This route is NOT the classic summer approach up Monitor Ridge, although the two do meet 700-1000 ft below the summit. This trailhead is the Marble Mountain SnoPark on Road FR 8300 just beyond FR 8100, NOT the Climber's Bivouac accessed by driving FR 8100 and feeders.
Take Exit 21 off I-5 onto WA HWY 503 East. At 23 miles NOTE Jack's Restaurant and Store. This is the permit place between Nov 1 and May 14, when i suggest you do this climb. After permitting (maybe it's $5 these days) continue on 503E through Cougar until it becomes Forest Road FR90. Five miles later turn LEFT on FR 8300. Drive FR8300 for 3-4 miles until you see the CLOSED ROAD sign on FR8100 leading to Climbers Bivouac. Shed no tears. For you are NOT going that direction. You're not going to eat that summer ash and dust in the wake of 100s of hikers and climbers and small children on Monitor Ridge. You are going to take the "high road" so to speak.
So continue past FR8100 on FR8300 for another mile or less, and find the Marble Mountain SnoPark. You need a SnoPark Permit, dude. Park there at 2700 ft and calculate the vertical elevation gain between you and the summit at 8365 ft... Weep, pack your stuff and get going.
The trail - the Swift Creek Ski Trail #244B on the Green Trails Map - leaves from the West side of the parking turn-out and this exit/entrance is sometimes not very distinct. Once on it, though, the way is big and paved - so to speak. The trail proceeds North and turns into Trail #244 - Swift Creek Trail (?!) - and actually crosses Swift Creek somewhere around 4400 ft or so at Chocolate Falls - this is the "end" of the trail. The Nelson book says it emerges from Timberline at 6600 ft but my last trip there, my altimeter said 4600 ft... I thought... Hmmm. You need a back-country permit to camp above 4400 ft, so get one and bivy up or GO LIGHT and push it all the way to the summit and back to the car. A one-day trip is a good 6-13 hours, depending on your fitness and on whether or not you are skiing out or glissading. If skiing or boarding, please be nice to the snoeshoe-bound and walking-impaired that share the trail with you.
There is enough traffic to usually discern a "trail" of sorts well beyond Chocolate Falls. My last trip in 1999 on the last weekend before the May permit season started saw ~80 other people on the route. We did kick steps for them, though. :-)
From Chocolate Falls. cross over to the left (west) side of Swift Creek and climb up and North along the first ridge system immediately west of Swift Creek. The ridge wiggles and dips and bends slightly westward (sort of like a Worm or something that Flows downhill or such), but stick to it and it will connect with the Monitor Ridge route about 700-1000 feet below the summit depending upon your navigation and preference.
If you get on the East side of Swift Creek there is a really large and steep rib that lots of people take. That's fine, but it's no better than the other rib and has a lot more objective hazard from slipping and falling. if snow conditions are good, you can push idrectly up the drainage between the two if the ridges get crowded. The Climbing Rangers discourage people from climbing the Swift Creek East Ridge.
There are some nice bivy spots all along the Worm Flows route from 4600-5600 ft including one just at the toe of the first steep-ish snow slope that is styling. Good luck finding it by these directions.
Be aware of snow conditions and possible avalanche danger. You may be the only one of 100 people to know whether the conditions are safe or unsafe. There are sometimes Climbing Rangers on the route during the weekends leading up to permit season, when the traffic gets heavy. Nice people usually!
Descend the ascent route. Skis, boards, plastic bags, snow shovels, or Gore-Tex butt-wear recommended.
Ice axe, crampons, harness, standard 10-13 essentials. Not many people take crampons. Many people wish they did. If you ski, take some skins for the old boards. The entire route can be ski-mountaineered.
Please be aware that the Swift Glacier is still active and may have crevasses, both seen and unseen, depending on conditions. As with all glacier travel, use appropriate caution.
If you have information about this route that doesn't pertain to any of the other sections, please add it here.