AdvisorySAAC04

02-05 to 06-05 Juneau Rounds & Mt Stewart

NE wind transport was heavy on Saturday morning, February 5, and we saw the many natural slab avalanches that we expected, but most were quite small. They were confined to small portions of lee slopes below the ridges that took the brunt of the wind. Most released early in the loading cycle, and stayed small and powdery. On Cross Bay Creek path T017 and on Mt Stewart there were some hard slabs whose debris was distinct, in contrast to the powder snow avalanches whose debris was so thin and spread out that it was usually undetectable. Heliboarders released several slides on Mt Stewart, and there were natural releases there as well.

On Sunday, we were out with the UAS Level II course on Mt Stewart, where we found mostly well bonded loose snow in the areas without windloading. But the NW to SW slopes that were loaded by wind transport the previous day were still very sensitive. We triggered two small R1D1 slabs on 40° rolls, both about 15 - 30 cm deep and 10 m or less in width. We kept our slope angles low and avoided windloaded areas wherever possible.

We dug out the string and creep column experiments we had set two weeks before, did dye percolation tests, and worked on routefinding and risk management skills.

The new snow, wind, and warming in the forecast are likely to build more sensitive slabs on the weak snowpack and trigger more slab avalanches in the next day or two.

Saturday's NE windloading caused numerous small natural releases throughout the area, wherever wind transported snow loaded onto lee slopes. A Juneau Police Department officer reported a slide crossing Thane Road and running out to the water on path D003 Union Oil at about 9:45 am, leaving a light dusting of debris. The debris was so thin we had a hard time confirming the slide, but binocular study from out on the rock dump revealed a small R2D2 size slab perhaps 20 - 60 cm deep and 70 m wide on the slope just below the cross on the Mt Roberts trail. The slide had become entirely a powder snow avalanche, with no flowing component and thus almost no debris by the time it reached sea level.

We noted similar slabs on T007 Satellite Dish and T008 West AJ, these three R2D2 size slabs on T011 Snowslide Creek, all at about 600 m, 20 - 80 cm deep and 20 - 80 m wide, and another similar slab on T014 Middle path. Cross Bay Creek path T017 had the only hard slabs, with clearly visible solid debris piles. They were similar R2D2 size, but stronger wind action there produced harder slabs.

On Sunday, we were out with the UAS Level II course on Mt Stewart. We dug out the string and creep column sites that we set January 23rd, two weeks ago. The profile from the N aspect 40° site on Fish Creek Knob is above, showing its RB6 Q2 test results (multiple hard jumps to release Rutschblock, average shear) and sizeable 14° departure from vertical on top 30 cm of the creep column. Students in the photo are digging out the Rutschblock on the S aspect 31° site, where we found that we could not get the Rutschblock to release at all (RB7), and we had only 4° of creep. The lower angle and south aspect probably caused both the stronger block test value and lower amount of creep on the S aspect slope.

Lucas Merli demonstrates Rutschblock test technique on a snowboard. Ski poles help boarders move smoothly onto the block. At this point, multiple hard jumps have failed to cause fracture, and the jumper has moved downslope as the hole he makes by jumping gets larger.