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Missoula Avalanche

Professional Observation

Basic Information

Observation Details

Observation Date:
December 20, 2021
Submitted:
December 20, 2021
Observer:
WCMAC - Ryan Sorenson and Todd Glew
Zone or Region:
Rattlesnake
Location:
Point 6

Signs of Unstable Snow

Recent Avalanches? 
Yes
Cracking? 
Isolated
Collapsing? 
None Experienced
We triggered a small (SS-AS-D1-R1) wind slab from lower-angle terrain on the ridge. It failed on a density change 6 inches to 1 foot deep. The slide was approximately 50 feet wide and 100 feet vertical fall on a 38 degree east and north-facing bowl near Point 6. We also experienced cracking on a southwest-facing cross-loaded test slope above Jenny Lake.

Bottom Line

We went to Point 6 from Snowbowl today to check today's avalanche hazard. We traveled on this ridge and intentionally triggered a small soft slab avalanche on a steep, northeast-facing slope around 7800 feet from flat terrain on the ridge.

We dug a snow pit on a protected northeast aspect around 7600 feet. We found generally strong, right-side-up layers that got harder closer to the ground. We did find a layer of rounding facets just above the December 3rd snow surface. We did not get any propagating test results.

Traveling one at a time with radios to check in, we carefully skied the southeast-facing terrain above Jenny Lake. The snowpack here was the thinnest due to previous wind scouring. It had recently been cross-loaded by north and easterly wind leaving small 6 inches to 1-foot drifts. We ski cut the small drifts to mitigate any hazard on the slope before proceeding.

On lower angle terrain near Jenny Lake, the snowpack got deeper, and we noticed the newer snow was denser than lighter old snow. To avoid storm slabs, we traveled on low-angle terrain back to Snowbowl.

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Advanced Information

Weather Summary

Cloud Cover:
Overcast
Temperature:
25°F
Wind:
Moderate , W

Increasing winds with elevation. We experienced moderate ridgetop winds with strong gusts out of the west. Previous winds were strong enough to transport snow to mid-slope on terrain above 7,000 feet. It was snowing less than an inch per hour (S1) throughout the day. Highest Precipitation Rate: Moderate Snowfall (S2).

Avalanche Observations

Small (SS-AS-D1-R1) wind slab from lower-angle terrain on the ridge

Snowpack Observations

Generally strong, right-side-up layers that got harder closer to the ground. We had an ECTN20 result on the new/old snow interface 10 inches deep on a northeast aspect at 7,800 feet.

Avalanche Problems

Problem Location Distribution Sensitivity Size Comments
Wind Slab
Isolated
Specific
Widespread
Unreactive
Stubborn
Reactive
Touchy
D1
D1.5
D2
D2.5
D3
D3.5
D4
D4.5
D5
Layer Depth/Date: 6 inch to 1 foot
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