Severe Weather Awareness: Tornadoes

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Meteorologist Trent Okerson

Although tornadoes can occur in many locations around the world, no area sees anywhere close to the number of tornadoes each year than the middle of the United States.  On average, nearly 800 of these destructive storms are reported in the US, resulting in around 80 deaths and over 1,500 injuries.  The most violent tornadoes are capable of incredible winds of up to 200 mph or more, and can have damage paths of over a mile wide and 50 miles long! 

What Causes Tornadoes?

Tornadoes are always associated with strong thunderstorms....thunderstorms that are not only capable of producing tornadoes,  but also large hail and damaging winds.  These thunderstorms usually develop in warm, moist air ahead of eastward-moving cyclones (areas of low pressure).  These cyclones typically have a warm front and a cold front associated with them, and more often than not, the best conditions for tornadoes are found in between the fronts, and area called the "warm sector" of the storm system. 

Thunderstorms that are capable of producing tornadoes develop in an environment where changes in wind direction and an increase in wind speed with height creates a horizontal spinning of the air.  As the thunderstorm grows, it develops an updraft that tilts that spinning air from horizontal (along the ground), to vertical (from the ground into the clouds).  As the thunderstorm becomes stronger, this rotating updraft also becomes stronger and faster, and may cause the base of the clouds to lower, forming a wall cloud.  This is the area where a tornado may develop if conditions are favorable.

Tornado Shapes & Sizes

Weak Tornadoes:

-69% of all tornadoes, Less than 5% of tornado deaths, Lifetime of 1-10 minutes, Winds less than 110 mph.

Strong Tornadoes:

-29% of all tornadoes, Nearly 30% of tornado deaths, Lifetime of 20 minutes or longer, Winds 110-205 mph.

Violet Tornadoes:

-Only 2% of all tornadoes, 70% of tornado deaths, Lifetime of up to 1 hour or more, Winds over 205 mph.

Frequency of Tornadoes

Tornadoes can occur at any time of the year.  However, in the middle and southern parts of the US, the peak of tornado season runs from March through May, and sometimes into June.  In addition, there is also a secondary peak of severe storms and tornadoes in the fall. 

Tornadoes are most likely to occur between the hours of 3PM-9PM when the heating of the day is greatest, but tornadoes can and have occurred at all hours of the day and night.  Overnight tornadoes are some of the deadliest due to the lack of warning since most people are asleep.

What to Watch For

If the threat of tornadoes has been mentioned in the forecast, you'll likely hear about watches or warning being issued for your area. 

Tornado Watch:  Conditions are favorable for thunderstorms to develop that may spawn tornadoes.  Stay alert for the latest forecast and monitor the sky for approaching storms.

Tornado Warning:  A tornado has been sighted by a trained spotter or law enforcement, or has been detected by Doppler radar.  If a tornado warning is issued for your area, move immediately to your designated place of shelter.

Severe Thunderstorm Watch:  Severe thunderstorms capable of producing large hail of at least 1" in diameter, damaging winds of at least 58 mph, or tornadoes are possible.

Severe Thunderstorm Warning:  Severe thunderstorms are occurring in your area, with the potential to produce tornadoes. 

Besides the dangers from possible tornadoes, severe thunderstorms are dangerous and potentially deadly for a number of other reasons as well. 

Flash Floods:  The top weather-related killed annually, responsible for an average of 146 each year.

Lightning:  Kills 75-100 people on average each year.

Damaging Straight-Line Winds:  Can reach over 100 mph and cause extensive damage and power outages.

Large Hail:  Can reach the size of baseballs and even grapefruits, causing several million dollars in damage to property and crops every year.

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