Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Possible
- Size
- 1–2
We are still seeing poor snowpack structure in our snowpits at all elevations. The poor snowpack structure is in the form of weak, unconsolidated faceted grains located relatively close to the ground. The facets or "sugar snow" were formed during cool dry weather that prevailed in November after our first major snowfall event. This "persistent " weak layer is now buried by a 2'-3' (or more ) slab of snow that we have received in December. This persistent weak layer problem has been reactive all month long. Observers in the Wallowas and Elkhorn mountains have been reporting avalanche activity and collapses as recently as last week. Take a look at the observations posted last week on this website. There you will find great information about avalanche activity that people have been documenting not only last week, but all month. Persistent slab slab avalanches are hard to predict. The likelihood of triggering a persistent slab avalanche is low but the consequences of triggering such a potentially large avalanche are high. Areas where the snowpack is thin (ie rocky areas and cliff bands) are places where a recreationalist is most likely to trigger a persistent slab avalanche. The best way to avoid this low likelihood but high consequence scenario is to stay off and out from under terrain steeper than 30 degrees. Manage the problem by making conservative decisions about the terrain you intend to travel through.
(12/17/2022)
Snowpit at 5400' on a north facing slope at the Spout Springs area in the Blue mountains. The snow was almost 4.5' at this location. For the most part , the snow in the upper part of the snowpit is right side up and strong. Unfortunately, the bottom 8" of snow at the ground is weak "sugar snow" or facets. The compression test I performed failed on this layer (ct12 sc) as did a Propagation Saw Test that I performed (PST 40/110 End). This is a deeply buried weak layer that would be hard to trigger. If triggered, the result would be a large and potentially destructive avalanche.
Photo: TG
