Avalanche: Moonlight

Observer Name
L. Dunn
Observation Date
Wednesday, January 10, 2024
Avalanche Date
Wednesday, January 10, 2024
Salt Lake » Big Cottonwood Canyon » Mineral Fork » Moonlight
Location Name or Route
BCC>Mineral Fork>Moonlight
Slope Angle
Trigger: additional info
Unintentionally Triggered
Avalanche Type
Soft Slab
Avalanche Problem
New Snow
Weak Layer
Buried - Partly
My partner and I were apparently the only people in Mineral Fork today. The overnight new snow was around 7" above 9k, and the total new snow in the past week was approximately 15-20", much less than the 40+ reported in LCC. There had been some wind overnight, but it didn't appear to be a lot, and the subsequent snow was low density with little wind effect. One of the reasons we chose to go up Moonlight was it often is somewhat wind sheltered, especially from west northwest flow and it didn't have so much new snow. It was snowing lightly all day with some heavier bursts, with accumulation of about 2" during our time there. The wind was light. For our first run on Moonlight we skied from a knoll that is at some distance from the overhead hazard above and to the West. We saw no signs of instability, no evidence of a slab, no cracking, no settlement, and no avalanches, not even sluffs. The skiing was excellent and we went back up for another run, but this time we decided to push a bit farther uphill to get a little longer run.
I broke trail to a group of trees that were at the base of a ridge where we planned to transition. I made it to just below the trees without incident. My partner followed a few minutes later and stopped to transition about 5 feet from me. I was directly below a large tree, he was not quite below the same tree. Within a minute after he stopped moving, he was hit by avalanche debris. The debris knocked him free of both skis (binding was in ski mode) and he stepped downhill as the debris piled up to just above his waist. He remained standing, and we commenced to dig for his skis. They were found uphill from him on the old skin track buried 2' deep exactly where he had been standing. We believe we triggered this slide as we arrived at our 'safe' spot, just by standing on the slope. The crown was about 100' above us in steep rocky terrain and was about 2' high, 150' wide, and the slide ran about 200' vertically. I remained untouched by the avalanche that hit my partner 5 feet away.
The first image shows the crown (red line) and the debris field (blue line) and the location of my partner and me (black stars). We were on the northern edge of the slide. The second image is a close up of the northern end of the crown. The 3rd image is my partner after the debris stopped moving, and the 4th image is of his skis as we found them.
We were doing just fine until we underestimated the overhead hazard in those steep rocky cliffs and pushed into a place we shouldn't have gone.