Route & General Observations
Went to Frenso to investigate the natural avalanche cycle that occurred early afternoon on 2/21 during a big outflow wind event. See Fresno observation from 2/21. Brief recap: On 2/21/19 between 11:15am and 12:40pm three avalanches released naturally on Frenso Ridge. The first was a small (D1) windslab noticed at 11:22am. The second occurred at 12:25pm, as we were about to descend at 2400′ due to deterring conditions and increasing avalanche hazard. This was a larger (D2) avalanche and debris filled the gully just North of our location. After we descended and regrouped around 1600′ on low angle terrain, we witnessed the third, even larger (D3) avalanche. This one hit the shoulder of the a sub-ridge we had just been on before deciding to turn around.
We determined the weak layer involved in the third and largest avalanche released on the ground on basal facets facet/crust combo. This was visually obvious based on the exposed ground and intact crust along most of the bed surface. The weak layer involved in the second avalanche is unknown, but suspect the MLK layer. This avalanche did not appear to release on the ground, but did propagate a large crown ~1000 wide. The smaller wind slab likely released on the new/old snow interface, possibly near surface facets buried on 2/16.
We confirmed that part of our route (skin track) on 2/21 was overrun by the third avalanche and destroyed part of a hemlock grove at 2200′-2300′. Some of the broken hemlocks were up to 8″ in diameter and one hemlock tree was completely uprooted. Our turnaround point at 2400′ was just outside of this zone.
The runout angle from the bottom of the debris pile to the starting zone was 24 degrees. We probed the bottom 200′ of debris, it averaged 60-70cm deep, but there were some places it was closer to a meter deep as it constricted towards the narrow gully above. Large hard slab chunks 70-90cm thick were sitting on top of the debris. The bottom of the debris was at ~1600′ in elevation and the starting zone was around ~3400′. The length of the path was ~3700′ and the vertical fall was 1900′. Based on path length and dozens of broken hemlock trees we determined the destructive force to be D3.
|Trigger||Natural||Avalanche Type||Hard Slab||Aspect||Southeast|
|Elevation||3400ft||Slope Angle||unknown||Crown Depth||unknown|
Obvious signs of instability
|Recent Avalanches?||No||Collapsing (Whumphing)?||Yes||Cracking (Shooting cracks)?||No|
Felt several collapses between 1500' and 1700'. Minor cracking just beyond length of skis on shallow wind slabs.
clear and sunny
temps in mid to upper 20F's
Variable wind affected surfaces: sastrugi, smooth wind slabs 10-40cm thick, and exposed melt freeze crust (MLK layer.)
After experiencing a large collapse at 1600' we found 1mm facets failing both above and below the MLK (martin luther king day) layer 30-35cm below the surface. MLK layer was easily recognizable with buried surface hoar sandwiched within a melt freeze crust. We also found tiny buried surface hoar mixed with 1mm facets below the crust. In this location the facets above and below this layer were more concerning then the buried surface hoar itself.
We didn't dig any formal pits due to not having access to a safe representative slope of any of the avalanches.