A PREPAREDNESS GUIDE Including Safety Information for Schools U.S. DEPARTMENTOF COMMERCE National Oceanic and Atmospheric AdministrationNational WeatherService September 1992 (NOAA,FEMA,The American Red Cross)
| Although tornadoes occur in many parts of the world, these destructiveforces of nature are found most frequently in the United States east ofthe Rocky Mountains during the spring and summer months. In an averageyear, 800 tornadoes are reported nationwide, resulting in 80 deaths andover 1,500 injuries. A tornado is defined as a violently rotating columnof air extending from a thunderstorm to the ground. The most violent tornadoesare capable of tremendous destruction with wind speeds of 250 mph or more.Damage paths can be in excess of one mile wide and 50 miles long. Oncea tornado in Broken Bow, Oklahoma, carried a motel sign 30 miles and droppedit in Arkansas! Thunderstorms develop in warm, moist air in advance of eastward-movingcold fronts. These thunderstorms often produce large hail, strong winds,and tornadoes. Tornadoes in the winter and early spring are often associatedwith strong, frontal systems that form in the Central States and move east.Occasionally, large outbreaks of tornadoes occur with this type of weatherpattern. Several states may be affected by numerous severe thunderstormsand tornadoes. |
|
| During the spring in the Central Plains, thunderstormsfrequently develop along a "dryline," which separates very warm,moist air to the east from hot, dry air to the west. Tornado-producingthunderstorms may form as the dryline moves east during the afternoon hours. Along the front range of the Rocky Mountains, in the Texas panhandle,and in the southern High Plains, thunderstorms frequently form as air nearthe ground flows "upslope" toward higher terrain. If other favorableconditions exist, these thunderstorms can produce tornadoes. Tornadoes occasionally accompany tropical storms and hurricanes thatmove over land. Tornadoes are most common to the right and ahead of thepath of the storm center as it comes onshore. | |
|
|
|
|
Before thunderstorms develop, a change in wind direction andan increase in wind speed with increasing height creates an invisible,horizontal spinning effect in the lower atmosphere. | Rising air within the thunderstorm updraft tilts the rotatingair from horizontal to vertical. | An area of rotation, 2-6 miles wide, now extends through muchof the storm. Most strong and violent tornadoes form within this area ofstrong rotation. |
|
|
|
| A lower cloud base in the center of the photograph identifies an areaof rotation known as a rotating wall cloud. This area is often nearly rain-free.Note rain in the background. |
| Moments later a strong tornado develops in this area. Softball-sizehail and damaging "straight-line" winds also occurred with thisstorm. |
|
|
|
| ||
Areas near rivers, lakes,and mountains are safe from tornadoes.
No place is safe from tornadoes.In the late 1980's, a tornado swept through Yellowstone National Park leavinga path of destruction up and down a 10,000 ft. mountain.
The low pressure with a tornadocauses buildings to "explode" as the tornado passes overhead.
Violent winds and debris slamminginto buildings cause most structural damage.
Windows should be opened beforea tornado approaches to equalize pressure and minimize damage.
Opening windows allows damagingwinds to enter the structure. Leave the windows alone; instead, immediatelygo to a safe place.
| ||
Carolinas Outbreak:
| Pennsylvania-Ohio Outbreak:
| Plains Outbreak:
|
Meteorologists rely on weather radar to provide information on developing storms. The National Weather Service is strategically locating Doppler radars across the country which can detect air movement toward or away from the radar. Early detection of increasing rotation aloft within a thunderstorm can allow lifesaving warnings to be issued before the tornado forms.
Tornadoes can occur at any timeof the year.In the southern states, peak tornadooccurrence is in March through May, while peak months in the northern statesare during the summer.Note, in some states, a secondarytornado maximum occurs in the fall.Tornadoes are most likely to occurbetween 3 and 9 p.m. but have been known to occur at all hours of the dayor night.The average tornado moves fromsouthwest to northeast, but tornadoes have been known to move in any direction.The average forward speed is 30 mph but may vary from nearly stationaryto 70 mph.The total number of tornadoes is probably higher than indicated in the western states. Sparse population reduces the number reported.
Months of Peak Tornado Occurrence
United States Totals 1961-1993
US Reported Tornadoes and Average Numberof Deaths Per Year 1961-1993
When conditions are favorable for severe weather to develop, a severethunderstorm or tornado is issued.
Weather Service personnel use information from weather radar, spotters,and other sources to issue severe thunderstorm and tornado for areas where severe weather is imminent.
Severe thunderstorm warnings are passed to local radio and televisionstations and are broadcast over local NOAA Weather Radio stations servingthe warned areas. These warnings are also relayed to local emergency managementand public safety officials who can activate local warning systems to alertcommunities.
The National Weather Service continuously broadcasts updated weatherwarnings and forecasts that can be received byNOAA Weather Radiossold in many stores. The average range is 40 miles, depending on topography.Your National Weather Service recommends purchasing a radio that has botha battery backup and a tone-alert feature which automatically alerts youwhen a watch or warning is issued.
Tornadoes are possiblein your area. Remain alert for approaching storms.
A tornado has beensighted or indicated by weather radar. If a tornado warning is issued foryour area and the sky becomes threatening, move to your pre-designatedplace of safety.
Severe thunderstormsare possible in your area.
Severethunderstorms are occurring.
Remember, tornadoes occasionally develop in areas in which a severethunderstorm watch or warning is in effect. Remain alert to signs of anapproaching tornado and seek shelter if threatening conditions exist.
Some tornadoes appear as a visible funnel extending only partiallyto the ground. Look for signs of debris below the visible funnel. | Some tornadoes are clearly visible while others are obscuredby rain or nearby low-hanging clouds. |
|
|
Contact your local National Weather Service office, American Red Crosschapter, or Federal Emergency Management Agency office for a copy of the"Thunderstorms and Lightning...The Underrated Killers" brochure(NOAA PA 92053) and the "Flash Floods and Floods...The Awesome Power"brochure (NOAA PA 92050). |  |
Occasionally, tornadoes develop so rapidly that advance warning is notpossible. Remain alert for signs of an approaching tornado. Flying debrisfrom tornadoes causes most deaths and injuries. |  |
Each year, many people are killed or seriously injured by tornadoesdespite advance warning. Some did not hear the warning while others receivedthe warning but did not believe a tornado would actually affect them. Thepreparedness information in this brochure, combined with timely severeweather watches and warnings, could save your life in the event a tornadothreatens your area. After you have received the warning or observed threateningskies, YOU must make the decision to seek shelter before the storm arrives.It could be the most important decision you will ever make.
People in automobilesThe elderly, very young, and thephysically or mentally impairedPeople in mobile homesPeople who may not understand thewarning due to a language barrier
| Your National Weather Service, Federal Emergency Management Agency,and American Red Cross educate community officials and the public concerningthe dangers posed by tornadoes. YOU can prepare for the possibility ofa tornado by learning the safest places to seek shelter when at home, work,school, or outdoors. You should also understand basic weather terms anddanger signs related to tornadoes. Your chances of staying safe duringa tornado are greater if you have a plan for you and your family, and practicethe plan frequently. |
|
Families should be prepared for all hazards that affect their area.NOAA's National Weather Service, the Federal Emergency Management Agency,and the American Red Cross urge each family to develop a family disasterplan.
Where will your family be when disaster strikes? They could be anywhere- at work, at school, or in the car. How will you find each other? Willyou know if your children are safe? Disasters may force you to evacuateyour neighborhood or confine you to your home. What would you do if basicservices - water, gas, electricity or telephones - were cut off?
Contact your local National Weather Service office, emergency managementor civil defense office, and American Red Cross chapter. Find out whattype of disasters could occur and how you should respond. Learn your community'swarning signals and evacuation plans.
Discuss the information you have gathered. Pick two places to meet: a spotoutside your home for an emergency, such as fire, and a place away fromyour neighborhood in case you can't return home. Choose an out-of-statefriend as your "family check-in contact" for everyone to callif the family gets separated. Discuss what you would do if advised to evacuate.
(1) Post emergency telephone numbers by phones;
(2) Install safety features in your house, such as smoke detectors andfire extinguishers;
(3) Inspect your home for potential hazards (such as items that can move,fall, break, or catch fire) and correct them;
(4) Have your family learn basic safety measures, such as CPR and firstaid; how to use a fire extinguisher; and how and when to turn off water,gas, and electricity in your home;
(5) Teach children how and when to call 911 or your local Emergency MedicalServices number;
(6) Keep enough supplies in your home to meet your needs for at least threedays. Assemble a disaster supplies kit with items you may need in caseof an evacuation. Store these supplies in sturdy, easy-to-carry containers,such as backpacks or duffle bags. Keep important family documents in awaterproof container. Keep a smaller disaster supplies kit in the trunkof your car.
Askquestions to make sure your family remembers meeting places, phone numbers,and safety rules. Conduct drills. Test your smoke detectors monthly andchange the batteries at least once a year. Test and recharge your fireextinguisher(s) according to manufacturer's instructions. Replace storedwater and food every six months.
National Weather Service
Back to theNSSL home page
