poor structure, shallow coverage VA Lakes

SNOWPACK OBSERVATION
Blue lake area
Submission Info
Forecaster
Thursday, December 31, 2020 - 2:45pm
Red Flags: 
38° 2' 54.8088" N, 119° 16' 23.8764" W
Snowpit Observations
More detailed information about the snowpack: 

I toured past Blue lake towards the north side of Black mountain in the Virginia Lakes basin today to get an idea of recent wind transport and to monitor the Dec 12 PWL.

The winds today were primarily out of the E-NE and significant flagging was present along ridgelines indicating wind transport and subsequent deposition on southerly and westerly aspects at upper elevations. I also found numerous terrain features both near and below tree line with reactive wind deposits on more northerly and easterly terrain. I Stomped on a cross loaded feature at around 10600’ on a    due north aspect which produced a small D1 avalanche that was certainly large enough to knock an unsuspecting rider off their feet but was not large enough to burry a person. The wind deposit in this location was 1 finger in hardness and sitting on top off a fist hard unconsolidated base. (see photos below)

I also experienced widespread collapsing and cracking today on northerly and easterly aspects the largest of which I would describe as “slope scale”.  I heard a loud whoomph and felt a noticeable drop as a spider web of cracks shot out both above and below me.  I chose to dig in near this location at 10,100’ on a NE aspect,32° slope, HS=70 cm. I found several things of note here beyond the poor structure that has been observed in many similar locations across the forecast area. Most notably I observed 14 cm of fist hard advanced facets just above the ground at this location with clear striations, some as large as 4mm in size. This depth hoar is something more typically found in a continental snowpack and may prove to be a concern well into the future. I also noted significant faceting in the “slab” of December snow which has led to the entire snowpack feeling relatively unconsolidated. (Boot penetration was all the way to the ground.) In simple English the weak layer is very weak and has the energy to propagate across a slope, however, the overriding slab is not particularly well condensed or cohesive and in fact seems to be losing structure. At least where I dug today. (see pit profile for more details)

I performed multiple tests in this location targeting the PWL and found propagation in all of them.  (ECTP-11, PST 27/100 END) I also performed two modified tests. One ECT, 120 cm wide which propagated across the column under moderate force. And a Propagation Saw Test where I added about 4” of 1F hard snow to the column before isolating it. (PST 5/100END) (see Video for more detail)

My thoughts regarding the PWL

We are continuing to find very poor structure, particularly in the northern part of the forecast area. While this is certainly something to keep in mind and be aware of, my observations today suggest the overriding slab is relatively thin and unconsolidated. I tried to sniff out an area today in steeper terrain with the same structure and more reactivity and was not successful. I did find many shallowly buried rocks, strips of bare ground, and wide swaths of un-rideable terrain. While I’m sure the conditions exist to create an avalanche on these basal facets somewhere, I believe it is an isolated problem at worst. From a 10’000-foot view, we just don’t have the coverage or depth yet to see this problem as widespread. That being said a significant new load is likely to trigger a cycle of increased hazard.

Snowpit or crown profile photo or graph: 
Snowpack photos: 
Snowpit videos (tests, etc): 
Weather Observations
Blowing Snow: 
Yes
Cloud Cover: 
25% of the sky covered by clouds
Air temperature: 
Below Freezing
Wind Speed: 
Moderate
Precipitation: 
Air temperature trend: 
Cooling
Wind Direction: 
Northeast
Accumulation rate: 
More detailed information about the weather: 
-- placeholder --

ESAC receives significant financial support from ...