I've lived and hiked in OR and WA for 28 years. Your Dry Ducks and umbrella will be fine for OR in August. I'd change to my full-on rain parka and rain pants (lots of wet brush) at either Cascade Locks or Snoqualmie Pass. If I wanted to push my luck, I'd do it at Stevens Pass. There's no way I'd go north of Stevens Pass in mid to late August without full-on rain jacket and pants. My insulating layer is always synthetic in the North Cascades.
September rains in the North Cascades can be vicious beasts. Multi-day continuous rains of varying intensities are common. Snow is not unheard of although it rarely stays. These are usually driven by pretty solid wind and with temperatures in the 40° to 50° F range. Read in "ideal hypothermia weather".
From Snoqualmie Pass north, the terrain gets steadily steeper, higher, and has fewer potential bail-out points. While rarely above treeline, it can be a nasty place to get caught in a storm.
Example: In early September, I was northbound, approaching Harts Pass from the south, and looked back to see storm clouds and rain coming up the small canyon behind me. Rather than camp at the bottom of that canyon, I elected to keep going (albeit at a faster pace) up the north side. I misjudged the speed of the storm's advance. It caught me about half-way up. Fortunately, there was a large treed bench there where I hastily set up camp. The rain poured and temperatures dropped.
The next morning, in full Gortex parka and rain pants, I continued north towards Harts Pass, expecting the storm to be short-lived. The wind-driven rain and cold temperatures meant that even hiking up-hill and wearing my fleece under my parka, I could not get warm. It turned out that a local was heading south at the same time and had also been forced to camp on the same bench, less than 100 yards from me. Neither of us saw the other due to the rain. He - and most of the local hunters - had seen this stuff before, knew what it portended, and turned back to Harts Pass to go home. I caught up to him at the pass, and he offered me a ride into Winthrop, 18 miles away. I wisely took it. There's no public transportation in that area so my wife drove 6 hours from Portland to rescue me.
This turned out to be a very strong storm. It poured steadily for 5 days up in the North Cascades. I returned the following year to complete the 35 miles to Manning Park.