In August you should look forward to rain. Last year was pretty dry, but expect some wet. Bring warm clothes, especially for halibut fishing. You'll be glad you brought long-johns and good rain gear. It may not be raining, but rain gear cuts the wind. And yes, bring a light stocking hat and light gloves. I am guessing since you mentioned Kenai that you will be trying for halibut out of Homer, Deep Creek, or Ninilchik. The fishing should be good, don't keep the chickens (little halibut) unless it is late in the day and the fishing has been slow. Keep going for the big boys. As for the Kenai River, you'll be targeting sockeye (Reds) and silver salmon. I am not a big rainbow fisherman (except during winter), but I know the Kenai holds plenty of big bows and dollies. Normal temps during the day should be in low-mid 60's to the mid 70's. It will dip into the 50's during the evening. Bring lots of film and spend lots of money!!! You will not regret a trip up here. You may even end up staying!
Icy Mike