Overview of the route (from Black Pk).
This route is longer than the east face route, but I think it is easier and makes a good combination with Black Peak. The only problem is it a long way back to Heather Pass if you go back the way you came. You will also have to climb back up the northwest ridge to get back to the Lewis Glacier. I did this route in spring when there was a lot of snow still there, and I descended via the east face route. This was not easy downclimbing and the snow was soft and ready to slide. If you are going to do this, I would recommend a rope for getting down the east face. Also, the rock I encountered on the east face was very loose (the SW ridge is solid).
For another picture of the route later in the season, see Paul Klenke's photo here
From Heather Pass, follow the route toward Black Peak. Descend to the north, then head ENE to Lewis Lake. Halfway between Lewis & Heather Lake, you will see the Lewis Glacier heading up and to the left (south). Ascend this to the top, just below the ridge. The upper part of the ridge is sheer and the easiest way up is to keep going up and to the left and ascend a steep ledge to the ridge. When I did it in the spring, there was a large cornice here, and once you get on the ledge itself there is extreme exposure on the east side down to the snowfield below the north face. If you slipped here, there was no stopping. Below it was about a 50° angle and then off a huge cliff. Be careful!
Once atop the ridge, follow the ridge west to the notch, where there is an easier route to descend 400' to the snow basin below the sheer west face. From here, there is a snow finger (dry gully in summer) going up to the ridge. Climb up the ridge to the summit. It is class 3-4. The scrambling is not that difficult, but very exposed on both sides. It is a narrow ridge, and staying right on top of it is your best bet.
Take the snow finger in the center.
Ice axe, may want rope, helmet. If you are returning by the east face route I would bring a helmet & rope.