There is no "cascade pass" between Seattle and Portland. The worst risk of snow and ice is usually just south of Bellingham, but there are several different weather zones along the way, including the dreaded "convergence zones" where bad weather sometimes funnels into narrow bands.
And people ride this route all year--I've done it myself in January and February. You just need to pay attention to the weather and choose your day wisely.
If you live in Vancouver, you already know what the seasons are like. Just multiply whatever you find outside your door times 300 miles/500 km or so.
Hope that helps.
Mark
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