Severe Thunderstorms


Photo of lightningSevere thunderstorms visit Iowa frequently each year. On average, Iowa sees close to 50 tornadoes, tens of flash flood events, hundreds of severe thunderstorms and thousands of non-severe thunderstorms.


By definition, a severe thunderstorm must contain hail that is one inch in diameter or larger, straight line winds 58 mph or stronger and/or a tornado. The National Weather Service issues severe thunderstorm and tornado watches and warnings for severe thunderstorms.

Supercell Thunderstorms

Storms possessing this structure have been observed to generate the vast majority of long-lived strong and violent (EF2 to EF5) tornadoes, as well as downburst damage and large hail. Supercells are thunderstorms consisting of one quasi-steady to rotating updraft which may exist for several hours.


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Before the Storm


  • Know the county in which you live and the names of nearby major cities. Severe weather warnings are issued by county. If you are not near home, know the county where you're located, in case the weather turns.
  • Check the weather forecast before leaving to be outdoors for extended periods.
  • Watch for signs of approaching storms.
  • If a storm is approaching, keep a NOAA Weather Radio or AM/FM radio with you.
  • Postpone outdoor activities if thunderstorms are imminent. This is your best way to avoid being caught in a dangerous situation.
  • Check on those who have trouble taking shelter if severe weather threatens.


When Thunderstorms Approach


If you are inside

  • Telephone lines and metal pipes can conduct electricity. Unplug appliances not necessary for obtaining weather information. Avoid using the telephone or any electrical appliances. Use phones only in an emergency.
  • Do not take a bath or shower.
  • Turn off air conditioners. Power surges from lightning can overload the compressors.

If you are outside

  • If you can hear thunder, you are close enough to the storm to be struck by lightning. Seek shelter immediately!
  • Move to a sturdy building or car. Do not take shelter in small sheds, under isolated trees or in convertible automobiles.
  • If lightning is occuring and a sturdy shelter is not available, get inside a hard-top automobile and keep the windows up.
  • Get out of boats and away from water.

If you are outside and no shelter is nearby

  • Find a low spot away from trees, fences and poles. Make sure the place you pick is not subject to flooding.
  • If you are in the woods, take shelter under shorter trees.
  • If you feel your skin tingle or your hair stand on end, squat low to the ground on the balls of your feet. Place your hands on your knees with your head between them. Make yourself the smallest target possible and minimize your contact with the ground.