Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Likely
- Size
- 2
Snowpack depths vary significantly across this zone. Unfortunatley, a disturbing constant exists on all snowy slopes: the weak snow buried in December forms the base of the snowpack. Snowfall over the past month has favored the western side of this zone (Soldier Mountains and near Dollarhide Summit). Here, the weak snow that developed in December is buried by a 1.5-2.5' thick slab of snow. On Friday, Soldier Mountain Cat Ski guides remotely-triggered an avalanche that appears to have failed on this weak snow (photo here).
Further east in this zone, in the mountains above the Big Wood and Little Wood Rivers, a very thin snowpack exists. On many slopes, what little snow has fallen has been removed by wind and snow. On shaded slopes and those sheltered from the wind, weak early season snow is creating dangerous avalanche conditions (photo at right/below). In areas sheltered from the wind, this weak snow is only buried about 1' deep. However, in areas loaded by the wind this slab can be thicker, as was the case in the avalanche pictured to the right.
The picture is simple, but its far from satisfying. Dangerous avalanche conditions exists and you are likely to trigger a large avalanche on most steep slopes. The only way to avoid the hazard posed by the weak snow at the base of the snowpack is by sticking to lower-angled terrain. Given the possibility of remote-triggering, you'll also want to avoid being below steep slopes.
ADDITIONAL DISCUSSION - WIND SLABS: Periods of strong to extreme winds have been the theme for the past week. Yesterday brought more of the same. Although wind slabs are becoming less reactive to the weight of a skier or rider, these slabs are adding stress to the buried weak layer of snow underneath. If you do trigger a wind slab, it could "step down", failing on layers deeper in the snowpack and becoming much larger than the initial avalanche.
(1/17/2021) This avalanche failed in the mountains above the Wood River Valley on a N facing slope at 8,000'. The culprit weak layer was the same weak, sugary snow that exists across our forecast areas. The slope had been heavily loaded by the winds of the past week.