Tornado Season

Tornados, according to the Glossary of Meteorology (AMS 2000), are “a violently rotating column of air, pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud.”

In order for a vortex to be classified as a tornado, it must be in contact with the ground and the cloud base. A tornado may not have a visible funnel.

Tornados are formed, according to NOAA, the most destructive and deadly tornadoes occur from supercells — which are rotating thunderstorms with a well-defined radar circulation called a mesocyclone. [Supercells can also produce damaging hail, severe non-tornadic winds, unusually frequent lightning, and flash floods.] Tornado formation is believed to be dictated mainly by things which happen on the storm scale, in and around the mesocyclone. Recent theories and results from the VORTEX program suggest that once a mesocyclone is underway, tornado development is related to the temperature differences across the edge of downdraft air wrapping around the mesocyclone (the occlusion downdraft).

Tornadoes can appear from any direction. Most move from southwest to northeast, or west to east. Some tornadoes have changed direction amid path, or even backtracked.

Rain, wind, lightning, and hail characteristics vary from storm to storm, from one hour to the next, and even with the direction the storm is moving with respect to the observer. While large hail can indicate the presence of an unusually dangerous thunderstorm, and can happen before a tornado, don’t depend on it. Hail, or any particular pattern of rain, lightning or calmness, is not a reliable predictor of tornado threat.

Tornadoes can last from several seconds to more than an hour. The longest-lived tornado in history is really unknown, because so many of the long-lived tornadoes reported from the early-mid 1900s and before are believed to be tornado series instead. Most tornadoes last less than 10 minutes. A barometer can start dropping many hours or even days in advance of a tornado if there is low pressure on a broad scale moving into the area. Strong pressure falls will often happen as the mesocyclone (parent circulation in the thunderstorm) moves overhead or nearby. The biggest drop will be in the tornado itself, of course.

A waterspout is a tornado over water — usually meaning non-supercell tornadoes over water. Waterspouts are common along the southeast U.S. coast — especially off southern Florida and the Keys — and can happen over seas, bays and lakes worldwide. They are smaller and weaker than the most intense Great Plains tornadoes, but still can be quite dangerous.

Most tornadoes — but not all! — rotate cyclonically, which is counterclockwise in the northern hemisphere and clockwise south of the equator. Anticyclonic tornadoes (clockwise-spinning in the northern hemisphere) have been observed, however — usually in the form of waterspouts, non-supercell land tornadoes, or anticyclonic whirls around the rim of a supercell’s mesocyclone. T

The F-scale Dr. T. Theodore Fujita developed a damage scale (Fujita 1971, Fujita and Pearson 1973) for winds, including tornadoes, which was supposed to relate the degree of damage to the intensity of the wind. This scale was the result. The original F-scale should not be used anymore, because it has been replaced by an enhanced version.

What does a tornado sound like? That depends on what it is hitting, its size, intensity, closeness and other factors. The most common tornado sound is a continuous rumble, like a closeby train. Sometimes a tornado produces a loud whooshing sound, like that of a waterfall or of open car windows while driving very fast.
P

TORNADO FORECASTING

Who forecasts tornadoes? In the U.S., only the National Weather Service (NWS) issues tornado forecasts nationwide. Warnings come from each NWS office. The Storm Prediction Center issues watches, general severe weather outlooks, and mesoscale discussions.

How do you forecast tornadoes? We look for the development of temperature and wind flow patterns in the atmosphere which can cause enough moisture, instability, lift, and wind shear for tornadic thunderstorms. Those are the four needed ingredients. But it is not as easy as it sounds. “How much is enough” of those is not a hard fast number, but varies a lot from situation to situation — and sometimes is unknown!

What is the role of Doppler radar in tornado forecasting? Each NWS forecast office uses output from at least one Doppler radar in the area to help to determine if a warning is needed. Doppler radar signatures can tell warning meteorologists a great deal about a thunderstorm’s structure, but usually can’t see the tornado itself. This is because the radar beam gets too wide to resolve even the biggest tornadoes within a few tens of miles after leaving the transmitter. Instead, a radar indicates strong winds blowing toward and away from it in a way that tells forecasters, “An intense circulation probably exists in this storm and a tornado is possible.”
TORNADO DAMAGE

How is tornado damage rated? The most widely used method worldwide, for over three decades, was the F-scale developed by Dr. T. Theodore Fujita. In the U.S., and probably elsewhere within a few years, the new Enhanced F-scale is becoming the standard for assessing tornado damage. In Britain, there is a scale similar to the original F-scale but with more divisions; for more info, go to the TORRO scale website. In both original F- and TORRO-scales, the wind speeds are based on calculations of the Beaufort wind scale and have never been scientifically verified in real tornadoes. Enhanced F-scale winds are derived from engineering guidelines but still are only judgmental estimates.
TORNADO SAFETY

What should I do in case of a tornado? That depends on where you are.

What is a tornado watch? A tornado watch defines an area shaped like a parallelogram, where tornadoes and other kinds of severe weather are possible in the next several hours. It does not mean tornadoes are imminent — just that you need to be alert, and to be prepared to go to safe shelter if tornadoes do happen or a warning is issued.

What is a tornado warning? A tornado warning means that a tornado has been spotted, or that Doppler radar indicates a thunderstorm circulation which can spawn a tornado. When a tornado warning is issued for your town or county, take immediate safety precautions. local NWS offices issue tornado warnings.

Long ago, I was told to open windows to equalize pressure. Now I have heard that’s a bad thing to do. Which is right? Opening the windows is absolutely useless, a waste of precious time, and can be very dangerous. Don’t do it. You may be injured by flying glass trying to do it. And if the tornado hits your home, it will blast the windows open anyway.

I’ve seen a video of people running under a bridge to ride out a tornado. Is that safe? Absolutely not! Stopping under a bridge to take shelter from a tornado is a very dangerous idea, for several reasons:

1. Deadly flying debris can still be blasted into the spaces between bridge and grade — and impaled in any people hiding there.
2. Even when strongly gripping the girders (if they exist), people may be blown loose, out from under the bridge and into the open — possibly well up into the tornado itself. Chances for survival are not good if that happens.
3. The bridge itself may fail, peeling apart and creating large flying objects, or even collapsing down onto people underneath. The structural integrity of many bridges in tornado winds is unknown — even for those which may look sturdy.
4. Whether or not the tornado hits, parking on traffic lanes is illegal and dangerous to yourself and others. It creates a potentially deadly hazard for others, who may plow into your vehicle at full highway speeds in the rain, hail, and/or dust. Also, it can trap people in the storm’s path against their will, or block emergency vehicles from saving lives.

So if I’m in a car, which is supposed to be very unsafe, and shouldn’t get under a bridge, what can I do? Vehicles are notorious as death traps in tornadoes, because they are easily tossed and destroyed. Either leave the vehicle for sturdy shelter or drive out of the tornado’s path. When the traffic is jammed or the tornado is bearing down on you at close range, your only option may be to park safely off the traffic lanes, get out and find a sturdy building for shelter, if possible. If not, lie flat in a low spot, as far from the road as possible (to avoid flying vehicles). However, in open country, the best option is to escape if the tornado is far away.

In a basement, the safest place is under a sturdy workbench, mattress or other such protection — and out from under heavy furniture or appliances resting on top of the floor

What was the biggest outbreak of tornadoes? 147 tornadoes touched down in 13 U.S. states on 3 and 4 April, 1974.

What was the biggest known tornado? The Hallam, Nebraska F4 tornado of 22 May 2004 is the newest record-holder for peak width, at nearly two and a half miles, as surveyed by Brian Smith of NWS Omaha.

What single month had the most tornadoes? The record for most tornadoes in any month (since modern tornado record keeping began in 1950) was set in May 2003, with 543 tornadoes confirmed in the final numbers. This easily broke the old mark of 399, set in June 1992.

TORNADO CLIMATOLOGY

How many tornadoes hit the US yearly? About one thousand. The actual average is unknown, because tornado spotting and reporting methods have changed so much in the last several decades that the officially recorded tornado climatologies are believed to be incomplete.

How many people are killed every year by tornadoes? How do most deaths happen in tornadoes? On average, tornadoes kill about 60 people per year — most from flying or falling (crushing) debris.

What is tornado season? Tornado season usually means the peak period for historical tornado reports in an area, when averaged over the history of reports. There is a general northward shift in “tornado season” in the U.S. from late winter through mid summer.

What city has been hit by the most tornadoes? Oklahoma City. The exact count varies because city limits and tornado reporting practices have changed over the years; but the known total is now over 100.

What’s the difference between a funnel cloud and a tornado? What is a funnel cloud? In a tornado, a damaging circulation is on the ground — whether or not the cloud is. A true funnel cloud rotates, but has no ground contact or debris, and is not doing damage. If it is a low-hanging cloud with no rotation, it is not a funnel cloud.

Signs of an Approaching Storm

Some tornadoes strike rapidly, without time for a tornado warning, and sometimes without a thunderstorm in the vicinity. When you are watching for rapidly emerging tornadoes, it is important to know that you cannot depend on seeing a funnel: clouds or rain may block your view. The following weather signs may mean that a tornado is approaching:

* A dark or green-colored sky.
* A large, dark, low-lying cloud.
* Large hail.
* A loud roar that sounds like a freight train.

If you notice any of these weather conditions, take cover immediately, and keep tuned to local radio and TV stations or to a NOAA weather radio.

Sighting a Funnel Cloud

If you see a funnel cloud nearby, take shelter immediately (see the following section for instructions on shelter). However, if you spot a tornado that is far away, help alert others to the hazard by reporting it to the newsroom of a local radio or TV station before taking shelter as described later. Use common sense and exercise caution: if you believe that you might be in danger, seek shelter immediately.
Taking Shelter

Your family could be anywhere when a tornado strikes–at home, at work, at school, or in the car. Discuss with your family where the best tornado shelters are and how family members can protect themselves from flying and falling debris.

The key to surviving a tornado and reducing the risk of injury lies in planning, preparing, and practicing what you and your family will do if a tornado strikes. Flying debris causes most deaths and injuries during a tornado. Although there is no completely safe place during a tornado, some locations are much safer than others.
At Home

Pick a place in the home where family members can gather if a tornado is headed your way. One basic rule is AVOID WINDOWS. An exploding window can injure or kill.

The safest place in the home is the interior part of a basement. If there is no basement, go to an inside room, without windows, on the lowest floor. This could be a center hallway, bathroom, or closet.

For added protection, get under something sturdy such as a heavy table or workbench. If possible, cover your body with a blanket, sleeping bag, or mattress, and protect your head with anything available–even your hands. Avoid taking shelter where there are heavy objects, such as pianos or refrigerators, on the area of floor that is directly above you. They could fall though the floor if the tornado strikes your house.
In a Mobile Home

DO NOT STAY IN A MOBILE HOME DURING A TORNADO. Mobile homes can turn over during strong winds. Even mobile homes with a tie-down system cannot withstand the force of tornado winds.

Plan ahead. If you live in a mobile home, go to a nearby building, preferably one with a basement. If there is no shelter nearby, lie flat in the nearest ditch, ravine, or culvert and shield your head with your hands.

If you live in a tornado-prone area, encourage your mobile home community to build a tornado shelter.
On the Road

The least desirable place to be during a tornado is in a motor vehicle. Cars, buses, and trucks are easily tossed by tornado winds.

DO NOT TRY TO OUTRUN A TORNADO IN YOUR CAR. If you see a tornado, stop your vehicle and get out. Do not get under your vehicle. Follow the directions for seeking shelter outdoors (see next section).
Outdoors

If you are caught outside during a tornado and there is no adequate shelter immediately available–

* Avoid areas with many trees.
* Avoid vehicles.
* Lie down flat in a gully, ditch, or low spot on the ground.
* Protect your head with an object or with your arms.

Long-Span Buildings

A long-span building, such as a shopping mall, theater, or gymnasium, is especially dangerous because the roof structure is usually supported solely by the outside walls. Most such buildings hit by tornados cannot withstand the enormous pressure. They simply collapse.

If you are in a long-span building during a tornado, stay away from windows. Get to the lowest level of the building–the basement if possible–and away from the windows.

If there is no time to get to a tornado shelter or to a lower level, try to get under a door frame or get up against something that will support or deflect falling debris. For instance, in a department store, get up against heavy shelving or counters. In a theater, get under the seats. Remember to protect your head.
Office Buildings, Schools, Hospitals, Churches, and Other Public Buildings

Extra care is required in offices, schools, hospitals, or any building where a large group of people is concentrated in a small area. The exterior walls of such buildings often have large windows.

If you are in any of these buildings–

* Move away from windows and glass doorways.

* Go to the innermost part of the building on the lowest possible floor.

* Do not use elevators because the power may fail, leaving you trapped.

* Protect your head and make yourself as small a target as possible by crouching down.

Shelter for People with Special Needs

Advance planning is especially important if you require assistance to reach shelter from an approaching storm (see specific instructions in the next section).

* If you are in a wheelchair, get away from windows and go to an interior room of the house. If possible, seek shelter under a sturdy table or desk. Do cover your head with anything available, even your hands.

* If you are unable to move from a bed or a chair and assistance is not available, protect yourself from falling objects by covering up with blankets and pillows.

* If you are outside and a tornado is approaching, get into a ditch or gully. If possible, lie flat and cover your head with your arms.

A tornado watch is issued when weather conditions favor the formation of tornadoes, for example, during a severe thunderstorm.

During a tornado watch,

* Stay tuned to local radio and TV stations or a National Oceanographic and Atmospheric Administration (NOAA) Weather Radio for further weather information.

* Watch the weather and be prepared to take shelter immediately if conditions worsen.

A tornado warning is issued when a tornado funnel is sighted or indicated by weather radar.

You should take shelter immediately.
Thunderstorms

Because tornadoes often accompany thunderstorms, pay close attention to changing weather conditions when there is a severe thunderstorm watch or warning.

A severe thunderstorm watch means severe thunderstorms are possible in your area.

A severe thunderstorm warning means severe thunderstorms are occurring in your area.

Keep fresh batteries and a battery-powered radio or TV on hand. Electrical power is often interrupted during thunderstorms–just when information about weather warnings is most needed.
Important Measures To Take

* Take a few minutes with your family to develop a tornado emergency plan. Sketch a floor plan of where you live, or walk through each room and discuss where and how to seek shelter.

* Show a second way to exit from each room or area. If you need special equipment, such as a rope ladder, mark where it is located.

* Make sure everyone understands the siren warning system, if there’s such a system in your area.

* Mark where your first-aid kit and fire extinguishers are located.

* Mark where the utility switches or valves are located so they can be turned off–if time permits–in an emergency.

* Teach your family how to administer basic first aid, how to use a fire extinguisher, and how and when to turn off water, gas, and electricity in your home.

* Learn the emergency dismissal policy for your child’s school.

* Make sure your children know–
o What a tornado is
o What tornado watches and warnings are
o What county or parish they live in (warnings are issued by county or parish)
o How to take shelter, whether at home or at school.

Walls and Roof Rafters

Strengthen the areas of connection between the wall studs and roof rafters with hurricane clips as shown in the above graphic.
Shutting Off Utilities
Gas

After a tornado, DO NOT USE matches, lighters, or appliances, or operate light switches until you are sure there are no gas leaks. Sparks from electrical switches could ignite gas and cause an explosion.

If you smell the odor of gas or if you notice a large consumption of gas being registered on the gas meter, shut off the gas immediately. First, find the main shut-off valve located on a pipe next to the gas meter. Use an adjustable wrench to turn the valve to the “off” position.
Electricity

After a major disaster, shut off the electricity. Sparks from electrical switches could ignite leaking gas and cause an explosion.
Water

*

Water may be turned off at either of two locations:
1.

At the main meter, which controls the water flow to the entire property.
2.

At the water main leading into the home. If you may need an emergency source of fresh water, it is better to shut off your water here, because it will conserve the water in your water heater.
* Attach a valve wrench to the water line. (This tool can be purchased at most hardware stores.)

* Label the water mains for quick identification.

*

General Safety Precautions

Here are some safety precautions that could help you avoid injury after a tornado:
* Continue to monitor your battery-powered radio or television for emergency information.

* Be careful when entering any structure that has been damaged.

* Wear sturdy shoes or boots, long sleeves, and gloves when handling or walking on or near debris.

* Be aware of hazards from exposed nails and broken glass.

* Do not touch downed power lines or objects in contact with downed lines. Report electrical hazards to the police and the utility company.

* Use battery-powered lanterns, if possible, rather than candles to light homes without electrical power. If you use candles, make sure they are in safe holders away from curtains, paper, wood, or other flammable items. Never leave a candle burning when you are out of the room.

* Never use generators, pressure washers, grills, camp stoves, or other gasoline, propane, natural gas, or charcoal-burning devices inside your home, basement, garage, or camper—or even outside near an open window, door, or vent. Carbon monoxide (CO)–an odorless, colorless gas that can cause sudden illness and death if you breathe it–from these sources can build up in your home, garage, or camper and poison the people and animals inside. Seek prompt medical attention if you suspect CO poisoning and are feeling dizzy, light-headed, or nauseated.

* Hang up displaced telephone receivers that may have been knocked off by the tornado, but stay off the telephone, except to report an emergency.

* Cooperate fully with public safety officials.

* Respond to requests for volunteer assistance by police, fire fighters, emergency management, and relief organizations, but do not go into damaged areas unless assistance has been requested. Your presence could hamper relief efforts, and you could endanger yourself.

Inspecting the Damage

* After a tornado, be aware of possible structural, electrical, or gas-leak hazards in your home. Contact your local city or county building inspectors for information on structural safety codes and standards. They may also offer suggestions on finding a qualified contractor to do work for you.

* In general, if you suspect any damage to your home, shut off electrical power, natural gas, and propane tanks to avoid fire, electrocution, or explosions.

* If it is dark when you are inspecting your home, use a flashlight rather than a candle or torch to avoid the risk of fire or explosion in a damaged home.

* If you see frayed wiring or sparks, or if there is an odor of something burning, you should immediately shut off the electrical system at the main circuit breaker if you have not done so already.

* If you smell gas or suspect a leak, turn off the main gas valve, open all windows, and leave the house immediately. Notify the gas company, the police or fire departments, or State Fire Marshal’s office, and do not turn on the lights, light matches, smoke, or do anything that could cause a spark. Do not return to your house until you are told it is safe to do so.

Safety During Clean Up

* Wear sturdy shoes or boots, long sleeves, and gloves.

* Learn proper safety procedures and operating instructions before operating any gas-powered or electric-powered saws or tools.

* Clean up spilled medicines, drugs, flammable liquids, and other potentially hazardous materials.

source NOAA and the CDC