Waves Size & Direction
Wave sizes, also known as swell, vary by time of year. The north swell that happens in the winter in Hawaii is well known as the best time to surf on the north shore of Oahu. The north swell also makes the Na Pali Coast on the north shore of Kauai inaccessible for the winter months. Obviously this means you won't be snorkeling those areas. There can be a south swell in the summer that is a result of the winter storms in the southern Pacific. But in our experience, the winter north swell is more consistent than the summer south swell, so once again, we prefer summer. With the tropical storms that have been happening in the summers since 2013, swell as a result of those storms is something to consider too.
There are always exceptions to this as it is weather related and not completely predictable, so you may luck out and have no swell in the winter on the north shore. When this happens you are bound to see surfers moping around wishing the waves were back.