Solitude may just be the theme of this trip. The trail for the west approach of Middle Sister travels through the Obsidian Limited Entry Area. Advanced permits are required to day hike and overnight camp here. Only 30 day hikers and 40 backpackers are permitted each day. Once on the climber's trail, some route finding may be needed, as the path is not so worn. The route travels through a significant variety of terrain and features including old-growth forest, recently burnt forest, a vast lava flow, wildflower meadows, a waterfall, springs, sub-alpine forest, rocky plains, alpine lakes, obsidian cliffs, less-than-welcome scree, and a glacier. Once above treeline, and if camping at Arrowhead Lake, views of the Cascade Peaks to the north all the way to Mount Adams may be had during the majority of the climb. There are also the up close views of the jagged North Sister and, of course, South Sister and the nearby peaks from the summit.
At 19 miles, it is recommended to camp on the trail for at least one night. After securing your overnight permit well in advance (they seem to sell out fairly quickly - especially on the weekends) and as early as May 1, ensure you also have a Northwest Forest Pass to park. The trailhead has very limited, individual parking spots. A gradual incline through thick forest meets hikers initially, quickly giving way to the starkly bleak burnt forest, a result of to the Milli fire of 2017. The trail travels in and out of lush and burnt forest a couple of times before ascending switchbacks and meeting the sidewall of the young lava flow. At nearly 3,000 years old, this laval flow is considered quite young. Marvel at the expansive desolation dotted with saplings. Spot the nearby Obsidian Cliffs to the south, which reflect the light as glass does. First views of the North Sister and Middle Sister will be had here. After meandering through the lava, drop down into wildflower meadows, cross the small White Branch creek before again ascending through thick old-growth forest. The meadows, obsidian cliffs, and lose chunks of obsidian along the trail become more abundant as the hiker gradually ascends nearer to the tree line.
Meet the PCT and follow it north, taking in views of the impressive Obsidian Falls, and then marvel at the Sister Springs, the headwaters of Obsidian Creek, in the plains below the Arrowhead Lake bench. Just above the south Sister Spring, in a gulley at the southeast nook of the plain, the somewhat obvious climber's trail may be spotted. However, for prime backcountry campsites, continue north on the PCT for another half a mile, then take the next right marked user trail to Arrowhead Lake. It is recommended to stop off at the Scott Spring at this trail junction to fill up your reservoirs (you may find filtering optional). Continue on the unmaintained user trail as it ascends above the trees through scree to the top of the apparent bench above and to the south where Arrowhead Lake resides. Spectacular campsites can be found along the lakeshore with expansive vistas just to the west from the edge of the bluff. View the pristine reflection of the intimidating North Sister and Middle Sister in the lake. To the north, note Mount Washington, Three Fingered Jack, and Mount Jefferson. Depending on your departure time, you may decide to proceed from here to the summit before camping. However, it is recommended to plan for an alpine start on your summit day.
From Arrowhead Lake, some cross-country travel is required to meet up with the summit trail. Be sure to adhere to the second princple of Leave to Trace and travel on durable surfaces, avoiding fragile meadows. Alternatively, you may chose to retrace the user trail from Arrowhead Lake back to the PCT and to Sister Spring to take the obvious gulley trail mentioned earlier. Otherwise, take the path of least resistance over the mostly featureless plains to the south-southeast from Arrowhead Lake searching for the climber's trail. A GPS may be necessary. For some guidance, proceed just past the jagged ridge that juts out of the plains to the east, separating North Sister and Middle Sister. The climber's trail ascends just south of this ridge, taking quick switchbacks through small boulders and trees. Note The Husband to the southwest. At the top of this slope, traveling parallel to the aforementioned ridge, reach a plateau and dry tarn bed that may be filled with snowmelt in late season. Here, scope your route up the gully to the toe of the Renfrew Glacier. A boulder moraine lies at the base of the glacier, providing a natural ramp-like route that skirts the south edge of the glacier. Climbers without an ax and crampons may prefer this short detour instead of the steep glacier. Otherwise, drop down onto the glacier as it levels out where the western ridge juts up near to a near vertical angle. Traverse the glacier south, rounding the rocky ridge back to the east. The summit will now be in view, lying above a hanging snowfield.
Continue along the gradual incline of the glacier, crossing a rocky island, sighting the Prouty Point saddle. Once cresting the saddle, you'll have views of the opposing Hayden Glacier on the eastern slope and may spot other climbers on the route - this is where the west and east approaches join up. The North Sister sits, crumbly and jagged, occupying the view to the north. Follow this saddle ridge to the south toward the summit as the trail traverses through a scree slope, becoming steeper. The scree here is incredibly loose. By scree slope standards, it may just be the loosest scree on any Cascade volcano's summit push. Fortunately, the terrain becomes slightly more stable nearing the summit. Abrupt views of the massive, glacier-clad South Sister (just 3.5 miles away) and her friends Broken Top and the Bachelor are enjoyed here at the summit.
Descend the route taken, perhaps opting to camp an additional night at Arrowhead Lake. Enjoy the accomplishment by marveling at the impressive Middle Sister on the shore of the lake. This is an ideal photo location with North Sister and the rest of Oregon's northerly Cascade peaks. Not to mention the unimpeded views of sunset from the edge of the west cliff. For a somewhat shorter return route, hiking out via the Glacier Way to the north is an option. Once returning to the PCT via the user trail, continue north for a half mile before taking the marked trail to the west. This trail follows Glacier Creek, where thick patches of wildflowers grow creekside. Then link back up with the Obsidian Trail after another 0.6 miles and hike out through the lava flow and subsequent live/burnt forests. The Glacier Way reduces the return trip by about a mile, as opposed to taking re-tracing the original route in.