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The Johnson County area is affected on an average by 40-60 thunderstorm days per year. The typical thunderstorm is 15 miles in diameter and lasts an average of thirty minutes. All thunderstorms are dangerous. The offspring of thunderstorms are strong winds, lightning, hail, heavy rain, flash floods and flooding, downbursts, and tornadoes.
Only about 10% of 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 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.
SEVERE THUNDERSTORM (or TORNADO) WATCH: Watches are issued by the Storm Prediction Center in Norman, Oklahoma when the threat appears. Watches are issued to heighten public awareness anywhere from 2-6 hrs before severe weather may develop in a specific geographic area that may include 30,000 square miles.
SEVERE THUNDERSTORM (or TORNADO) WARNING: Warnings are issued by the Weather Service Forecast Office in their area of responsibility when the threat poses an imminent danger to life and property to those in the path of the storm. A warning is issued based on information reported by spotters or indicated by radar. The WSFO serving our area is at Pleasant Hill, Missouri.
Lightning occurs with all thunderstorms and is the result of a sudden discharge of the electrical potential between the positive and negative charges generated in a thunderstorm. Lightning produces an electric charge or current that generates an enormous amount of heat - up to 50,000 degrees Fahrenheit. The rapid heating and cooling of the air near the lightning channel causes a shock wave that results in thunder. More deaths are attributed to lightning each year than tornadoes and hurricanes combined. Most lightning deaths and injuries occur when people are caught outdoors.
A cloud to ground lightning strike begins as an invisible channel of electrically charged air moving from the cloud toward the ground. When one channel nears an object on the ground, a powerful surge of electricity.
If Caught Outdoors and No Shelter Is Nearby...
The 30-30 Rule
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 6 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!