Development and Detection of Tornadoes

Tornadoes are defined as violently rotating columns of air in contact with the ground. When they are not in contact with the ground, they are called funnel clouds. Tornadoes comes in different sizes, many as narrow rope-like swirls, others as wide funnels. Across the Plains, tornadoes can be seen from miles away. Across the eastern United States and the Deep South, tornadoes are often hidden in large swaths of rain and hail, making them very difficult to see, and thus even more dangerous.

A majority of tornadoes are labelled weak with wind speeds of 100 mph or less, and are usually on the ground for a few minutes. Even weak tornadoes can produce substantial damage. Some tornadoes intensify further and become strong or violent. Strong tornadoes produce winds up to around 200 mph. Most of the Mississippi tornadoes that produce major damage are labelled strong. Only a few tornadoes across the country each year are labelled violent with winds of 200 to 300 mph. These tornadoes produce catastrophic damage, and can be on the ground for half an hour or longer.

The key atmospheric ingredients that lead to tornado potential are instability(warm moist air near the ground, cool dry air aloft) and wind shear (change in wind speed and direction with height). An unstable airmass promotes the development of strong updrafts. Wind shear not only increases the strength of the further, it also promotes storm rotation from which tornadoes are spawned.

All thunderstorms have the potential to produce tornadoes, but the type of storm that is most commonly tornadic is the Supercell. This very severe and long-lived thunderstorm contain a circulation aloft mesocyclone that grows upward through the storm and downward toward the ground. When conditions are just right tornadoes rapidly spin up from mosocyclones.

Dopplar radar can detect the circulation associated with a tornado-producing storm. Once the circulation is identified, a warning is issued. Dopplar radar is not perfect though. In fact, the radar only indicates rotation aloft, and does not indicate what may be occurring at ground level.

This makes the work of storm spotters very important. Only feedback from spotters can confirm whether the radar signature is associated with a tornado. The National Weather Service relies on the help of trained spotters. Working together, we can save lives.

Several other states across the nation have a greater yearly number of tornadoes, however, since 1950 Mississippi has had 385 tornado deaths. The reason for this is that Mississippi has a higher percentage of strong or violent tornadoes. Also tornadoes are not as easily spotted in Mississippi as they are in the Midwest where flat land and few trees make tornadoes visible from many miles away. Many times tornadoes in Mississippi are associated with heavy precipitation thunderstorms, and this adds to the spotting difficulty.

Tornadoes have occurred in every month of the year, but have a higher frequency during the period from February through May, as well as the month of November.

They can also occur at any time of the day or night, but are most frequent during the late afternoon and early evening hours. This is when the earth's surface heating is at a maximum, and it is this heating that fuels the thunderstorms that produce tornadoes.

1999 Tornado Stats

Number of Tornadoes........49

Number of Injuries..........6

Number of Deaths............1

Annual Stats 1950 to 1999

Average Number............25

Greatest Number...62 in 1988

Least Number.......5 in 1964

Average Deaths.............8

Tornado Frequency: 4.82 per 10,000 square miles.

Tornado Facts

FACT: Tornadoes travel at an average speed of 30 mph...but speeds up to 70 mph have occurred.

It is very dangerous to attempt to flee in your automobile, especially in populated areas. In addition, a car offers little protection against the strong tornadic winds. It is better to seek shelter in a sturdy building...but even a ditch or ravine offers more protection than an automobile. Move well away from your vehicle so that the strong winds do not roll the vehicle on top of you.

FACT: Tornadoes usually move from the southwest to the northeast...but have been observed to move toward the east or southeast on occasion...and can move very erratically at times.

The typical movement from southwest to northeast makes the southwest portion of a structure the the most susceptible to damage by a tornado's very strong winds. The safest portion of a sturdy structure is usually the northeast portion.

FACT: Opening windows may not protect your home from tornado damage.

It was once thought that opening windows prevented damage due to the sudden drop in atmospheric pressure as a tornado passed overhead. The atmospheric pressure within a tornado plays only a minor role at most in the damage process. Most structures have enough venting to allow for the sudden drop in pressure. Opening windows may lead to more damage inside structures...not less...due to very strong tornadic winds.

FACT: Tornado wind speeds increase with height within the tornado.

Storm cellars and basements offer the greatest protection from tornadoes.

FACT: Tornadoes are not the only weather phenomenon that makes a "loud roar".

At night or in heavy rain, the only clue to a tornado's presence may be the loud roar it makes, which sounds like a freight train. However, violent straightline winds may also produce this roar.

FACT: Large hail may or may not precede a tornado.

Tornadoes often follow large hail. Often tornadoes develop within storms that do not produce hail. Large hail may also follow a tornado rather than precede it. Do not assume you are safe from tornadoes if you do not observe hail.

FACT: Tornadoes may occur at any time of the day or night.

It is important to have access to tornado warning information even when you are asleep. NOAA Weather Radio with a warning alarm device is the best tool to have to be notified when watches and warnings have been issued for your general area of the state.

FACT: Tornadoes can occur throughout the year.

While a Mississippi tornado can occur in any month, the peak season is February through May and again in November. Tornadoes are most likely to occur between 2 p.m. and 8 p.m., but have been known to occur at all hours of the day and night. The least active hour since 1950 has been 4 a.m., when 19 tornadoes have occurred.

FACT: Tornadoes are ranked according to strength and severity by the Fujita Scale.

"Weak" tornadoes (F0/F1) account for 70 percent of all tornadoes where winds are less than 113 mph. Strong tornadoes(F2/F3) account for 29 percent of all tornadoes where winds are 113 to 206 mph. Violent tornadoes(F4/F5) account for only one percent of all tornadoes, but cause 70 percent of all tornado deaths. Wind speeds are in excess of 206 mph.

FACT: Early warning is the key to survival.

Radio and television stations operate the Emergency Alert System that rebroadcasts NWS watches and warnings. Additionally, 12 NOAA Weather Radio Stations guard the state. These stations are the "voice" of the National Weather Service.

When A Tornado Threatens...Planning is the key to Surviving Disaster

Tornadoes develop very quickly; the violent ones, responsible for most deaths, tend to move very fast. In fact, forward speeds of over 60 mph have been observed! Once a tornado is underway, time for planning has passed and immediate action must be taken to protect ourselves. Our plan has to be ready to meet the test. Preparing for severe weather is the theme of this week, so how do we go about it?

Preparedness plans come in all sizes as dictated by our individual and collective needs, but it always comes down to the individual. Betting that the storm will not strike you may be the most costly wager you ever make. Questions you should ask: Do you know the basic safety rules? How about your home shelter area; would your children know what to do if home alone? Area plans ready to move elderly or disabled people to shelter quickly? Do you know the best source for obtaining warning information?

Lets think about these things and PLAN AHEAD.

Basice severe weather preparedness plans must include:
• A thorough knowledge of safety rules.
• Selection and designation of the best shelter you have.
• A reliable method of receiving warning information.
• Instructions for every person in the proper procedures to follow when a watch or a warning is issued, or if threatening weather should develop without advance warning.
• Holding drills to test and practice the plan.

Your local Emergency Management Agency, the National Weather Service, or your local Red Cross Chapter can help you with your planning. Brochures on severe weather safety are available upon request. Some safety films are also available from the National Weather Service Offices and other organizations. It is especially important that school officials understand the severe local storm warning procedures and the importance of tornado drills. School plans must include procedures for buses to follow because they are vulnerable to overturning in high wind.

The primary mission of the National Weather Service is to warn of impending hazardous weather. But even with today's knowledge and technology, we simply cannot warn for every storm when we are dealing with something as volatile as tornado development or flash floods. Storm spotters, radar, and satellite reports all help, but tornadoes can and do develop without being detected. Advance warning time is often only a few minutes and may be only a few seconds. Preparedness is the other key in dealing with the threat.

Additional preparedness information has been provided to all schools in Mississippi by MEMA and local emergency management programs. These classroom kits contain a "Teachers Toolkit" with reference material and ready-to-duplicate handouts of preparedness materials.

Tornado Terms and Safety Tips

A TORNADO WATCH is issued by the National Weather Service when atmospheric conditions are favorable for tornado development. Listen to NOAA Weather Radio or commercial radio and television stations for the latest weather information. Stay Informed! Review safety precautions wherever you are. Keep an eye out for threatening or rapidly changing weather conditions and be prepared to go to a safe place.

A TORNADO WATCH is normally issued for a large area covering a whole state or larger part of several states. The Watch is intended to give you time to prepare, time to review safety rules. The sky may be blue, but weather changes could be just over the horizon. Begin preparing when you hear the watch.

A TORNADO WARNING is broadcast when a tornado has been detected by radar or a tornado has been detected by radar or a tornado has been spotted. A TORNADO WARNING is usually issued for one or two counties. If the tornado warning is in your county, then you should go to a safe shelter...Remember, tornadoes form quickly! You may only have a few seconds to react and find shelter. When a tornado threatens, your immediate action can save your life! Seconds do save lives! Know what to

Homes or Other Small Buildings

In general get as lows as you can. A basement below ground level or at least the lowest floor of a building offers the greatest safety. And, put as many walls between yourself and the outside as possible. Avoid windows at all costs!

Go to the basement or an interior room or hall, or a small room such as a closet or bathroom on the lower level. Get under something sturdy like a heavy table or a bed. Protect yourself from flying debris with pillows, heavy coats, blankets, or quilts.

Schools, Nursing Homes, Hospitals, Factories, and Shopping Malls.

Go to predesignated shelter areas. Interior hallways on the lowest floor are usually the best. Avoid large windows or glassed areas. Stay away from large rooms like a dining hall or gymnasium. Protect your head with your hands.

High-Rise Buildings:

Go to an interior room or hallway. Protect your head with your hands.

Mobile Homes, Automobiles(Including Buses):

Evacuate it and go to a substantial structure. If there is no shelter nearby, lie flat in the nearest ditch, ravine, gully, or other low spot and shield your head with your hands.

Remember...stay away from windows, doors, and outside walls, and protect your head. Determine what you would do if it were an actual emergency. Knowing what to do when a tornado approaches or a warning is issued and doing so quickly and calmly, will save your life!

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