Many companies, in an effort to reduce bulk, will forgo storm flaps in favor of waterproof zippers. Technically, no zipper is waterproof, due to very small gaps in the teeth from bending or stretching the surrounding fabric. However, zippers can get pretty close. A coating, often polyurethane, is applied to the exterior zipper surface, to minimize leakage to the inside. And indeed, I own a jacket that has a water-resistant coating applied to the zippers, and have never noticed leaking from the zippers; although, to be honest, it does have small storm flaps on either side of the main zipper’s exterior, and a stiffer, interior storm flap. Typically, as the price of a jacket increases, the quality of the zipper coating will increase; some less expensive jackets might have the coating degrade, due to abrasion from the zipper pull. Waterproof zippers are a pretty good idea, but some sacrifices are made when storm flaps are eliminated; the zipper, due to the coating, becomes stiffer, and more difficult to operate; clearly, the storm flaps did all the work of keeping the elements out, thus allowing the zippers to be easy to manipulate. But the stiffness and difficulty of operation generally decrease with age and usage of the garment’s zippers.