Its Hawaii .. its tropical, so the weather can change a lot in a short period of time.
Don't get me wrong, there are showers and spells of cold quite a bit in kailua kona, but its a nice temperature throughout most of the year, but at night it can get rather cold. Around now, the temperature is around 80 degrees and an occasional shower shows up. Here is something i found on hawaii weather in general:
Overall, summer temperatures between May and October are warmer, peaking in August, and drier than the winter months. February and March are the coolest months when night temperatures on the coast fall to 15C.