The trade winds hit the east side of the islands first, where they build up clouds along the peaks of the mountains and drop precipitation. Hilo, on the windward side of the big island, for example, is the nation's wettest city. The leeward, or western sides, of the islands are in the lee of the trade winds, so they're drier and warmer. For the most part, towns like Waianae, Lahaina, Kona, and Kekaha, which are all on the leeward sides, stay very dry and sunny. It could be pouring in Kula, and sunny just down the mountain in Wailea.
Temperatures drop with altitude, even here.
In the winter, you'll see frost above 4000 feet. Haleakala, Maui, occasionally gets snow. Dangerous wind conditions sometimes close these summits, so check the weather report before you head up. In the past 50 years there have been 6 tsunamis over 20 feet. If you hear the sirens at any time other than the first day of the month at noon, get to higher ground.