Avalanche on Red Lk Bowl -Virginia Lakes

AVALANCHE OBSERVATION
Red Lake Bowl
Submission Info
Christopher F. Engelhardt
Forecaster
Tuesday, December 22, 2020 - 11:45am
Red Flags: 
Recent avalanche activity
Avalanche Type: 
Slab
Slope: 
35degrees
Trigger type: 
Natural
Crown Height: 
2 ft
Aspect: 
North
Avalanche Width: 
160ft.
Terrain: 
Above Treeline
Elevation: 
10 602ft.
Bed Surface: 
Old Snow
Avalanche Length: 
700ft.
Number of similar avalanches: 
2
More detailed information about the avalanche: 

It was another clear and sunny day, with light SW winds in the Virginia lakes area. From the end of the road I saw a slab avalanche that had occurred in the Red Lake Bowl so I headed up to investigate. Compared to this past Saturday when there was a soft 25cm layer of surface powder throughout the bowl, today the entire zone had been wind stripped with drift development, and significant surface crust’s and wind board… in some places supportable and in other not. Out of the sheltered east facing trees, surface conditions were not pleasant, as there was little consistency from turn to turn. No doubt the violent southerly winds last night played part in awakening some avalanche dragons.

 There were multiple crowns and avalanches in the north facing cirque above Red Lake, with the most significant avalanche measuring approximately 150ft across with a 1-2ft crown depth. The crown may have even been a bit deeper (3ft) initially but the bed surface had already been filled back in  a bit with wind deposited snow.  As far as categorizing the deepest pocket, it was an R4D2, the slide relative to its path was large and pulled much of a concave starting zone above a prominent gully feature and had a Destructive force of 1.5/2. The slide certainly would have injured someone severely if not proving fatal as it swept out all the snow through the gully down to bare rock and deposited it 600 vertical feet down on the apron. Debris was not significantly deep, but it did fill several small terrain trap features where a person could have been buried. It was a hard slab and looked like possibly a shallower wind slab had initiated 50ft or so above the main crown, and as it slid across the gut of the starting zone, the avalanche stepped down to the deeper persistent slab causing the bigger avalanche. This is an assumption, but certainly a possibility. There were several other shallower crown lines that measured around a 1ft in depth and extended 40-50ft right adjacent to the base of the headwall of the cirque which gets the heaviest wind loading from the S-SW winds.

I was able to skin up and dig a snow pit real close to the same elevation and aspect as the avalanche and get a closer look.  It looked like the deepest part of the persistent slab (newer firmer December  snow) certainly failed and slid on the weak faceted layer ( old November snow) at the bottom of the snowpack.

Bottom line is that snowpack in the upper north facing terrain is pretty crappy. Thinner shields (30-40cm) that I booted through are super faceted and loose, now with a thin wind board encapsulating them. In deeper areas ~90-100cm there now is a firm surface wind board on top of a 4F-1F/P robust slab, sitting on hollow airy 20-30cm facets at the base. Everything felt very drummy and hollow.

The firm Pencil hardness surface wind slab was deepest ~25cm right adjacent to the base of the cliffs and then became much thinner downslope, averaging around 5cm in depth. I performed the whole gamet of block tests. A CT failed in the basal facets on a 22, the ECT did not propagate, but the PST had a full propagation at 35/100cm end. Propagation Saw Tests (PST) describe propagation propensity in persistent weak layers, and as I drug my snow saw up slope through the weak faceted layer  (75cm deep)the whole block propagated and shot off down slope when I reach 35cm up a 100cm long block…A score of less than half the distance of the 100cm block shows some poor structure and overall stability.

 

Snow pit @ 10,602ft, 31 N Aspect, 32deg slope, wind loaded area

HS: 95cm

CT22 Q2 22cm 75 cm down in basal facets.

ECTX

PST 35/100 End

 

Avalanche Photos: 
Snowpit or crown profile photo or graph: 
Avalanche observation video: 
red lk bowl 38° 2' 21.9156" N, 119° 16' 7.0356" W
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