Severe Thunderstorms

The typical thunderstorm is 15 miles in diameter and lasts an average of 30 minutes. All thunderstorms are dangerous. Thunderstorms may include strong winds, lightning, hail, heavy rain, flash floods and flooding, downbursts, and tornadoes.

Only about 10 percent of the storms that occur each year in the United States are classified as severe. The National Weather Service considers a thunderstorm severe if it produces hail at least ¾ inch in diameter, winds of 58 mph or higher, or tornadoes.

Thunderstorms frequently occur in the late afternoon and at night in the Plains states. Although they are most likely to happen in the spring and summer months, they can occur year round and at all hours.

Thunderstorm safety tips

  • If you can hear thunder, go to a sturdy building or car immediately. Do not take shelter in small sheds, under isolated trees, or in convertible automobiles.
  • If lightning is occurring and a sturdy shelter is not available, get inside a hard-top vehicle and keep the windows up.
  • Telephone lines and metal pipes can conduct electricity. Unplug appliances not necessary for obtaining weather information. Avoid using the telephone or any electrical appliance. Use phones only in an emergency. Turn off air conditioners. Power surges from lightning can overload compressors, damage electrical appliances, and cause fires.
  • Do not take a bath or a shower during a thunderstorm.
  • Get to higher ground if flash flooding or flooding is possible. Once flooding begins, abandon cars and climb to higher ground. Do not attempt to drive through water. (Most flash flood deaths occur in automobiles.)
  • If you are caught outdoors 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.
    • If you are boating or swimming, get to land and find shelter immediately!

Lightning Kills — Play it Safe

Lightning can be fascinating to watch, but it is also extremely dangerous. During the past 30 years, lightning killed an average of 67 people per year in the United States.

During a thunderstorm, each flash of cloud-to-ground lightning is a potential killer. In addition to the visible flash that travels through the air, the current associated with the lightning discharge travels along the ground. Although some victims are struck directly by the main lightning stroke, many victims are struck as the current moves in and along the ground.

Lightning can strike as far as 10 miles away from the rain area in a thunderstorm, but at that distance it may even be difficult to tell a storm is coming. The first stroke of lightning can be just as deadly as the last, so if the sky looks threatening, take shelter before hearing thunder.

Use the 30-30 rule where visibility is good and there is nothing obstructing your view of the thunderstorm. When you see lightning, count the time until you hear thunder. If that time is 30 seconds or less, the thunderstorm is within six miles of you and is dangerous. Seek shelter immediately. The threat of lightning continues for much longer period than most people realize. Wait at least 30 minutes after the last clap of thunder before leaving shelter. Don’t be fooled by sunshine or blue sky!

For more information about lightning, visit www.lightningsafety.noaa.gov.

Hail and high winds

Much of the damage caused by storms in the Midwest comes from hail and high winds.

  • Large Hail: The strong rising currents of air within a storm, called updrafts, carry water droplets to a height where freezing occurs. The resulting ice particles grow in size, finally becoming too heavy to be supported by the updraft, and fall to the ground. Large hailstones can fall at speeds faster than 100 mph. Hail causes nearly $1 billion in property damage in the United States every year.
  • Straight-line winds: Straight-line winds are responsible for most thunderstorm wind damage. Winds can exceed 100 mph. One type of straight-line wind is the downburst.
  • Downbursts: A downburst is an area of rapidly descending air beneath a thunderstorm. The strong winds usually approach from one direction and may be known as “straight-line” winds. They can, in extreme cases, cause damage equivalent to a strong tornado causing significant damage to some buildings. A downburst greater in size than 2.5 miles is called a macroburst. A downburst smaller than 2.5 miles is called a microburst. Microbursts may be wet or dry.

The 30-30 rule:

If there is less than 30 seconds between a flash of lightning and the sound of thunder, you need to seek shelter.

Wait until at least 30 minutes after the last clap of thunder before leaving shelter.

Watches & Warnings

A severe weather WATCH means that the risk of storms in your area has increased significantly. Find out what counties are in the watch area by listening to your NOAA Weather Radio or local radio and television stations.

A severe storm WARNING means that a hazardous weather is imminent for your area. Take shelter immediately.