What do you say if your friend asks you why you are carrying an umbrella on a cloudy day? Maybe you are carrying the umbrella because you want to be prepared. In this situation, you can say "Just in case (혹시 모르니까)." The expression can be used like, " I don't think it will rain, but i will take an umbrella just in case it rains." However, you can't say future tense after the phrase '(Just) in case.' So you can't say "Just in case it will rain." I hope you will take your umbrella during the rainy season just in case it rains! So, why don't you try to use this expression?
L.L.F reporter You-Jin Kim