Pro Field Report

Basic Information

Observation Details

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Observation Date:

December 17, 2022

Submitted:

December 18, 2022

Observer:

SAC - VandenBos, O'Connor

Zone or Region:

Soldier and Wood River Valley Mtns

Location:

East Fork Big Wood (6,300-9,500, all aspects)

Signs of Unstable Snow

Recent Avalanches? 

None Observed

Cracking? 

Isolated

Collapsing? 

Isolated

We experienced 5 impressive, booming, rumbling collapses and associated cracking. These were not occurring frequently, but the collapses were so large that we collapsed a good portion of the terrain we traveled through. One collapse traveled an estimated 2,000' in one direction. It wrapped around a large meadow and then came rocketing back at us from the other side. Wild.

Snow Stability

Stability Rating: 

Fair

Confidence in Rating: 

Moderate

Stability Trend: 

Steady

Bottom Line

We experienced 5 impressive, long-traveling collapses. These shook snow off dozens of trees, visibly dropped the snowpack, and traveled many hundreds to thousands of feet. These were occurring less frequently than they were closer to the big loading event, but the nature of the collapses speaks to the strong possibility of remotely triggering slopes at great distances. We also observed the aftermath of an impressive, widespread avalanche cycle. Most of the the terrain that we might have remotely triggered today had already avalanched.

Advanced Information

Weather Summary

Valley fog pushed up the Wood River Valley in the early morning hours and receeded by late morning. During our tour (1200-1800) skies were clear except for some streaming high clouds earlier in the day. Winds were calm to light all day long. Temps remained cold.

Avalanche Observations

We observed dozens of slab avalanches large enough to bury a person in the terrain we were in (D2), as well as multiple D2.5s and a few D3s. Glassing revealed many more avalanches. Slides were most common on slopes facing E-NE-N-NW-W. Many of these started on 12/8 weak layer and stepped down to 11/27. Others initiated on 11/27. Many occurred early in the storm and many occurred later.

Snowpack Observations

Surface/upper snowpack: I've been tracking and mapping a pretty interesting/ugly surface that's developing in our southern areas. On the morning of 12/13, the first clear day, I observed a 1-1.5mm rain crust all the way to the top of Baldy. If you were skiing fresh snow up there you likely felt it/heard it/seen it flying through the air as you skied. The crust exists on all aspects and is noticeably thicker the higher you climb. It presents as a true IFrc, with a narrow water-ice lens, accompanied by melt-layer recrystallization facets on either side (it snowed a bit more after the rain event, which must have been right near the tail end of the storm). I've found this on Baldy up to 9,000' (12/13), further out Warm Springs up to 8,500' (12/15), in Baker Creek up to 9,700' (12/16), and today in the East Fork up to 9,500' (12/17). Public observations from Croy and the Soldiers describe a similar layer. I'm calling this layer the 12/11 in my head, it either happened late 12/11 or early 12/12. Can't let a year go by without having a 12/11. I've spent some time pondering what else may have built this (high humidity, riming, wet snow, etc.) but a spit of rain seems like the best answer. It is accompanied by a feature that I've come to think of as distinctive of rain-on-dry-snow events: rain wrinkles. You may have seen these near objects and concavaities on slopes, they resemble the wrinkles in a thin fabric as it is draped over an object. SH exists on top of the 1-2cm of snow on top of the 12/11, and dusting of snow over the past few days along with additional SH growth cap this. We observed recent SH growth extending up to 9,500', the crystals resembled 1cm tall Christmas trees. There you have it, kind of an ugly mess in the upper 5cm, but pretty neat if you love snow.

Pit at 6800', N aspect:
12/8: presented as SH overlying FCsf. Produced ECTNs with moderate force and rough shears. SH was penetrating overlying slab and underlying FCsf.

11/27: presented as 1.5-2.5mm dry FC and produced unstable results in all snowpack tests. ECTPV, ECTP16, ECTP21. CPST 8/100, 15/100, 25/200, all to END.

The slab: snow overlying 11/27 grades smoothly from F at the top to 1F at the base. In this location it was 50-60cm thick, and HS was <1m. The slab is faceting and losing strength here, unlike many locations I've observed a bit further north. Will be tricky if deeper portions of this zone trend toward healing while shallower portions facet.

Avalanche Problems

Problem	Location	Distribution	Sensitivity	Size	Comments
Persistent Slab	N E S W	Isolated Specific Widespread	Unreactive Stubborn Reactive Touchy	D1 D1.5 D2 D2.5 D3 D3.5 D4 D4.5 D5	Layer Depth/Date: down 50-60cm here Weak Layer(s): Nov 27, 2022 (FC) Comments: This is grading towards a deep persistent problem in other areas, still a solid persistent problem here
Persistent Slab	N E S W	Isolated Specific Widespread	Unknown	D1 D1.5 D2 D2.5 D3 D3.5 D4 D4.5 D5	Layer Depth/Date: down 25-30 Weak Layer(s): Dec 8, 2022 (FCsf) Comments: This layer is widespread. It seems to be trending towards unreactive, but I have very low confidence in that. Where SH exists I've observed it penetrating the overlying slab.

Terrain Use

We avoided all consequential avalanche terrain, including runouts below both small and large slopes.