If you watched the video, you might have notice the Propagation Saw Test (PST) stability test. The score was PST30/100(End) - 80 cm of depth at 8000 feet on a slope with 28 degrees with a north aspect. This is evidence of instability. The snowpack is capable of propagating a fracture over significant distances. The risk of avalanches with propagation potential relates to the much larger amount of snow involved in the avalanche. Check the next picture of what propagation means, and how an avalanche propagated readily through terrain at Banner Summit.
|Avalanche at Bull trout Point|
The 30-35 cm interface seems to be sintering well, confirmed by lack of reactivity during yesterday's tour at Copper, and few days earlier at Pilot peak. We keep searching for buried surface hoar but have not detected this structure during the last tours.
I should comment that CT scores were CTMQ1(SC) at 80 cm. The sudden collapse (SC) fracture character did not revealed a well developed gliding surface. But the PST failure resulted in a very clean gliding surface along the fracture plane. I have to admit that I was quite surprised by it! The weak layer is close to 10 cms in thickness, and I could NOT distinguish a gliding surface from CT, ECT, or snow pit work. In the video referred above notice how easy the slab block glides along a well defined fracture plane produced by the PST failure.
Bottom line is that Pedro and I are being diligent in measuring the angles of every slope before skiing it, and making certain that we only ski slopes below 35 degrees in steepness, and that are not connected to steeper slopes either. Even in subalpine terrain the snow depth is highly variable, and the likelihood of finding a trigger point is not negligible. As an example, yesterday as we skinned out of a narrow valley bottom, with highly consequential terrain traps, I detected a hollow wind slab undermined by advanced facets. The slope was approximately 30 degrees, but it sends shivers along my spine, and I found myself rapidly but carefully skinning backward to safety. Notice that the wind slab was very localized, and it was created not by prevalent winds, but by cold air sinking along a steep narrow drainage!
The skiing was fantastic. But you knew that from the video. North aspects snow skied creamy with 20 cms of dense and loose snow consisting of small facets. Aspects with sunlight were skiing much better, with a silky satisfying feeling typical of large diurnal recrystallized crystals covered by massive amounts of large surface hoar crystals.
|Pedro & Chago tracks at lookers right. Diurnal recrystallized snow skiing!|
Back to surface hoar - this layer is going to be next problem once they get buried. And it is unlikely that warm temps or wind can destroy all of the surface hoar now resting in top of the snow at Banners Summit area. Enjoy the sequence of pictures for surface hoar crystals observed yesterday.
|Festival of Surface Hoar!|
- T at surface = -15 deg-C
- T at 5 cm = -18 deg-C
- T at 10 cm = -14 deg-C
- T at 15 cm = -10 deg-C
- T at 20 cms = -8 deg-C
|Skin transitions at creek bottoms looked cold and felt cold ... brr!|
|Copper Mountain South Face - Nice ski tracks!|
|One of my favorite ski tours, south of Copper, we call it 9218.|
|ID21 Avalanche Activity during mid-January|