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It appeared to me that access to Grayback Mountain was possible to summit from the South staying off of Yakima Nation Lands. I also found a trail map with access and photos of the summit from 2019 here: http://www.willhiteweb.com/washington_fire_lookouts/grayback_mountain_lookout/map_300.htm
Although you can get a very pleasant eight mile RT hike in, the road/trail at the turnoff to access the summit of Grayback Mountain is signed with very clear No Trespassing signage. I contacted the website owner for wa fire lookouts and asked him to update his site.
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