Pro Field Report

Basic Information

Observation Details

Observation Date:
December 18, 2020
Submitted:
December 18, 2020
Observer:
Chris Lundy
Zone or Region:
Banner Summit
Location:
Langer Area (7000-9000, S-E-N)

Signs of Unstable Snow

Recent Avalanches? 
Yes
Cracking? 
Isolated
Collapsing? 
Widespread
Most collapses were fairly localized (extending out less than ~20 feet), but a few felt like they traveled much farther.

Snow Stability

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

Bottom Line

10-12 inches of settled new snow has begun to build a slab atop the extremely weak faceted old snow surface, but isn't quite enough to produce widespread instability. It would likely take added loading from wind transport or a very steep slope to trigger an avalanche. Still, signs of instability such as widespread collapsing and very poor test scores have me traveling conservatively and provide indications of things to come with the next load.

Media/Attachments

Small avalanches along Highway 21 near Cape Horn. May have been plow-triggered.

Advanced Information

Weather Summary

Cloud Cover:
Overcast
Wind:
Light , SW

Skies were initially broken this morning, and it looked as though at least some clearing was imminent. Late morning, cloud cover increased again and it socked in at times.

Avalanche Observations

 #  Date Location Size Type Bed Sfc Depth Trigger Photos Details
5 Dec 17, 2020
(+/- 1 day) 		Langer area
NE 8000ft 		D1.5 SS-Soft Slab N-Natural
Avalanche in the Langer Creek area in the Banner Summit zone. It failed on very weak faceted snow that formed during the prolonged dry spell. 8000', NE
Small avalanches in the Langer Creek area in the Banner Summit zone. These failed on very weak faceted snow that formed during the prolonged dry spell. 8000', NE
Report
1 Dec 17, 2020
(+/- 1 day) 		Langer area
NE 9000ft 		D2 SS-Soft Slab N-Natural
Crowns in very steep terrain near Langer Peak in the Banner Summit zone. These crowns failed on very weak faceted snow that formed during the prolonged dry spell. 9000, NE
Debris of avalanche near Langer Peak. Heavily edited due to poor light.
Report
Langer area
Langer area

I also triggered a small hanging snowfield with a cornice drop on a 9000', NE facing slope that produced a D1 slide. In addition to the avalanches listed, there were several small (likely) plow-triggered slides on cut banks along Highway 21. The visibility today was poor, and I suspect with better light more small persistent slab avalanches would have been apparent.

Snowpack Observations

In the terrain observed, 25-30cm of settled recent snow sits atop the extremely weak 12/11 near surface facets. Collapsing was widespread, although the slab was soft enough that cracking was isolated. Some of the collapses traveled a significant distance. Collapsing was experienced where there was a thin crust between the facets and the slab, and where there was no crust. I did not spend much time on due S aspects, but I didn't feel any collapsing on those slopes where a stout crust existed.

@8400', E: HS 95cm. 28cm of settled snow atop the 12/11 FCsf. The base of the slab was F+, and the FC are F--. Test results on 12/11: ECTPVx2, ECTP2, PST9/100end. No basal weak layer present.

Despite a number of collapses while walking along a ridge, I did not remotely trigger any 35-38 degree slopes below me. Based on the observed avalanche activity, it appeared that wind loading and/or a 40+ degree slope was needed to trigger an avalanche.

Avalanche Problems

Problem Location Distribution Sensitivity Size Comments
Persistent Slab
Isolated
Specific
Widespread
Unreactive
Stubborn
Reactive
Touchy
D1
D1.5
D2
D2.5
D3
D3.5
D4
D4.5
D5
 Weak Layer(s): Dec 11, 2020 (FCsf)
Comments: The layer exist at lower elevations as well, but there is less of a load. D1-1.5 is the expected size, although D2's are possible.

Terrain Use

I would have felt very uncomfortable on any slope steeper than 35 degrees, especially if wind loaded. I avoided avalanche terrain today due to solo travel.