Professional Observation

Observation Details

Signs of Unstable Snow

Blowing snow and fresh wind deposits made for touchy D1 wind slabs in some NTL lee start zones. Small cornices from today’s wind were also touchy.

Key Points

Went out to McGee Mountain to investigate a recently reported large natural slab avalanche that failed on 3/11 or 3/12. We have seen some impressive crowns, but wanted to check if the Feb. PWL was the culprit, or if this was just a deep wind slab at a stomr interface. I wanted to know if the Feb. facets were still producing avalanches ahead of the next round of loading.

• Moderate blowing snow on top of McGee Mountain deposited small, but touchy, wind slabs in upper start zones.
• Solar warming was beginning to melt the upper snowpack by late morning. We noticed roller balls and wet surface snow on our descent.
• The avlanche was a Deep Persistent Slab – Natural D3, N aspect, 10,000′. It failed on 1mm rounded facets (4F+ hardness).
• Crown was 225cm high. The average slab hardness was ~ P-
• The avalanche ran ~ 2500′ and stopped at the bottom of the track wher the slope angle was ~15 degrees.

This avalanche fits a pattern of deep slab activity that has continued in the past 48 hours despite decreasing loading. The Feb. PWL is still sensitive to larger triggers. In this case the trigger was likely a natural cornice fall triggering a large wind slab in the upper storm layers that then stepped down.