We're going to stay in Troutdale the night we fly into Portland and get an early start (about 6a.m.?) and take the Columbia River Scenic Hwy. all the way to The Dalles. We're debating whether to take the 197 to Sisters and enjoy the McKenzie and Santiam Pass Scenic Byways to Salem and then ending the trip in Portland OR once we're in The Dalles, backtrack to the 35 and take the Mt. Hood Loop (Fruit Loop?)ending the road trip in Portland? Can this be done in one day? We have the car rented until 12 midnight. Thanks for any help.

Is there a reason you are including The Dalles?

From the Dalles to Sisters, unless you like sagebrush, it is not very scenic, IMO.

I would completely pass on The Dalles and just do the loop around Mt. Hood unless you can make an overnight stop somewhere along the way.

From your other post it sounds like you'll be in the area soon. My understanding is that McKenzie Pass isn't open yet due to high snow levels in the mountains (I just googled it and it looks like it won't be open until mid-late July). Even if it were open that would be a LOOOONG day to go all the way down to Sisters--so I'd do the Gorge, Hood River, Mt Hood (Timberline) loop.

I don't think there's any reason to go out as far as the Dalles, unless you're interested in seeing Maryhill and/or the replica of Stonehenge across the river in WA.

What considerations must we take given inclement weather conditions? Thanks in advance.

The only weather you might encounter will be rain. As long as you can drive in rain, you will be fine