Severe Weather Information

Thunderstorms, Tornadoes, and Lightning  - Nature’s most violent storms.

Thunderstorms affect relatively small areas when compared with hurricanes and winter storms. The typical thunderstorm is 15 miles in diameter and lasts an average of 30 minutes. Despite their small size, ALL thunderstorms are dangerous! Of the estimated 100,000 thunderstorms that occur each year in the United States, about 10 percent are classified as severe.

Tornadoes - Although tornadoes occur in many parts of the world, they are found most frequently in the United States.....

  • A tornado is a violently rotating column of air extending from a thunderstorm to the ground.
  • Tornadoes cause an average of 70 fatalities and 1,500 injuries in the U.S. each year..
  • The strongest tornadoes have rotating winds of more than 250 mph.
  • Tornadoes can be one mile wide and stay on the ground over 50 miles.
  • Tornadoes may appear nearly transparent until dust and debris are picked up or a cloud forms within the funnel. The average tornado moves from southwest to northeast, but tornadoes have been known to move in any direction.
  • The average forward speed is 30 mph but may vary from nearly stationary to 70 mph.
  • Waterspouts are tornadoes which form over warm water. They can move onshore and cause damage to coastal areas.


  • Causes an average of about 60 fatalities and 300 injuries each year.
  • Lightning occurs in all thunderstorms; each year lightning strikes the United States 25 million times.
  • The energy from one lightning flash could light a 100-watt light bulb for more than 3 months.
  • Most lightning fatalities and injuries occur when people are caught outdoors in the summer months during the afternoon and evening.
  • Lightning can occur from cloud-to-cloud, within a cloud, cloud-to-ground, or cloud-to-air.
  • Many fires in the western United States and Alaska are started by lightning.
  • The air near a lightning strike is heated to 50,000°F--hotter than the surface of the sun!
  • The rapid heating and cooling of the air near the lightning channel causes a shock wave that results in thunder.

Straight-line Winds...

  • Straight-line winds are responsible for most thunderstorm wind damage.
  • Winds can exceed 100 mph!
  • One type of straight-line wind, the downburst, is a small area of rapidly descending air beneath a thunderstorm
  • A downburst can cause damage equivalent to a strong tornado and can be extremely dangerous to aviation.
  • A “dry microburst” is a downburst that occurs with little or no rain. These destructive winds are most common in the western United States

Flash Flooding...

  • Is the #1 cause of deaths associated with thunderstorms...more than 140 fatalities each year
  • Most flash flood fatalities occur at night and most victims are people who become trapped in automobiles.
  • Six inches of fast-moving water can knock you off your feet; a depth of two feet will cause most vehicles to float.


  • Strong rising currents of air within a storm, called updrafts, carry water droplets to a height where freezing occurs.
  • Ice particles grow in size, becoming too heavy to be supported by the updraft, and fall to the ground.
  • Causes more than $1 billion in damage to property and crops each year.
  • Large stones fall at speeds faster than 100 mph.

Severe Weather Warnings

The National Weather Service considers a thunderstorm to be severe if it

produces hail ¾ inch or larger, or wind speed 58 mph or more.


Watches are issued when conditions are favorable for tornadoes, severe

thunderstorms or flash floods. If you are in a watch area, make plans to seek

shelter if necessary.


Warnings are issued when severe weather has been reported or is imminent.

Seek shelter immediately if you are in or near the path of the storm

Sirens are meant to warn those who are outdoors. If you hear a siren, seek

shelter immediately and turn on a radio or TV to hear safety information.


Tornado Siren: The Town of Selma utilizes the old siren located on top of the fire station to alert citizens of any TORNADO WARNING ISSUED FOR JOHNSTON COUNTY.  When citizens hear the siren, they should immediately seek cover and tune into the local forecast by radio, television, or internet for exact paths of the storm. The siren is meant to aid in notification of a warning. The best alerting system is a weather radio. The siren may not be audible for the entire town due to wind direction and weather conditions.


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