Sawtooth Avalanche Center

Pro Field Report

Basic Information

Observation Details

Observation Date:
January 13, 2020
Submitted:
January 13, 2020
Observer:
SAC - Savage
Zone or Region:
Soldier and Wood River Valley Mtns
Location:
Croy Canyon - Rotarun "sidecountry", 5400-5900'

Signs of Unstable Snow

Recent Avalanches? 
Yes
Cracking? 
Widespread
Collapsing? 
Widespread
Cracking was 12-30" deep in new wind drifts, shooting up to about 50'. Collapsing was mostly small areas (less than 100' diameter) on a variety of layers (see snowpack discussion). Slopes continued cracking and collapsing on multiple passes/tracks.

Snow Stability

Stability Rating: 
Poor
Confidence in Rating: 
High
Stability Trend: 

Bottom Line

Cracking and collapsing was widespread on fresh wind-drifts. We were in 30-32* terrain at its steepest, almost entirely on slopes not steep enough to slide - but I would have expected 35-45* wind-loaded slopes to be fairly easily triggered. Fortunately, the wind-loading patterns are obvious in this area...until they get buried. The slope above the old animal shelter alongside the road naturally cracked and moved a little bit - this area received much less wind-loading recently than all the other activity I observed.

Media/Attachments

Slope that cracked (naturally) above the old animal shelter site. Based on the appearance and location of the cracks it likely failed on weak snow near the ground.
Wind-loading patterns on Carbonate Mountain.
Wind loading patterns at Rotarun
More wind-loading patterns at Rotarun.

Advanced Information

Weather Summary

Cloud Cover:
Overcast
Temperature:
20s F
Wind:
Calm

About 8-10" of low-density new snow (20-25cm) from last night. Generally westerly winds loaded N-E-S aspects, focused on E. The wind-loading was mostly confined to at and just below ridgelines and cross-loaded gullies. There is still a fair amount of snow available for transport.

Avalanche Observations

 #  Date Location Size Type Bed Sfc Depth Trigger Comments Photo
20 Croy Canyon
6000-7000'
D1.5 SS 1' N-Natural Many smaller slides (D1-1.5) on N-E-S aspects. The slides broke at ridgelines where the wind-drifts were largest and the slope angle was steepest. None
2 Bullion Gulch - Peak 8435'
E 8300'
D2 SS 1-2' ? N-Natural Two slides in adjacent gullies/bowls that looked like persistent slab avalanches failing in wind-loaded terrain. Hybrid crown/flanks spanned at least 400'. Aspect was a touch south of due east, probably E/SE.
Bullion Gulch natural avalanches.
2 Kelly Mt
SE 8000'
D2 U N-Natural Slides were in the W Fork of Elk Ck drainage, Red Elephant Gulch. Aspect was SE/S. They failed in adjacent gullies, cross-loaded terrain on a sub ridge of Kelly. None

Snowpack Observations

Rotarun area observations: 12-30" wind drifts, soft to stiff in nature but not hard slabs. E-SE cracking and collapsing was on a variety of layers: a hard interface below the recent snow (wind event after New Year's), a thin crust+facet combo under the newer snow (guessing 1/10 but could have been 1/7?), trapped lower density snow/decomposing fragments, facets/depth hoar near the ground (thin areas where the "old snow" was less than a foot before the recent snowfall), and crust/facets in the lower part of the pack but off the ground. On N aspects, the thin crust+facet combo (1/10 or 1/7) produced collapsing and cracking. Slopes continued cracking and collapsing on multiple passes/tracks on E/SE aspects and on more northerly slopes.

The variability in old snow structure over short distances (less than 10') was impressive.

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: several, see notes above
Comments: Could call it a persistent slab problem or a wind slab problem: recent wind loading above a variety of weak layers.

Terrain Use

We dabbled in about 30-32 degree terrain on small slopes with no consequences (terrain traps, trees, rocks, etc).