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Tornadoes

A tornado is defined as a violently rotating column of air extending from a thunderstorm to the ground. Tornadoes can occur at any time of the year but in our area, they are most common between May through the summer months. They are usually associated with strong thunderstorm activity and fronts. The average tornado moves from southwest to northeast, but tornadoes have been known to move in any direction. The average forward speed is 30 miles per hour but may vary from nearly stationary to 70 miles per hour.

The most violent tornadoes are capable of tremendous destruction with wind speeds of 250 miles per hour or more. Damage paths can be in excess of one mile wide and 50 miles long.

Signs of a Tornado

Some tornadoes are clearly visible while others are obscured by rain or nearby low-hanging clouds. Some appear as visible funnels extending partially to the ground. Look for signs of debris below the visible funnel. Other signs include:

  • Dark, often greenish sky
  • Wall cloud
  • Large hail
  • Loud roar (similar to a freight train)

Causes

Before thunderstorms develop, a change in wind direction and an 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 rotating air from horizontal to an area of rotation, two to six miles wide, which extends through much of the storm.

Tornadoes in the winter and early spring are often associated with strong, frontal systems that form in the Central States and move east. Occasionally, large outbreaks of tornadoes occur with this type of weather pattern. Tornadoes occasionally accompany tropical storms and hurricanes that move over land. Tornadoes are most common to the right and ahead of the path of the storm center as it comes onshore.

  • Some tornadoes may form during the early stages of rapidly developing thunderstorms.
  • Tornadoes may appear nearly transparent until dust and debris are picked up.
  • Occasionally, two or more tornadoes may occur at the same time.
  • Waterspouts are weak tornadoes that form over warm water and occasionally move inland, becoming tornadoes.

Tornado Shapes and Sizes

Weak Tornadoes

  • 69 percent of all tornadoes
  • Less than five percent of tornado deaths
  • Lasts one to 10 or more minutes
  • Winds less than 110 mph

Strong Tornadoes

  • 29 percent of all tornadoes
  • Nearly 30 percent of all tornado deaths
  • May last 20 minutes or longer
  • Winds 110 to 205 mph

Violent Tornadoes

  • Only two percent of all tornadoes
  • 70 percent of all tornado deaths
  • Lifetime can exceed one hour
  • Winds greater than 205 mph

Watches Versus Warnings

No place is safe from tornadoes. Violent winds and debris slamming into buildings cause most structural damage. Immediately go to a safe place and keep any windows closed. Listen to NOAA Weather Radio, commercial radio, and television for the latest tornado watches and warnings. When conditions are favorable for severe weather to develop, a severe thunderstorm or tornado watch is issued.

Tornado Watch: Tornadoes are possible in your area. Remain alert for approaching storms.
Tornado Warning: A tornado has been sighted or indicated by weather radar. If a tornado warning is issued for your area and the sky becomes threatening, move to your pre-designated place of safety.

 

Revised November 4, 2016        

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