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Archived

Avalanche Forecast

Jan 1st, 2019–Jan 2nd, 2019
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate

Happy New Year! Human triggered avalanches were reported Monday from this zone resulting several near misses. Recent avalanches are a clear sign of the dangerous avalanche conditions. This is a good time to give avalanche terrain a wide berth and stay off of and out from under all slopes greater than 30 degrees.

Discussion

Avalanche and Snowpack Discussion

Monday was an active day in the Crystal backcountry. Avalanche were reported in Cement Basin, East Peak, Pickhandle Basin, Three-Way Pack, Sourdough Gap, Elk Pass, and Cash Run.  Many of these were human triggered avalanches, including some remote triggers. These avalanches were generally large and acted in unusual ways. They failed on lower angle slopes, propagated widely, crossed terrain features, and occurred low in the startzones. Observations like this scream buried surface hoar.  

12/31/18: A large avalanche that was remotely triggered in the Crystal backcountry. Photo: Jeremy Allyn. 

We have not received observations from other locations in the West-South zone. That doesn’t mean that buried surface hoar isn’t there. Areas such as White Pass and Chinook Pass are likely areas to encounter a similar, weak snowpack.

Snowpack Discussion

New Regional Synopsis Coming Soon

Avalanche Problems

Persistent Slabs

Evidence continues to mount that a layer of buried surface hoar survived the most recent storms. The number of human triggered avalanches is clear proof that this layer is present, will fail, and can kill you. Surface hoar can cause avalanches to act in unusual ways. Remote triggering, wide propagation, and failure on low angle slopes are all characteristics of surface hoar avalanches. When we see signs like this, it's time to step way back. Even 30 degree slopes can avalanche in these conditions. Put big buffers between you and any avalanche terrain. You are most likely to find a layer of buried surface hoar 1-2 feet below the snow surface on slopes above 6000 ft.

It is hard to say where exactly the buried surface is and is not. Snow profiles can demonstrate its presence, but they cannot prove its absence. The growing list of avalanches suggest this layer is more widespread than previously thought. We will continue to monitor these conditions and keep you informed.

Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.

 

The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.

 

This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.

Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood: Likely

Expected Size: 1 - 1

Wind Slabs

Wind slabs formed over the weekend are gaining strength, but have not fully healed. Backcountry travelers and local professionals reported finding wind slabs in unusual locations such as low on the slope and in areas often wind-stripped. You may still trigger wind slabs on convex rollovers, near the sides of cross-loaded gullies, and on wind drifted snow well below ridgeline. In some locations, wind drifted snow may sit over a layer of buried surface hoar. In these locations, an avalanche may propagate widely and surprise you.

Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

 

Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..

 

Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.

 

Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood: Possible

Expected Size: 1 - 1