August can be rainy in Southcentral Alaska and the Alaska range. You will likely be hiking and camping above treeline much of the time, especially in Denali. (Treeline up here is around 2000' depending on aspect and latitude.) I've used a tarp tent and 0F WM down bag in Denali State Park at 3000' in early July and, even with a very short night, still been pretty cold. Think low 40s and windy, with no real shelter from either. It could also snow and frost at night, especially in Denali in late August. No way would I use an umbrella. I take a full set of 200-300 weight fleece plus fleece gloves and hat, plus Marmot Precip jacket and pants, for wearing in camp (and wear all the other layers I have with me, too). If you are anywhere near a glacier, catabatic winds will make it colder still. If you have access to a single-wall expedition tent that might be the best alternative, since condensation is not usually a big problem here. Air temps are too low and it seldom rains hard for hours at a time. Cloud ceiling is low, though, so you are in the clouds a lot getting damp and chilled.
It could be sunny part of the time, of course. Bugs are really not a big deal anywhere you'll be hiking or camping. I seldom use DEET. Bring long nylon or softshell pants and a light-colored nylon shirt -- latter will provide some wind protection and also keep the bugs off your arms if you have to pump water in a boggy spot.
Take something to wear on your feet during river crossings. Water will be glacial melt. Learn safe crossing techniques. I use cheap and lightweight plastic shoes that are like enclosed Crocs with Velcro. With socks on they double as camp shoes.
You are required to carry all your food and toiletries in a bear resistant barrel while in DENA -- make sure it fits inside your pack as they are hard to secure to the outside and doing this is terrible for your center of gravity.
Hope this helps, and enjoy our beautiful state!