In The Imitation Game there's this aha moment where they realize IF they know the contents of the morning weather report they can use that to break the enigma. They can use the common phrases found in the message, like "weather" or "heil Hitler" to decider the code. It makes sense that they used these common phrases to help break the code. But how did they have the weather messages decoded in the first place before they were able to break the code?