As far as I know, SH weather is generated "on the spot", every time player's sub moves from zone to zone, or when the curren "weather change interval" elapses.
The only way I see for having weather forecasts, is by making the game to read either the current or the upcoming weather from a matrix of data stored on file. This one would be an hard task and would require a deep knowledge of the game code, since weather is hardcoded. I remember there was a discussion on it, but I guess the project was abondened even before starting.
If you are using Dynamic Environment, the closer you can get to a weather forecast is by looking at these tables:
They show real wind average speeds respectively for the months of January and July, and DynEnv's wind patterns were based on them.
Indeed, you cannot use them for forecasting the exact in-game wind speed, but they will let you know when and where you can expect high, low or medium winds.
See this post for more details and charts on the subject:
http://www.subsim.com/radioroom/show...43#post1662143