Final answer:
A common first sign of an approaching thunderstorm is dark clouds. These come before lightning, thunder, strong winds, and rain as the storm develops.
Step-by-step explanation:
A common first sign of an approaching thunderstorm is often dark clouds. These can be seen as the sky darkens and the atmosphere takes on a heavy and ominous quality.
Thunderstorms typically develop in environments where warm, moist air is present, and as this air rises and cools, it forms cumulonimbus clouds, which appear dark due to their density and the shadows cast by their thickness. Lightning and thunder follow as the storm develops, but they are usually not the first signs. The darkening sky often precedes other elements of the storm such as strong winds, which can intensify as the storm approaches, and rain, which will start falling as the storm is upon you.