Professional Observation

Observation Details

Signs of Unstable Snow

Key Points

I kept moving in the Mammoth Lakes Basin trying to keep myself warm while looking for loose snow avalanches and wind slabs. I found both…and almost stayed warm enough.

• I saw a few small natural sluffs and it was easy to trigger loose dry avalanches on just about any slope steeper than 35 degrees near and below treeline. These were up to 8 inches deep and long running on recently buried hard surfaces or facets. They never grew large enough to bury someone in the terrain where I tested them, but they let me know I should watch out for terrain traps below.
• I found soft (F+) wind slabs on many aspects near treeline. These were up to 16 inches thick at the top and tapered quickly down the slope. I got shooting cracks walking around in a few of them on both east- and west-facing slopes. Above treeline wind slabs were up to 2.5 feet thick and stiffer (4F). The distribution up higher seemed predominately from northeast winds – east slopes were stripped down to old sastrugi, northerly slopes were cross-loadded, and westerly slopes were loaded. These also tapered as they went from the ridgeline down the slope. When I triggered wind slabs on test slopes they would break on F- precipitation particles and crack at the crown. Then the debris was so soft it behaved like a sluff. They did not propagate widely.
• Winds were light when I reached the Mammoth Crest, but they picked up quickly from the northeast and began moving more snow onto south and west aspects as I left. I saw the same pattern on higher peaks and near the Sierra Crest on my drive back down to Bishop. The wind chill was truly cold.
• I stopped to test the upper snowpack in a sheltered area below treeline. On a northwest-facing slope it was easy to identify the interface between the unconsolidated new snow (about 3 to 7 inches of it) and the recently buried facets in a shovel tilt test. I got easy small column results that collapsed suddenly at the same interface. The new snow was too soft to give any feedback in long column tests.