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"Middle" in difficulty between South and North Sister, this is a nice climb with a great approach - just about 5 miles and 1500 vertical feet to great camping at treeline (from the east). A bit of snow travel, gentle slopes, and one pitch of steep near the top make this a good route for those looking for a moderate multi day trip. This approach to Middle Sister is from the Frog Camp/Obsidian Trailhead at 4,750 feet. You will find a beautiful camp area at Arrowhead Lake (44.1705, -121.8127)
From camp you will follow a fairly gentle, 30 degrees or less, grade in snow up to the saddle between north and middle sister. From there, ascend an exposed rock spine to the summit. You are likely to encounter loose rock and one steeper snow pitch. An alternative route from the West side is more of the NW Ridge, more of a boulder-hop staircase than the scree of the North ridge proper.