Bulletin d’avalanche

West Slopes Central

Northwest Avalanche Center
Voir sur la carte

Danger d’avalanche

Thu
Alpin
2 · Modéré
Limite forestière
2 · Modéré
Sous la limite
1 · Faible
Open full map
Périmé — ce bulletin a expiré 5 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Slabs of wind drifted snow will build at upper elevations on Wednesday. Bypass steep, freshly wind loaded terrain and choose a mellower route to reduce the risk of triggering an avalanche. Older, larger, and more dangerous slabs are difficult to trigger but still a concern in extreme terrain. 

Problèmes avalancheux

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1–2

    Observers on Tuesday reported reactive pockets of fresh wind slab near and above treeline. Strengthening north winds will transport abundant new snow and potentially build bigger slabs. Stay vigilant in wind affected terrain and be on the lookout for new wind slabs on south or west aspects where they are not normally found. Drifting snow will be the most obvious sign of wind transport. Also, note changes like wind erosion or a more supportable layer at the snow surface to identify areas of wind loaded snow.

    Larger avalanches involving most or all of the new snow since December 21st have been reported from various locations in the Cascades. Triggering one of these larger avalanches is unlikely but not impossible in extreme terrain or with a large trigger like a smaller avalanche or cornice falling from above. Factor in the uncertainty around these avalanches before committing to wind loaded slopes.

    (12/28/2021)

    Westerly wind textured surfaces near the top of Big Chief Mountain on December 27-28th.

    Photo: Joe Dellaporta

Avalanche Discussion

A handful of very large natural avalanches - plus a few more from avalanche mitigation work - were reported since snowfall backed off on the 26th. They are scattered around the Cascades from the Stevens Pass/East Central boundary to Snoqualmie Pass to up north around Baker.  At this point, there is no discernable pattern other than they are in alpine terrain,  deep and probably released at or around the Solstice Crust. We do know weak snow grains - surface hoar and facets - were observed at the snow surface prior to the 21st in some areas. On the other hand, the warm front and associated precipitation of the 21st were observed to have melted back and destroyed most of these weak grains. Are these avalanches related to persistent grain types or just big loads of wind slab sitting on a crust or lighter density snow? If they released on buried surface hoar or facets, what is the distribution?  And finally, can a single person on a slope actually trigger one?

Well, none were reportedly triggered by a human simply moving through avalanche terrain. None were reported in the past 24 hours despite plenty of voluntary slope testers nearly everywhere in the Cascades. Are they a risk? Maybe not - but no one can really say for certain, especially in big, complex, rocky terrain at upper elevations. Make sure to talk about this looming piece of uncertainty before stepping onto big, high-consequence slopes.

Small loose snow and soft slab avalanches were triggered on Tuesday but reported to present limited hazard on open slopes. Terrain with deep, hollow tree wells, open creeks, or gullies in the track or runout still poses a risk if caught. Strong sun on Wednesday could trigger a round of small loose snow avalanches on steep, southerly aspects.

Colder than normal temperatures will continue through the week so plan and prepare for backcountry travel accordingly. Though the upper snowpack of deep, loose snow is consolidating, becoming fully immersed in a tree well is still possible. Keep your partners in visual or at least verbal contact. 

Régions couvertes