The Bottom Line: Sub-zero temperatures in the alpine have rotted out buried weak layers below and around melt-freeze crusts. The probability of triggering one of these widespread layers could increase with warming temperatures and today’s inversion. A shallow and variable early season snowpack, along with the unpredictable nature of the problem makes human triggered avalanches possible especially from thin spots around rocks, trees, or cliffs.

Mid-Pack Persistent Layers:

The unique weak layer in the midpack below a melt-freeze crust is buried about 50-70cm deep and formed Nov. 17 when temperatures spiked in the alpine and then refroze. The “Big Warmup” Layer has faceted, or become weak due to the arctic outflow and cold temperatures last week. With the low predictability of this unusual weak layer, a wide safety margin is necessary.

Test results from 12/1 in the Lutak zone found high strength within the “Big Warmup” layer but still failed and propagated. We know it to exist at all elevations above 1000ft — even up to the highest summits. We expect this layer (or other midpack crust-facet layers) to be an ongoing issue going forward.

Depth Hoar Formation:

Late October storms and November cold snaps created basal facets in thin areas above 1000ft. These facets haven’t been active yet, but the possibility still exists that an avalanche could fail at the ground in thin areas with a slab on top. This concern may grow into a deep slab problem over time if these facets weaken into depth hoar with single digit temperatures like we had last week.

Snow pit photo from Lutak zone 12/1, and profile from 11/26.

Very stiff wind slabs, from several days of strong NW winds are still at the surface of our snowpack on SW-S-SE-E aspects and terrain features like cross-loaded gullies. Wind slabs 1-2ft thick remain possible for human triggering. These surface wind slabs could also step down to persistent weak layers.

Wind loaded slopes and cross-loaded gullies should be avoided or carefully managed. You can tell if a slope is wind loaded by the filled-in, rounded or pregnant shape of the snow. Probe around for hollow, slabby surface snow. Minimize your exposure to these slopes, and always travel one-at-a-time through any suspect areas. Rocks will be a major safety hazard as well, so take it slow and easy, and keep your avalanche eyeballs on, especially in wind loaded areas and convexities! Avoid terrain traps!