Can you guess what conjecture means? It's a word to use when you are not sure of something and have to "guess or surmise."
You can see how the word conjecture means that you create a theory or opinion about something without basing it in fact because the original definition of conjecture, from Old French, is "interpretation of signs and omens." Since signs and omens are pretty subjective, it makes sense that the word would then move to its current meaning. However, even though it only seems like weather reports are conjectures, they are actually based on evidence!