Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Possible
- Size
- 1–2
Last week, extreme winds scoured ridges, reshaped the landscape, and deposited stiff, thick slabs of snow on a layer of weak facets. The resulting problem is referred to as a Persistent Slab for reasons relating mostly to the grain type of the weak layer, but as the name suggests, the slab is likely to be with us for a little while.
Winds aloft were predominantly east and southeast, loading mostly west through northerly aspects. But closer to the terrain, winds appeared to have deposited snow in a chaotic manner, accumulating on a variety of micro and macro terrain features on every aspect at mid and upper elevations. The remaining drifts, ribbons, and lenses of smooth snow are now suspect and can be found on every side of the compass.
You'll find this problem exists more often where slabs are stiff and can support your weight. Whumping, collapsing, and shooting cracks are indications that the slope you're on is unstable. It will be easier to trigger these slabs on slopes 30° and steeper.
Slabs are still sensitive, as seen by this recent observation of a party who triggered an avalanche near the summer trail to April Bowl.
Avoidance of suspect slopes and safe travel protocol will help you manage this problem. Spread out, only place one person on the slope at a time, regroup in safe areas, and avoid terrain traps.
The recent mild temperatures are working to help increase stability, but this problem will likely be slow to heal. Expect the reactivity to trend from sensitive to stubborn in the coming days. If you're getting out this weekend be ready to encounter rocks, crusts, and ice as you navigate. Expect winds to be on the rise again today
Some large blocks close to the ridge from a natural avalanche on Hatch Common from 12/5.
Photo: Tim Rogers