One of the things that makes Hawaii such a popular year-round vacation destination is its incredibly mild climate. If you are cold in the wintertime, you can come here and get warm. If you are hot in the summertime you can come here and cool off. Many Maui residences have no air conditioning and no heater. Windows are almost always left open and ceiling fans and our cooling trade wind breezes provide all the air conditioning we need here in paradise.
In order to understand Hawaii weather, you have to realize the different micro-climates that exist here. The windward side of the islands receive lots of rain ... the trade winds blow in the moist ocean air, the mountains lift the air and cause it too cool, and it condenses and causes clouds and rain. The leeward side of the islands are in a rain shadow ... they are much drier and hotter than the windward side. The summit area of the West Maui Mountains is the second wettest spot on the face of the earth ... receiving 400 inches of rain a year! Yet in Lahaina, at the base of these mountains on the leeward side, the annual total is a sparse 12 inches or so! The trade winds typically blow from the northeast, leaving the southwest sides of the island dry and warm and producing rain forests on the north facing slopes.
Most of the resorts on Maui are located in the leeward areas. The closer you are to the windward side, the more rain you will get at your hotel. You might find yourself doing a little more driving to go to a beach that is sunnier. This line between the windward and leeward sides of the islands is what gives rise to the many rainbows that are seen in Hawaii. And of course, the 400" of rain causes spectacular waterfalls and deeply carved valleys. (you have to take a helicopter flight to really see the most spectacular falls)
We often get phone calls from people who have looked at a 10-day forecast and are worried it's going to be raining the entire time they are on vacation. If you are looking at a 10-day forecast on WEATHER.COM you need to realize that it doesn't account for the different weather on the dry and wet sides of the island. Since it rains in the rain forest almost 300 days a year, their long-range forecast tends to skew toward reporting all that rain. It is not unusual for people to come here and find out that they don't see a drop of rain all week at their resort, even though the WEATHER.COM extended forecast looked ominous.
We recommend that you look at http://hawaiiweathertoday.com for a forecast that more accurately accounts for the true weather conditions around here. Local weather guru Glenn James is based on Maui, and he'll give you a great local perspective on what the weather is really going to be like.
The other factor that contributes to our microclimates is elevation. The summit of Haleakala is 10,000' and it gets cold up there! On rare occasions it even snows. The town of Kula is much cooler in the daytime than at sea level. If you are going to be exploring these upcountry parts of the islands, bring some clothing that you can layer.