We visit Hawaii for 2-1/2 weeks every August. DW is a school teacher, and also has summer classes, so August is the only time window we have available to us.
Weather will depend on the trade winds. Trade winds blow most of the time, and are from the northeast. They keep the islands relatively cool - temps in the 80's with relative humidity about 60%. Generally quite pleasant, particularly given the latitude. If your experience is Florida or the Caribbean, Hawaii is a wonderful change.
When the trade winds falter the southerly Kona winds prevail, and it becomes much more tropical - hotter, more humid, and little air movement. Still not as bad as the Caribbean can be in August. Generally you can get by without AC, but it's very nice to have when Kona.
Hawaii does not have nearly the hurricane threats that the Caribbean has (because the Pacific Ocean is cooler), but August is the peak month for tropical storms. About one-half the time there are significant storms being tracked, and perhaps one-third of the time there is a day with significant weather - usually a day or rainy weather. Even if there is no storm, there may be some elevated surf on south and east facing beaches.
One advantage of going to Hawaii in August is that the ocean on the North Shore is calmer. That means that you can be in the water at places such as Hanalei and the Princeville beaches. (Note that there are some beaches where it is not safe to swim any time. Know before you go.)
Because of the school vacations situation, Hawaii is relatively crowded in August. You will see that in air fares and car rental rates, especially.