Note: After discussing this with Paul (Klenke) who recently adopted the Thomson page, we thought it might be beneficial to revamp the West Ridge route description page as well. So here's my attempt at it (11/11/2004).
Follow the approach directions on the main page to reach Bumblebee Basin (directly below the north face of the mountain) via Bumblebee Pass. The Basin looks as though it might offer good bivy options.
The West Ridge is one of two popular ways of reaching the top of Mt. Thomson (the other being the East Ridge, rated class 4). Given that the climb can be done car-to-car in a day, it can get crowded on summer weekends (we had two or three other parties on route when we climbed it). However, given the abundance of belay ledge options and climbing options, passing other parties is generally a non-issue.
The West Ridge route begins from a small notch between a gendarme and the start of the ridge (left and below the "0" in this photo; "0" marks the rope-up point). To get to it, scramble up from the Bumblebee Basin on scree aiming for the lowest point in the ridge (it's left of where you see the beginning of the ridge from below). Once you top out on the ridge, scramble over the small gendarme (i.e. traverse rightward toward the ridge). The going is about class 4. The route begins from the small notch - big enough to accomodate about 3 people at most.
The first pitch begins with a step left into a well-featured chimney. This is possibly the crux of the standard West Ridge climb. Reaching the summit takes about 4 or 5 pitches depending on how you break them up. Plenty of belay ledges to pick and choose from so the details of "how" you break up the pitches are not that important. Climbing is not sustained - mostly 4th to low 5th class.
Basically in about two pitches (with only one or two 5.6-ish moves, see here), you should reach the low-angle slab visible from Bumblebee Pass and Basin. The slab is probably only 3rd to 4th class but quite exposed. After traversing the slab, move up in one pitch (easy, 5.4 - 5.5; also see here) to the west sub-summit. Walk east over the sub-summit (slab followed by dirt) and descend the dirt trail (exposed but probably class 2 or 3) down to the notch below the true summit. Move up (we did it on the left side) onto the summit ridge via one of the low angle chimneys (low 5th class). Walk the unexposed summit ridge east.
Standard way to descend is down the East Ridge (class 4). We stayed on the ridge crest until we got to a gendarme on the ridge. We then moved right and down. There were rap slings around a tree but the terrain below looked easy enough to downclimb. Rapping would probably be faster though as the downclimb was more hairy than expected. More class 3 down scrambling brings you to another steeper section (again, it's probably a good idea for most to rap here). It's realtively smooth sailing after that as you reach a broad slope with lots of vegetation and a climbers' path heading down and right toward the basin. Just follow the climbers' path past the first "tower" and it'll take you down the scree slopes to the bottom of the Basin. Hike back up to Bumblebee Pass.
A light traditional rack (nuts and cams up to perhaps 3 inches) is all you will need. Bring a helmet. Single rope will do esp. if you're planning on the standard, East Ridge descent. The climbing is not sustained and most of the time you're running it out anyway on class 3-4 terrain.