What causes tornadoes?
Tornadoes most commonly form from large-scale storm systems that traverse the U.S. from west to east. Typically, severe thunderstorms will develop in the warm, moist airmass ahead of the storm system, and can produce tornadoes. Tornadoes can result from isolated thunderstorms or as part of a large squall line that forms along a cold front. Severe thunderstorms and tornadoes need an unstable (warm, humid) airmass, cold air at mid-levels of the atmosphere, and strong wind shear (turning and strengthening of winds with height in the atmosphere) to develop. In a thunderstorm, the tornado begins as a circulation several thousand feet above the ground, and slowly descends to the ground.
Organized tropical systems like tropical storms and hurricanes can also produce tornadoes, but usually smaller and shorter-lived than those spawned by mid-latitude systems.
How do tornadoes impact Southeast Texas?
Texas averages 137 tornadoes each year, the highest number of any state in the U.S. Tornadoes are most common during the spring and summer months nationwide, but due to our proximity to the Gulf of Mexico as a moisture source, tornadoes can occur in southeast Texas any time of year. In fact, some of the worst tornado outbreaks on record in southeast Texas have actually occurred in the late fall and winter months. Strong to violent tornadoes (those EF-2 or larger) are fairly uncommon in southeast Texas, but they can occur. Tornadoes in southeast Texas are usually isolated incidences, but occasionally can occur in "outbreaks" of several in one day.
What is a waterspout?
Waterspouts are weak tornadoes that form over water. Along the upper Texas coast they are most common during the morning hours of the summer months, and do not require a thunderstorm to be present to form. Usually, waterspouts are short-lived, do little if any damage, and have winds well under 100 mph. On rare occasions, they can move inland as tornadoes and cause damage.
How are tornadoes rated?
Weak tornadoes, those classified as EF-0 or EF-1, comprise approximately 75% of all tornadoes nationally and 93% of tornadoes in southeast Texas. These tornadoes typically last only a few minutes and produce winds less than 100 mph. Usually they will cause only spotty minor damage, such as downing trees or power lines. Tornadoes in the EF-0 or EF-1 category contribute to less than 5% of tornado deaths annually.
Strong tornadoes are those classified as EF-2 or EF-3, with winds of 111 to 165 mph. They comprise approximately 24% of all tornadoes and account for nearly 30% of all tornado deaths across the entire U.S.. They may last twenty minutes or longer. Tornadoes of this magnitude are uncommon in southeast Texas, accounting for only 7% of all tornadoes that have affected the area.
Violent tornadoes, those classified as EF-4 or EF-5, account for approximately 1% of all tornadoes nationwide. These tornadoes account for approximately 65% of all tornado deaths nationally, and their lifetime can exceed one hour. Violent tornadoes are rare in southeast Texas, accounting for less than 1% of local tornado reports. Only one violent tornado has been recorded in our area since records began (Channelview, Texas - November 1992 Outbreak).
Tornado rating has nothing to do with the size of the tornado. Each tornado is rated after it touches down, based on the damage that it has done. Some large tornadoes can cause little damage, while some small tornadoes are quite violent.
Note: Tornado percentages above obtained from www.spc.noaa.gov/publications/mccarthy/tor30yrs.pdf and Ashley, Walker S., 2007: Spatial and Temporal Analysis of Tornado Fatalities in the United States: 1880–2005. Wea. Forecasting, 22, 1214–1228.
|Enhanced F Scale for Tornado Damage
||Wind Speed (3 sec gust in mph)
What are some tornadoes and tornado outbreaks that have impacted Southeast Texas?
- February 10, 2013
- A short-lived mid-morning EF-0 tornado in the Livingston area (Polk County) injured a mad while he was seeking shelter from his trailor.
- January 25, 2012
- Six tornadoes and a lot of wind damage were produced in this severe weather outbreak. There were tornadoes from the Caldwell area to Brenham to Huntsville to Pearland.
- January 9, 2012
- A severe weather outbreak produced wind damage and several tornadoes from around Mission Bend to near Brazos Bend State Park to Texas City.
- August 30, 2009
- An EF-1 tornado with wind speeds in a 80 to 90 mph range produced a damage path around 1/3 of a mile long and 100 feet wide on Galveston Island. The tornado originated as a waterspout then moved onshore near the intersection of 29th Street and Seawall Blvd. The tornado caused a couple injuries and damaged several structures along its path.
- May 3, 2009
- An EF-1 tornado produced a 2 mile long and 50 yard wide damage path, destroyed an outdoor shed, shifted a double wide trailer and flipped a pickup truck. A small metal boat was carried 300 yards across an open field. Numerous trees were either uprooted or snapped off close to the ground. Damage was estimated at around $60,000.
- December 29, 2006
- An F1 tornado moved south to north across the central College Station area (Brazos County) and produced a damage path that was five miles long and at times 200 yards wide. The majority of the damage was concentrated along the Harvey Mitchell Parkway with several businesses and an apartment complex receiving significant damage.
- October 4, 2004
- An F0 tornado struck the Friendswood (Galveston County) area in the afternoon. An 18 wheeler was overturned at the intersection of FM 2351 and FM 518. Trees were snapped and numerous fences were damaged. Widespread power outages occured in and around city. A 16 foot boat was flipped over in a residential yard.
- March 30, 2002
- An F3 tornado struck the La Porte area on this evening. The tornado began its path three miles south of La Porte and then moved northeast and partially demolished a mini mart and caused major roof damage to an apartment complex. The path of this tornado was 3/4 of a mile long.
- October 23, 1997
- An unseasonably strong storm system spawned 11 tornadoes around southeast Texas. Two F2 tornadoes were reported. One tornado tracked 12 miles from Lake Houston through western Liberty County. Another tornado tracked 25 miles through Montgomery and Liberty Counties, resulting in major damage to several homes and mobile homes. An F1 tornado struck Sugarland damaging 109 homes. Total damage for the day was near $2.0 million.
- The November 1992 Outbreak
- Perhaps the worst tornado outbreak on record in Southeast Texas occurred on November 21, 1992. Over a dozen tornadoes resulted from a powerful storm system moving across the area. Most of the tornadoes occurred during a 2 hour period during the early afternoon that day. Widespread damage occurred in the Houston area that day, with hundreds of homes and businesses affected. A total of 34 injuries and over $12 million in damage resulted from this outbreak. Amazingly, there were no fatalities.
An F4 tornado struck Channelview, tracking on the ground 20 miles into Liberty County. At one point, the tornado widened to over a mile in diameter! This tornado resulted in 15 injuries, 200 homes destroyed, and 1000 homes damaged.
An F3 tornado was reported which tracked from Luce Bayou in northeastern Harris County into Liberty County for a total of 28 miles. Two F2 tornadoes occurred in Harris County. One tornado tracked right through the middle of Houston, traveling from Hermann Park northward to near Intercontinental Airport damaging 600 buildings and injuring 6 people. Another tornado struck the Kelliwood subdivision in Katy causing significant home damage and injuring six. It then went on to hit the West Side Airport, tracking a total of 21 miles and damaging a dozen aircraft.
National Weather Service Tornado Products:
- TORNADO WATCH
Issued by the Storm Prediction Center in Norman, Oklahoma. Usually covers a fairly large area (such as all or a portion of southeast Texas) and runs from 6 to 8 hours. A TORNADO WATCH is usually issued a few hours in advance of severe weather, and means conditions are favorable for severe thunderstorm development.
- TORNADO WARNING
Issued by your local National Weather Service office. Typically runs for a short time span, usually 30 minutes to 1 hour and covers a relatively small area of one to a few counties. Means a tornado has been sighted or a strong circulation has been detected by radar. If a TORNADO WARNING is issued for your area, seek shelter immediately!!
Remember, tornadoes can occur even if a TORNADO WATCH or a TORNADO WARNING are not in effect!!
What can you do ahead of time to prepare your home or business for a tornado threat?
- Develop a plan for your family or coworkers at your place of residence or business.
- Do frequent tornado drills.
- Know the county where you live. All National Weather Service warnings are issued by county.
- Purchase a NOAA Weather Radio with a warning alarm tone and battery back-up. It will automatically be activated when a warning is issued.
What do you do when a tornado is approaching or a TORNADO WARNING is issued?
- In a home or business, go to the safest place, usually on the lowest floor of the building in a central room away from windows. Get under a sturdy piece of furniture.
- Stay away from windows.
- Get out of automobiles immediately! They will become flying debris in a tornado. Do not try to outrun a tornado in your car.
- If caught outside, lie flat in a nearby ditch or depression.
- Abandon mobile homes. Even if tied down, they will offer no protection in a tornado.
- Remain alert for signs of an approaching tornado.
- Be aware of rapidly darkening skies.
- Listen for approaching strong winds or the sound of a freight train.
- Look for rotation in clouds or a "wall cloud", a distinct lowering from the back of a thunderstorm. In southeast Texas, hazy skies and tall trees make sighting tornadoes difficult. Also, tornadoes are often obscured by rain or can occur at night.
- Remember, NEVER try to outrun a tornado in your car.
What should a school do to prepare for a tornado?
1. Develop a severe weather action plan and conduct frequent drills. Children should kneel on the floor, putting their head on the ground and covering their neck with their hands. The neck and lower head are the most vulnerable parts of the body to flying debris.
2. Each school should be inspected and shelter areas designated by a registered engineer or architect. Schools without basements should use interior rooms and hallways on the lowest floor and away from windows.
3. Have a compressed air horn or megaphone to activate the alarm in case of power failure.
4. Make special provisions for disabled students or those in trailers.
5. Have someone on hand to turn off the gas or electricity if the school is damaged.
6. Prepare to keep children at school beyond regular hours if threatening weather is expected. Children are safer in school than traveling on the roadways during severe weather. Students should NOT be sent home early if severe weather is approaching.
7. Large, high rooms are dangerous when a tornado is approaching. Gymnasiums, cafeterias, and auditoriums offer no protection in a tornado. Other large facilities can take similar measures for tornado preparedness.
Tornado Statistics for Southeast Texas (1992-2012 graphs)
- Tornado Events (by month)
- Tornado Events (by time)
- Tornado Intensity Statistics