Severe Weather Awareness
Severe Thunderstorms

A typical thunderstorm is 15 miles in diameter and lasts an average of 30 minutes. Despite their small size, ALL thunderstorms are dangerous. Every thunderstorm produces lightning, which kills more people each year than tornadoes. Heavy rain from thunderstorms can lead to flash flooding. Strong winds, hail, and tornadoes are also dangers associated with some thunderstorms. We, here at the National Weather Service, consider a thunderstorm 'severe' if it produces hail at least one inch in diameter, wind 58 mph or higher, or tornadoes.

Large hail causes nearly $1 billion in damage to property and crops annually. The costliest hailstorm was in Denver, Colorado, July 11, 1990. Total damage was $625 million. Strong rising currents of air within a storm, called updrafts, carry water droplets to a height where freezing occurs. The ice particles grow in size, finally becoming too heavy to be supported by the updraft and fall to the ground. Large hailstones fall at speeds faster than 100 mph. In the midwest, we sometimes experience lines of severe thunderstorms with tremendous winds, called squall lines. These often occur in the evening and overnight and can produce winds over 100 mph. The most devastating of these storms are called derechos. Derechos can scour the countryside for literally hundreds of miles.

Thunderstorms are most likely to occur in the spring and summer months and during the afternoon and evening hours, but can occur year-round and at all hours. In this region, thunderstorms frequently occur in the late afternoon and at night. On the average, there are between 35 and 45 thunderstorm days each year in the Quad Cities Service Area.

When skies darken or thunderstorms are forecast LOOK and LISTEN for... increasing wind, flashes of lightning, the sound of thunder, and static on your AM radio. To find out how many miles you are away from the lightning flash, count the seconds between the lightning and the sound of thunder and divide by five. For example, if you see lightning and 10 seconds go by before you hear the thunder then the lightning flash is about 2 miles away from you.

When thunderstorms approach your area take these precautions:

  1. Go inside! Flying debris, hail, and lightning are all hazards which you need to avoid.
  2. Stay away from windows! They can easily shatter in severe thunderstorm winds, especially if the winds are also driving large hail or debris.
  3. Abandon mobile homes for a shelter or nearby building!
  4. Don't use electrical appliances and telephones! Lightning can travel through the wires.
  5. If outside, go inside a sturdy building. If no building is available you can go into a hard top automobile to escape the hail. But beware... strong winds can down trees onto cars and if there is a tornado, you should leave the vehicle for a nearby ditch.
  6. After the storm passes, don't touch any downed power lines! Report them to your local power company.
  7. Get out of boats and away from water! Go to land and find a safe shelter.
  8. Go to higher ground if flooding is possible!
  9. Keep an eye out for tornadoes... a tornado is always possible in severe thunderstorms.

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