Pro Field Report

Basic Information

Observation Details

Observation Date:
December 17, 2020
Submitted:
December 17, 2020
Observer:
SAC - VandenBos, Davis
Zone or Region:
Sawtooth and Western Smoky Mtns
Location:
McDonald (7,000-9,200', primarily N-NE-E-SE-S)

Signs of Unstable Snow

Recent Avalanches? 
Yes
Cracking? 
Widespread
Collapsing? 
Isolated

Snow Stability

Stability Rating: 
Fair
Confidence in Rating: 
Moderate
Stability Trend: 
Steady

Bottom Line

Even with just 6" of new snow there were plenty of signs of instability, including natural avalanches, cracking, collapsing, and unstable snowpack test scores.

Media/Attachments

Small, likely plow-triggered avalanche on the Stanley side of Galena Pass.
This avalanche was triggered by a cornice. It started out small, but entrained a significant amount of snow as it traveled down slope. You can see the powder cloud in the trees near the bottom of the slope.
This was triggered as a stiff wind pillow. It started out small, but entrained a significant amount of snow as it traveled down slope.
This avalanche failed on an E facing slope at 9,500' after just 6" of snow fell. Visible are several different pockets that failed, the largest is on the left side of the frame. It appears that this heavily wind-loaded portion was able to trigger slides in more sheltered terrain as it traveled downslope.
Ben looking down on some cracks that formed from a cornice collapse.

Advanced Information

Weather Summary

Cloud Cover:
Mostly Cloudy
Wind:
Moderate , NW

15cm HST. Warm in the morning with a period of S1 precip on the drive over the pass. Snowfall tapered quickly without much more than an additional skiff. Temperatures cooled quickly and winds picked up speed as they shifted from S-SW to NW in the early afternoon. Winds seemed to ease off again after about 60-90 minutes, around when we exited the field.

Avalanche Observations

 #  Date Location Size Type Bed Sfc Depth Trigger Photos Details
5 Dec 17, 2020
(+/- 1 day) 		McDonald
N 9000ft 		D1.5 SS-Soft Slab AC-Cornice Drop
This was triggered as a stiff wind pillow. It started out small, but entrained a significant amount of snow as it traveled down slope.
This avalanche was triggered by a cornice. It started out small, but entrained a significant amount of snow as it traveled down slope. You can see the powder cloud in the trees near the bottom of the slope.
Report
1 Dec 17, 2020
(+/- 1 day) 		McDonald
E 9500ft 		D2 SS-Soft Slab N-Natural
This avalanche failed on an E facing slope at 9,500' after just 6" of snow fell. Visible are several different pockets that failed, the largest is on the left side of the frame. It appears that this heavily wind-loaded portion was able to trigger slides in more sheltered terrain as it traveled downslope.
Report
McDonald
McDonald

From safe locations and with clear views of the terrain below, we were able to trigger a handful of slides that started either as very soft slabs near ridgetops or with cornice blocks. These entrained a significant amount of debris as they descended. I anticipate that we will see some odd debris piles with this set up (deep stack of facets), thanks to the relatively dense, cohesionless facet stack that developed. We could hear these facets hissing their way down the slope from many hundreds of feet above.

Snowpack Observations

15cm of relatively low-density new snow were enough to produce lots of signs of instability, including natural and human triggered avalanches. At ESE at 8,300' on 15 degree slope: ECTP1 and ECTP3 down 20cm on 12/11 FCsf. New snow had been ever so slightly stiffened by the wind where we dug. Localized cracking with just about every step on the up, with the occasional rumbling, far-traveling collapse. Collapses occurred in either northerly terrain where S winds had deposited thicker, stiffer drifts, or on solars and solar margins where a sun crust built in the first week of December was buried. This crust was buried by new snow on 12/11 and 12/13 with faceting occurring on 12/12 and 12/14 (based on observations in Headwaters from 12/14).

New snow was lacking stiffness for widespread activity, but not by much. Impressive for such a soft slab. Will be interesting to watch if a day or two of settlement and some additional wind loading are enough to change this picture.

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: down 20-40cm
Comments: Fair bit of wind affect down into middle terrain where we were.
Persistent Slab
Isolated
Specific
Widespread
D1
D1.5
D2
D2.5
D3
D3.5
D4
D4.5
D5
 Layer Depth/Date: down 20cm
Weak Layer(s): Dec 11, 2020 (FCsf)
Comments: Cohesionless, sand castle style facet stack on shaded slopes. FCsf + MFcr on solars.

Terrain Use

We avoided all avalanche terrain steeper than ~35 and all wind loaded avalanche terrain steeper than 30, including terrain exposed to these hazards from above. Traveled with the possibility of remote triggering on the mind.