The skiing above 7000 feet was sweet. East aspects unaffected by wind had 20 cm very low density snow that have experienced some level of "near surface facetting" due to warm snowpack underneath and cold temperatures during the evening.
|Nice Pow at 7800 feet - Pilot Peak, Lamar Ridge Area.|
|Trinity Mountain Snotel|
But the snowpack had also a much dark face. A very sensitive weak layer was buried under a very cohesive slab with a thickness 50-60 cm. Below a picture of a series natural releases at Pilot Peak on a very popular slope some call "Almost Top of the World".
Very large whumpfs were present pretty much everywhere though-out the whole day, and collapses of snowpack in the range of 2-4 cm were not rare either.
Stability tests were not surprising considering the observation of natural avalanches and significant wumpfing and collapses. Multiple "Extended Column Tests" (ECT) triggered a propagation with fewer three hand taps, and released with Q1 quality (rapid and clean release, along with sudden collapse). Compression Test (CT) also failed "Easy" with Q1 scores.
A test pit at an East aspect, 7700 feet, 24-26 degree steep showed the following results:
- ECTP3Q1 55 cm (facets - 2mm)
- ECTP8Q1 55 cm (facets - 2mm)
- CT3Q1 (SC) 55 cm (facets - 2mm)
The snowpack structure we found is summarized next:
- HST (snowpack depth) = 110 cm.
- 20 cm of "Non-Cohesive" (very skiable snow) with FIST hardness.
- 35 cm of dense slab progressing in hardness from 4-FINGER to Pencil at the weak layer interface. Snow crystals were round and small (less than 1 mm).
- 55 cm of heavily faceted grains (2 mm) with FIST hardness. This layer was very moist! Free water can be easily detected and squeezed from the snow.
With the above snowpack structure it is not hard to identify a WEAK snowpack structure with many elements present in most avalanches:
- Slab is present! Difference of hardness larger than 1 step (Pencil to Fist).
- Persistent Grain Types are present = FACETS!
- Difference in snow grains larger than 1 mm between slab and weak layer.
- Weak layer is within the top meter of snow (55cm).
Thus what are commonly called "lemons", four lemons were detected. The other lemon, relates to thin weak layers (less than 10 cm), but there is a very thick layer of facetted grains. The high reactivity and Q1 scores of the snowpack is most likely the result of a very cohesive slab, but in particular a thick (5+ cm) and pencil hard rain crust right above the weak layer.
As aside note, yesterday, the conditions reminded me a lot about the well documented instabilities caused by capillary barriers, were free water flows within the snowpack until it finds a structure that acts as sponge absorbing water responsible for "wet slab avalanches".
Rain events during December 28th and December 30th heavily soaked the facetted and now buried weak layer with rain. The snotel chart for Mores Creek Summit included below shows the extent rain in the snowpack Notice the precip line (red) augmenting but not the snowdepth line (blue).
It is reasonable to expect that the water soaked basal facet layers will assit in the stabilization of the weak basal structure by promoting rounding, and above all once the free water freezes deeper in the snowpack.
Stability test results for Friday december 30th by another observer on a North aspect slope at 6200 vertical feet were:
Back to the topic of ski conditions, the lower 500 feet of the mountain require some careful navigation to avoid brush. However the top 1000-1500 feet are skiing incredibly well. Check the next video: