Persistent Slabs
- Hochgebirge
- Waldgrenze
- Unterhalb der Waldgrenze
- Wahrscheinlichkeit
- Possible
- Größe
- 1–2
What is it? In terrain exposed to the wind, expect to find 1-2 foot thick, soft and stiff slabs perched above weak faceted snow and crusts. In the western part of this zone (Soldiers, Dollarhide area) where more snow will fall on a deeper snowpack today, the chances of triggering avalanches in sheltered terrain will rise as slabs form 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 weak, sugary faceted snow is better developed. Wind-loading should occur primarily near ridgetops and in chutes and gullies. 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, most slopes currently lack 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 in zones farther north; 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 when you dig in the snow. You may find weak snow near the ground, near yesterday's snow surface, or surrounding thin crusts.
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 stiffer snow exists 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: Relatively small avalanches you trigger could "step down" to weak faceted snow buried near the ground, causing a larger avalanche. Read Ben's Conditions Blog entry for more information about this weak layer.
(11/23/2022)
This avalanche was remotely triggered on Nov 23rd from the ridgeline above. It failed on weak snow near the ground in an area with a thin, wind-affected snow pack.