Commonwealth Basin is a fun place to explore during the winter. Once the snow melts away, though, the bushwacking gets pretty tough.
With year-round access and essentially flat terrain, Commonwealth is a reasonable choice for times when avalanche conditions make it unwise to go anywhere else.
The easiest way to get into the Basin goes through the private property of the Sahalee Ski Club. Don't go that way without their permission.
The public access route is from the Pacific Crest Trailhead, near where Alpental Road branches off from the old pass highway. The trailhead is not plowed during the winter, so you may have to park at Snoqualmie Ski Area and walk under I-90. All of the parking on Alpental Road is private.
Shortly after leaving the trailhead, turn left from the PCT and follow an old trail leading into the basin. The goal is to cross into the basin above the Commonwealth Creek waterfall.
Once in the basin, be very careful about routefinding. There may be many trails through the snow, and some of them are made by people who are lost. Commonwealth Creek has four or five large forks with identical looking hills between them, so it is hard to pinpoint your position.
In the winter there are usually plenty of snowbridges spanning the creek forks. In fact, you may not be able to tell where the creek runs. But watch out during the spring, when the creek starts melting itself out. It would be bad to fall through the snow into the creek.
The area is pretty much safe from avalanche, except near the bases of the surrounding peaks. The avalanche danger becomes higher when you leave the timber in the north end of the basin.
Go back the way you came.
There are usually lots of tracks leading into and out of Commonwealth Basin. Be sure to follow the correct trail back out.