We are heading there this next December, so havent experienced it for ourselves just yet, but my understanding is that the average temp/weather in Hawaii does not change all that much with the seasons. The high temp in Dec avrgs around 82 degrees with a low of 67.
In Hawaii a cloud will pass over and drop some rain and then it will be nice and sunny again. Most of the time when this happens we just stay on the beach get a refreshing rain shower and then enjoy the sun again.
This chart on Weather.com shows the avrg temps each month and the rainfall for Dec (the highest month) at 4.1 inches:
http://www.weather.com/weather/wxclimatology/monthly/graph/USHI0026