The Forecast Towers in Fortnite are set locations on the map that allow you to gain information about the next storm circles.
Fortnite is all about survival, as the storm circles force players to always keep moving to a smaller area or get eliminated. While you can’t usually know the future circles in the match, using the Forecast Towers allows you to have insight into the next circles, allowing you to make better strategies than your opponents. In the recent Diamond in the Raft story quests, you will need to locate one of these Forecast Towers on the map and use it to get to the next stage.