Start at the Obsidian Trailhead off Highway 242 and hike in 3.4 miles crossing the Jerry Lava Field which brings you to Sunshine camp. Take the path to the left which leads .6 miles to the PCT. Cross the PCT and take the climbers trail heading straight (where the creek turns south). Follow the trail as best as you can using the cairns and head to the gap between North and Middle Sisters keeping Folding Rock on your right. After a fairly steep part you will come to the middle of the Collier Glacier.
From the Collier Glacier, head towards the Col staying to the right of the crevasses. You may want to use crampons here depending on the condition of the glacier. Just to the left of the Col, where the glacier runs up on the base of the mountain you will see a trail running diagonal up to the left if in summer (if snow-covered, switchback up through easy slopes to gain the ridge). Follow that up the west side of the ridge until it flips over the the east side. The route traverses the first gendarme on the west side and back over to the east for the second gendarme. Traverse the last one on the west to get to the traverse on rock/scree to a ridge and the start of the Terrible Traverse to the base of Prouty Peak. Once around that, head up through the chute (the Bowling Alley) which is a loose class 4 scramble with no snow up dinner plate rock (can be steep snow, icy snow or water ice when not melted out). Ascend the Bowling Alley slightly right of center and exit right at the top of the chute. From there, scramble left onto the higher of the two pinnacles to the summit.
Helmets are a must. Almost continuous rockfall in the warm months. Snow conditions call for pickets, preferably two tools, some rock pro and a few ice screws. You'll want two 60 meter ropes to rap the Bowling Alley in one rap.
dkantola recommends the following:
Two tools, crampons, pickets, screws, rock pro, and a 50+ m rope until early July,
then an ice axe, crampons, pickets, rock pro, and a 50+ m rope until mid August,
then rock pro and a 50+ m rope if you want to protect or rap the Bowling Alley until snow comes in the autumn.
One 50 m rope is enough to rap past the fourth class section of the Bowling Alley
Total distance is just a bit over 7 miles from trailhead to summit with almost 5400 feet of elevation gain.
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