You will very likely hear "scattered rain showers and sunshine" as the Kauai weather forecast nearly every day. This is especially apparent as you move around the island because there is more rainfall on the north shores than on the south resulting in is lush green vegetation on the more arid conditions in some areas on the south shore. As on most islands, you will find the "windward" side has more precipitation and the "leeward" side is dryer.
Photo credit: HTJ
You may find more rainfall during the winter, although there is no true "rainy season" in Kauai, and there is not a season when you can expect prolonged periods of rain. If it's raining in one area, you can continue driving several miles to find clear skies.
Through most of the year, weather patterns are affected primarily by high-pressure zones in the north Pacific that pump cool, moist trade winds down onto the island's northeastern slopes. This holds true for most of the summer and approximately half of the time in the winter. These winds are forced up-slope by the mountain heights where their moisture condenses into clouds that produce rain. Most of the rain falls in the mountains and valleys on the windward (northeastern) side of the islands.
It is this weather phenomenon that creates the rich tropical environment for flowers and verdant greens. The wettest months are from November - March, but winter rains do not generally disrupt plans, since the weather is localized. This means that if it is raining where you are, there is almost always a sunny spot to be found around the coast.
Because of its location, one factor contributing to flooding on Kauai are its openness to the sea surges, making the island susceptible to flooding during hurricanes and tropical storms.
The action of trade winds here means that there is always a cooling breeze. The strength of this wind builds as the heat of the day rises and reaches a peak in the afternoon, only to diminish in the evening and start again the next day. Several times during the year the trade winds will stop completely and the wind will switch around to come out of the south or west, bringing stormy or hot sticky weather. This is sometimes called "Kona" weather, because kona means leeward or south, and this points to the direction from which these weather systems arrive.