Bulletin d’avalanche

Stevens Pass

Jan 3 – 4, 2019Northwest Avalanche Center
Voir sur la carte
Archivé · expiré 8 years ago

Danger d’avalanche

Sat
Alpin
3 · Marqué
Limite forestière
3 · Marqué
Sous la limite
2 · Modéré
Ouvrir la carte complète

Points saillants

Strong wind and snow have created dangerous avalanche conditions at upper elevations. Use caution near steep slopes where the wind is drifting thick slabs. Stay out from under large avalanche paths that start above treeline.

Problèmes avalancheux

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1

    Use extra caution at upper elevations on leeward, wind loaded slopes steeper than 35 degrees. Watch for fresh drifts and areas of variable height storm snow as indicators that you could trigger a wind slab avalanche. North through east through south aspects may be especially prone to wind slabs. Steer around thickly pillowed areas and convex rolls where the terrain gets steeper. Use small, inconsequential test slopes to clue you in to how well the new snow is bonded. In some locations, you may find a poor bond between the new and old snow or within the new snow.

  • Storm Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1

    Expect a new layer of warm, wet snow by Friday morning. Storm slabs could be up to 1 foot thick below treeline. If the storm stays on the warmer side, you could trigger wet avalanches in the upper snowpack. Either way, avalanches near the surface will be small but could have enough weight to push you around. Use caution near terrain traps like gullies, cliffs, and rocks where a small avalanche could have more serious consequences. Use small test slopes to check the bond between the new snow and old surfaces.

Snowpack Discussion

Stevens Pass

Before the most recent storm started at Stevens Pass, late on January 2nd, observers found a variety of surface conditions including crusts, small facets, and transformed surface hoar. These layers were not well-developed, but they could make for a slick surface or poor bond for the new snow. This is a similar set up to a small avalanche cycle reported December 29-30th, when the wind loaded east through south aspects at upper elevations. A couple of these avalanches were surprising in how widely they broke across a slope or the distance from which they were triggered. In Highlands Bowl, observers confirmed one of these avalanches ran on a slick crust topped with a layer of small facets. On Friday, look for similar weak layers or slick old surfaces below the new snow. Use quick snowpack tests or small test slopes to check the bond between then new and old snow.

Cascades Region

Thanks to all of you who volunteer, send observations, and support NWAC in various ways - we appreciate it.   

December of 2018 was fun (from a forecasting perspective) with three pronounced avalanche cycles, a couple different persistent weak layers, some rain events, and a flurry of human triggered avalanches to ring in the New Year. Most importantly, it seems that we made it through the last days of 2018 without anyone getting seriously hurt by an avalanche.

The deep (Dec 9) layer responsible for many of the avalanches early in the month no longer seems to be a problem in the western zones. That said, it is still possible to trigger an avalanche on its counterpart (or basal facets) in the eastern areas.

A widespread layer of surface hoar formed around Christmas. Late December storms preserved this layer in areas above the rain line and we have numerous (more than a dozen) reports of people triggering avalanches on it in the last three days. At least 4 people were caught and carried during this period, but so far we have no reports of serious injury. Most of these avalanches were soft slabs, D1-D2+, but there were several harder wind slabs in the mix.

It appears that the layer is most reactive and/or prevalent in the Crystal Mountain backcountry and in the mountains around Leavenworth and west of Mazama.

Surface Hoar can be an especially tricky and persistent weak layer. Read more about it here.

Régions couvertes