Watch for Developing Thunderstorms
All thunderstorms have lightning--that's what causes the thunder.
Thunderstorms are more likely to develop on humid spring or summer days but can occur year round.
As the sun heats the air, pockets of warmer air start to rise and cumulus clouds form. Continued heating can cause these clouds to grow vertically into towering cumulus clouds.
Clouds that grow vertically into towering cumulus clouds are often the first sign of a developing thunderstorm.
Distant thunder means the storm has already formed...

Check out the Severe Weather Links for additional information.