Daytime high temperatures on Maui range from about 78 degrees in the wintertime to about 93 degrees in the summertime. 

Because the weather on the islands is usually very mild and pleasant, there is really no bad time to visit. WInter in Maui brings more rain to the island, normally for the months between December and March. These are normally "Kona" storms, because they come from the south, bringing muggy weather and sometimes very heavy rains. Storms can be selective, only striking areas like Hana and the Northeast coast, or wider in size that then affect the whole island. 

The majority of the resorts are along the leeward sides of the island. The leeward sides include most of West Maui (Lahaina through Napili) and the south side (Kihei through Makena). These sides will receive the least amount of rain except in storm situations. 

The water that surrounds the Hawaiian Islands, including Maui, is fairly warm all year. In the winter, water temperature averages 74° and in the summer it can reach 82°. 

There are between 26 and 29 micro climates on Maui. A 10 minute drive can place you in sun instead of rain-or the other way around! Most places experience winds during the day, which are either "Kona" or "Trades". Kona winds are from the south and southwest, and herald muggy, hot conditions. They also bring vog from the Big Island to Maui. "Trades" are tradewinds, and come from the north and northeast. These winds keep the humidity lower, and make even high temperatures feel cooler. Trades will bring light showers to the mountain and windward areas, mostly at night.

When to go 

Whale season starts in mid-December and generally is over by the end of April. February is the peak of the season which provides visitors with the ability to see many pods of whales in the ocean, from shore or from boats. You'll see them breach out of the water from viewpoints on the Pali highway (road between Lahaina and Ma'alaea). Stopping along this road is not allowed, but there is one scenic point with a large parking lot. Inbetween the Pali and Lahaina, there are also numerous beach areas that one can pull into and enjoy the views.

Hawaii Weather today is a good site for local conditions. It can be found here: . Another good place to check is NOAA, especially if any storms are brewing:  Remember that areas like Hana will have much higher rainfall, almost anytime of year.  Here's a good chart that shows the rainfall and temperature averages for each area of Maui:  .