During the summer months, the ever present "20% chance of afternoon thunderstorms" is on the forecast map.
A few of these storms do grow to become severe with the combination of strong afternoon heating and high humidity values.
One of the questions we get into the weather center is why one thunderstorm can be deemed severe while another one will not be.
The National Weather Service, who will issue a severe thunderstorm warning, has criteria to do so.
One common misconception is if a storm has a lot of lightning and heavy rain it should be classified as severe. These two things actually have nothing to do with whether or not a storm is classified as severe.
By the National Weather Service's standard, for a storm to be classified as severe, it should possible one of the following:
- Winds in excess of 58 miles per hour.
- Hail at least one inch (quarter-sized) in diameter.
- A tornado (in which instance, the warning would be upgraded to a tornado warning)
Most of the storms we have during the summer months can be intense, but many of them fail to reach severe criteria.
At any rate, whether it be a typical garden variety storm or soemthing that becomes severe, when you hear thunder, you should seek shelter immediately.