Hilo the nation's wettest city, is on the windward side of the Big Island of Hawaii and shadowed by the state's tallest mountain, Mauna Kea. The western facing sides of the islands are in the lee of the tradewinds, so they're drier and warmer. Waianae Lahaina, Kekaha and Kona are all on the leeward sides of islands. Mountain slopes and valleys get more rain than leeward coastal areas. It could be pouring in Kula and yet be sunny right down the road in Wailea.
Temps drop with altitude even in Hawaii.
In fact, winter brings frost at elevations above 4,000 feet. Generally, dress warm if going up.