Hawaii winter rains may cause some human plans to change, but they provide for very happy plants and animals. The hills above the Hamakua Marsh in Kailua are many shades of green right now, due to the winter moisture. In the summer, they are tinged with beige as vegetation dries out.
Although it is sunny and warm today (78 degrees), we are still in the winter rainy season. Hawaii’s heaviest rain from storms is between October and April. The drier areas of the islands get most of their annual moisture during this time. Kailua is on the windward coast; our moisture comes from both the winter storms and from trade wind showers that spring up year-round. (You won’t find a “marsh” on the leeward side of the island.)
Hawaii overall is the rainiest state in the U.S. with a state-wide average of over 60 inches. However, that average means nothing to either people or plants. Some weather stations receive less than 20 inches of rain per year while others receive over 100. The micro-climates of all the islands are affected by mountains, wind and interaction with the sea.
If you are concerned about the weather during a vacation to Hawaii, you can be fairly certain that it is rainier in the winter and drier in the summer. But to get much more accurate than that, you need to know where you will be. Each location in each island has a different weather pattern. It also matters how long you plan to stay. Even through the rainiest times of this past winter, I don’t think it rained for more than four days in a row. The longer you can stay, the greater the odds that you’ll get good beach weather.