The phrase "you don't need a weatherman to know which way the wind blows" is used to convey the idea that the signs are clear and it doesn't take an expert to figure out what is happening or what is going to happen. There are various synonyms for this phrase, including "it's obvious," "it's plain to see," "it's apparent," and "it's self-evident." All of these phrases convey the same message that there is no need for specialized knowledge or expertise to understand what is happening. They imply that the situation is so clear that anyone can see what is going on without any extra assistance.
