Persistent Slabs
- Alta quota
- Limite del bosco
- Sotto il limite del bosco
- Probabilità
- Likely
- Dimensione
- 1–2
What is it? In terrain exposed to the wind, expect to find 1-3 foot thick, soft and stiff slabs perched above weak faceted snow and crusts. In more sheltered terrain, the chances of triggering avalanches will rise this afternoon as snowfall piles up on top of weak faceted snow and thin crusts in the upper part of the snowpack.
Where is it? Fortunately, this is pretty simple. You are far more likely to trigger avalanches big enough to bury you 1) where the wind has drifted or stiffened the snow (see photos) and 2) on shadier aspects facing west-north-east where the snow is weakest in the upper part of the snowpack. Wind-loading should occur primarily near ridgetops and in chutes and gullies. In the Sawtooth Mtns, expect "spindrift" to build slabs in bowls and cirques below large cliff faces and couloirs. The weak layer exists at all elevations, but winds will form thicker and stiffer slabs as you climb in elevation. While the snow is weakest at lower elevations, many slopes are currently lacking the slab component of the avalanche equation (weak layer + slab + steep slope).
How can you recognize it? Multiple groups reported snowpack collapsing, cracking, or recent avalanches yesterday; these red flags are obvious signs of instability. Wind-loaded areas will feel stiffer under your skis or sled and are more supportable. The weak, faceted snow is fairly easy to see by digging in the snow with your hands (see photo below). You'll also find weak facets surrounding the thin crusts in the upper part of the snowpack on sunnier, SW-S-SE facing slopes.
How can you manage it? Persistent slab avalanches are tricky. You could trigger them remotely (from gentler terrain above, below, and to the sides of steep slopes), and they may break wider than expected or higher on the slope above you. Avoiding areas where slabs exist above weak faceted snow or thin crusts is the conservative way to manage this problem. While today's avalanches should not be huge, it's easy to get injured in small slides when the snowcover is so thin.
ADDITIONAL DISCUSSION: Avalanches you trigger could "step down" to weak faceted snow buried near the ground, causing a larger avalanche. While avalanches are much more likely to release in the upper part of the snowpack, we can't rule out the possibility of slides failing on the large, grey, sugary snow crystals near the ground. Read Ben's Conditions Blog entry for more information about this weak layer.
(11/28/2022)
Remotely triggered avalanche in the Alturas Lake Creek drainage. The crack extended about 500 feet from where Ben was able to remotely trigger the slope while walking on the ridge.