The cat flap will keep the majority of the wind and rain outside. In particularly exposed areas or doors that face the weather, you may have wind and rain ingress on particularly bad days.
Cat flaps have to balance keeping the weather out with not being so strong that the cat cannot use it. We think the Glass Fitting Cat Flap and the Large Glass Fitting Cat Flap offer the best balance between being easy to use for your cat and being weatherproof.