All praise be to Allah, peace and blessings be upon His Messenger:
The weather forecast isn`t a kind of pretending to know The unseen; rather, it is from knowing the seen since satellites take pictures of clouds, lows, highs, and winds, and there is no pretending of knowing The Unseen in doing so. Imam Malik mentions in his [Mo`atta`, vol.1/pp.192] that "the Prophet (PBUH) used to say that if a cloud appeared from the sea side, and moved towards Sham (Syria) then, there will be a great deal of rain."
This is a solid evidence that predicting the rain based on observation and experience isn`t pretending to know The Unseen. Since sending the rain is from the keys of the invisible that none knows but Allah means that before they become visible, Allah, The Almighty, knows The Hour (Day of Resurrection), the sending of the rain, what is in the wombs, what man will earn on the morrow, and in what land a person will die. Allah knows all of these before their causes and preludes could appear since this is The Domain of Allah, The Exalted. But, after these appear, or the beginning of their causes, the role of man begins. And Allah Knows Best.