STS to RNT

I'm flying STS (Santa Rosa CA) to RNT (Renton WA, near Seattle), VFR
direct in early September. This takes me over some (7000 ft) hills with
higher terrain nearby, but in most cases the valley is not far away.
I'm thinking of crusing at about 8500 (unless the wind is strong, in
which case I'll go higher or sidestep to the valleys) This lets me
clear the peaks and ridges by 1500 feet, although higher terrain would
exist some ten miles away.

I'll be in an old Cutlass. For those familiar with the area, do you see
anything glaringly wrong with this plan? What cloud cover can I expect
in early September? I'm IFR capable, but that high ice would likely be
an issue.

Jose
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