Professional Observation

Observation Details

Signs of Unstable Snow

Snow Stability

Key Points

This morning, while discussing wind slab avalanches in the forecast, I wrote: “Avalanches may break across start zones. Reconsider terrain adjacent to steep slopes that you may typically think of as ‘safe’.” Today I spend a few hours testing storm instabilities on Panorama Dome. I used small test slopes I’ve used many times. I mostly found what I expected, until I didn’t.

• I triggered small, sluffs and soft storm and wind slabs near and below treeline on southeast, east, and northeast aspects. These were typically 8 inches thick and failing on a melt-freeze crust buried yesterday. On my last test slope, I triggered an 8 inch deep wind slab that stepped down 3.5 feet to a deeper wind deposit. The resulting avalanche was 200 feet wide, ran 75 feet into the trees, and left me feeling very anxious.
• We got many shooting cracks and one big collapse on adjacent test slopes. The cracks were breaking at the new/old snow interface. The collapse was the result of the melt-freeze crust breaking under a 2-foot thick wind slab. We confirmed that layer in a snow pit where we got propagation in fist hard decomposing-fragmented grains above and below the crust. The crust exists under the new storm snow in all exposed terrain. We did not find it in sheltered terrain below treeline. There we found dense, 4 finger hard snow under the new.
• We trudged through intense snowfall – rates over 2 inches per hour – and moderate to intense blowing snow.

Reconsider terrain you may typically think of as “safe,” because sensitive wind slabs can bread across start zones and be very large. Have a safe holiday weekend.