In order to gain the NW ridge of Thunderbolt Peak, you would need to first climb to Winchell Col (between Winchell and Thunderbolt). Winchell Col can be ascended from the east (Sam Mack Meadow) or west (Dusy Basin) sides. The west side would be easier since it does not involve snow / glacier travel, at least later in the season. If you come from the east side, you may have to negotiate the bergschrund.
Winchell Col consists of two notches separated by a rock formation called "Dolphin Fin". From the west side, it is easier to climb the higher notch located closer to Thunderbolt Peak so you don't have to go over "Dolphin Fin" on the way to the summit.
Since the west side approach is easier, it will be described in more detail. After starting at the South Lake trailhead, go over Bishop Pass (6 miles, 2K' gain). Instead of dropping down into Dusy Basin, traverse the talus slope on the west side of Mt. Agassiz, past Agassiz Col and Mt. Winchell. Once you get to below Winchell Col, climb up into the right-hand notch. For the easiest going (Class 3-4), stay on ledges and large blocks on the left side of the notch. Instead of going to the lowest part of the notch, you can start traversing right towards Thunderbolt Peak and gain the NW ridge.
The NW ridge of Thunderbolt is a classic route and arguably one the better route on this mountain. The rock quality here is excellent, the climbing is sustained, and the exposure is tremendous! Even though this route looks straight-forward and short from below, don't let it fool you. It is pretty long and may take several hours to complete from Winchell Col, especially if you decide to rope up. This route is a great start for the popular Thunderbolt to Sill traverse.
Once you are on the ridge above Winchell Col, negotiate a 5th class headwall and continue up, staying as close to the ridge proper as possible. Here climbing will be anywhere from 3rd to mid-5th class for hundreds of feet. Approximately 2/3 of the way up the ridge, you will come across a drop off. You can either downclimb it (Class 5+) or rappel. There are existing rappel slings but you may want to bring your own just in case. One rappel will bring you back onto easier climbing. Continue to follow the ridge over blocks and headwalls until you get to below the north summit (Lightning Rod). Here you can either climb Lightning Rod (Class 5-5.8), or go around it on the right side towards the main summit.
Depending on how you choose to scale the main summit block
, you may want to go around it on the left (East, 5.9) or right (West, 5.8) sides.
: You can take any of the easier routes down (i.e. Southwest Chutes #1 and #2, West Face, Underhill Couloir), continue traversing to Startlight Peak or go back the way you came.
Light alpine rope for the rappel, several slings for the rap station, small alpine rack if needed to protect Class 4-5 sections. However, I'd strongly recommend to solo this route if possible.
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