losses and Percent Loss Ratio for Tornadoes, by County, in the Lower Colorado River Basin Annualized Expected Property Losses ($) Annualized Percent Loss Ratio Jurisdiction Bastrop 26, % Blanco 1,146, % Brown 154, % Burnet 169, % Colorado 1,029, % Fayette 88, % Lampasas Negligible % Llano 17, % Mason 167, % Matagorda 113, % Mills 7, % San Saba 11, % Travis 8,022, % Wharton 43, % Williamson 3,617, % TOTAL 14,616,891 LCRA facilities (described starting on page 3-34) are also at risk from this hazard. However, no estimate is currently available of potential damages and losses to those facilities. Page Creating a Disaster-Resistant Lower Colorado River Basin