Or maybe he watched the weather forecast where the reporter mention that its gonna be heavy rain outside with danger of falling lightning dragon...so don't went outside...sasuke knew that but itachi don't
He would look upon the the dim clouds when it was raining hard and practiced taming the natural lightning sort of like Zuko did in Avatar the Last Airbender.
He sent his Dragon fire jutsu into the upper atmosphere to create the thunder clouds then used the dispersed chakra from that jutsu to manipulate the generated lightning. It's basically the same way earth and water style users manipulate their elements.
He would look upon the the dim clouds when it was raining hard and practiced taming the natural lightning sort of like Zuko did in Avatar the Last Airbender.