Most Maui rain is a passing trade-wind shower. Occasionally it is not, and the difference matters.
Flash flooding. East Maui's streams respond within minutes to rain falling miles upstream in weather you cannot see. The pools at ʻOheʻo Gulch in the Kīpahulu district are closed to swimming more often than they are open for exactly this reason, and the stream crossings on the Pipiwai Trail can become impassable. If water darkens, rises or starts carrying debris, get out and get high. Do not wait to see what happens.
The Hāna Highway in heavy rain. Sixty-four miles, roughly 600 curves and 59 mostly one-lane bridges, with rockfall and mudslide potential on the cut slopes. It is not a road to learn in a storm. Postpone the day.
Thunder and lightning. Get out of the water. Hawaiʻi gets far less lightning than the mainland, which means people take it less seriously than they should.
Kona lows and winter fronts. Between roughly October and April a system arriving from the southwest can rain on the leeward coast for a day or more, which is the one scenario where the whole island is wet. These are also the systems that bring heavy north and northwest swell, which closes Honolua to snorkelling, shuts the beach-loading catamarans at Kāʻanapali, and produces dangerous surf on the north shore.
High surf advisories. Take them literally. Maui's north-shore breaks in winter are genuinely large, and the shorebreak at Mākena and Baldwin is dangerous even on an ordinary day.
Hurricane season. The central Pacific season runs June through November. Direct hits on Maui are rare, but passing systems generate swell and rain, and if one is forecast, follow county emergency guidance rather than a weather app.