Severe thunderstorms are common, fast moving, and more dangerous than many people realize. Here are the key facts, straight and simple.
What counts as severe
The National Weather Service calls a thunderstorm severe when it produces at least one of these:
- Hail one inch in diameter or larger (about the size of a quarter).
- Wind gusts of 58 mph or higher.
- A tornado.
A storm can be dangerous with heavy rain and lightning without meeting the severe criteria, which is why a warning is specific and worth paying attention to.
How often they happen
- The United States sees roughly 100,000 thunderstorms in a typical year.
- About 10 percent of those reach severe levels.
- They are most common in the spring and summer, and most often fire up in the afternoon and evening.
What makes a thunderstorm form
Thunderstorms need three things to come together: moisture in the air, unstable air that wants to rise, and something to lift that air, such as a front or heating during the day. When the lift is strong and the air is very unstable, storms can grow tall and turn severe.
Lightning
- Every thunderstorm produces lightning. If you can hear thunder, you are close enough to be struck.
- Lightning can strike more than 10 miles away from the rain, sometimes from a sky that looks clear.
- The simple rule: when thunder roars, go indoors. There is no safe place outside during a storm.
Hail and damaging wind
- Hail forms when strong updrafts carry raindrops high into the freezing parts of a storm. Large stones can fall at highway speeds.
- Straight line winds from a collapsing storm, called downbursts, can top 100 mph and cause damage that looks like a tornado.
The hidden danger: flooding
Flash flooding is one of the deadliest thunderstorm hazards, and most flood deaths happen in vehicles. If you come to a flooded road, turn around, do not drown. It takes surprisingly little moving water to sweep a car away.
Warning vs watch
A watch means conditions are right for severe storms, so stay alert. A warning means a severe storm is happening or about to happen, so take action now. A severe thunderstorm warning usually covers a small area for about 30 to 60 minutes as the storm moves through.
Not sure about your area right now? Check if a severe thunderstorm warning is active for your ZIP code.