The North Face of Chair Peak rises above Snow Lake and in the winter months often makes for a very enjoyable steep snow and ice climb. Under favorable conditions, cold and clear, this route is a great way for a competent climber to gain winter climbing experience due to the ease of access, relative shortness of the route, and abundance of fixed protection on the climb and descent.
In winter there are generally two options to reach Source Lake from the Alpental ski area. The summer trail heads up the South Fork Snoqualmie valley on the north side of the creek, but often acquires a compacted boot track in the winter, and the so-called winter trail takes a path up the south side of the creek. Both of these trails are exposed to avalanche terrain at various points and both meet at Source Lake. Local avalanche conditions should be of primary concern when deciding which path to use, but if conditions allow, you would be best served to follow whichever track is best compacted; most often this is the winter trail on the south side of the valley.
From Source Lake head north-northwest towards the Source Lake-Snow Lake divide below the northeast side of Chair Peak. Drop any gear you don't plan on using during the climb in the basin below the east side of Chair before you head up to the divide. The most common descent deposits you below the East Face of the peak, so this is the most logical point to return to any belongings you left behind.
From the Source Lake-Snow Lake divide head climber's right, north, to reach the beginning of the North Face. As there are few good staging areas to gear up beneath the face, it is wise to rope and rack up at the divide before starting the climb. A ~100 meter traverse around the face from the divide will bring you to the base of the route. A wide moat often develops here forcing either awkward moves to get over it, or a high traverse to stay above it. This traverse will most likely be entirely on 45-55 degree snow and ice.
The first ropelength of technical climbing starts in a left-facing dihedral where ice usually forms. If belaying, make sure it is protected, because falling snow and ice will funnel into this chute. A full sixty meters of WI 2-3(if ice covered), with a fixed sling about halfway, will bring you to an open slope where good protection is hard to find. The leader should do their best to make an anchor out of available spots for pickets, small rock protection, and ice screws and belay the follower up; otherwise simul climbing this section to the next available tree anchor is recommended.
Climbing from the open slope sixty meters above the base of the route, continue up 50-degree snow to a group of trees on the right-hand side of the slope another full sixty meters. From these trees climb up and left past more evergreens on steep snow, past some exposed rock where you can find a fixed pin, over a short vertical step, then a short ways up steep snow to another solid tree.
The last ropelength heads straight up the final 45-degree snow slope to the corniced top of the face. There is often a way around the cornice by sticking to snow very close to the summit block on the right, or by traversing left to the top of the NE Butress. Otherwise, due to the poularity of this route, you should be able to find a spot where others have cut through or find a likely spot to tunnel over the top yourself.
From here follow the summit block and descent information found on the Northeast Buttress route page to finish.
The conditions on this route will dictate everything regarding the gear brought and used, belay and protection possibilities, and whether or not this is a steep snow and ice climb or just a snow-covered rock route.
Ice tools, crampons, rope, ice screws, pickets, small rock protection to 1 inch with an emphasis on pitons, a few double-length runners for slinging trees, avalanche beacons, probes, training for their use, and a shovel should you need one for the cornice.
This has been skied, read all about it, here.