|Location:||Seattle Ridge, BBQ hill, Warmup bowl, -2 Bowl|
Route & General Observations
Road up Seattle Ridge to investigate an avalanche triggered by a snowboarder yesterday (4/14/19) on a NE aspect off of BBQ Hill in Warm-up Bowl (-1 Bowl.) This avalanche was triggered upon descent into a chute with a crown ~30′ wide – the rider was not caught. We also found a second avalanche on a NW aspect of BBQ Hill in -2 Bowl. The -2 Bowl avalanche was also very recent – we suspect from yesterday. No tracks were seen entering/exiting the crown or debris, but three snowboard tracks were seen 5-10′ away on the same slope. This avalanche was likely remote triggered. Both of these avalanche were 1-2′ deep and had a starting zone just below 3200′. We could not safely access the avalanche in Warm-up Bowl, and dug a pit on a similar aspect and elevation and found a reactive layer of buried surface hoar sandwiched between two melt-freeze crusts. We could access the crown of the second avalanche and found the weak layer to be buried surface hoar on a melt freeze crust. The avalanche in -2 Bowl had a crown ~80′ wide, that also initiated a wind slab 10-20cm deep where the debris ran over a second lower steep feature. The crown of this wind slab was ~150′ wide.
Two recent human triggered avalanches at 3200' on NW and NE aspects. D1.5-D2 in size.
Sunny and clear
light East winds
New 3-4mm surface hoar was found above 2500' and was melting on South aspects by the afternoon. A melt freeze crust found on all aspects below 3000'. Above 3000' there was 1-3" melt freeze sun crust found on W-S-E aspects. By 2pm all Southerly tilted aspects had a wet surface. About 12" of loose dry snow found on North aspects, but even some steep NW and NE aspect had a 1cm sun crust that was not softening by 3pm.
We dug two pits.
Pit #1 - NE aspect at 3100' (same aspect and ~200' from Warm-Up Bowl avalanche.) HS=120cm, 34* slope. The slab was composed of 30-40cm of new snow and several melt freeze crusts. Weak layer was 6-8mm buried surface hoar sandwiched between two melt freeze crusts. Tests failed on this layer: ECTN14, ECTP16, CT18, CT16 We also noted a density change 10cm below 1F hard wind packed powder with a sun crust on surface.
The bed surface along the crown of the -2 Bowl avalanche was a 1cm melt freeze crust with 40-50cm crown. The slab was mostly 1F hard. There was a 1" melt freeze crust 35cm cm below the surface. We did a CT at the crown, but couldn't get the weak layer to fail at the crown. We moved 20' away and dug a pit and found the same structure - easy to identify these two melt freeze crusts - similar depths.
Pit #2: NW aspect at 3200', HS=deeper than 280cm probe, 28* slope. Compression test revealed 3-4mm buried surface hoar on a melt freeze crust. Some of the buried surface had pieces of Melt freeze crust stuck to them. There was also some near surface facets mixed with the buried surface hoar. Tests: CT16, ECTP24. We also noted a 4F density change (broken precip particles) 10cm below wind packed snow that had formed a sun crust on top. The crown also revealed this same 10cm wind slab on the surface.