The key thing to keep in mind is that there is the possibility that you will be unable to get over the passes. Last winter, all three passes over the Cascades were closed for several days around new years - and, at one point, the route up the Columbia Gorge was closed to. My son and I had tickets out of Seattle but there was absolutely no way to get there. As for getting from Seattle to Weanatchee be aware that if you go over 90, you then have to take Blewett Pass as well (unless you do a detour through Gorge.) Keep an eye on the weather and be prepared to change plans.