Avalanche Forecast

With strong winds, fresh snow, and cool temperatures, wind slabs will be the main concern for Sunday. Use extra caution at upper elevations on leeward, wind loaded slopes steeper than 35 degrees. Look for fresh drifts and areas of variable snow depth as indicators that you could trigger a wind slab avalanche. Use surface texture and feel to help identify where you can find these on the slopes. Steer around thickly pillowed areas and convex rolls where the slopes steepen. Use small, inconsequential test slopes to clue you in to how well the new snow is bonded. In many locations, the new snow may be sitting over a firm, refrozen crust and may not be well bonded. 

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.

On the 31st, a highly reactive layer of buried surface hoar was found above Leavenworth in the Icicle Creek drainage. The instability was obvious above 6,400ft where shooting cracks ran long distances. A number of avalanches were triggered from up to 200ft away while walking on a ridgeline. Activity appeared to be mainly on North through Southeast aspects. These avalanches were surprising in how widely they broke across the slopes. This layer is likely much less reactive at this point, but surface hoar is still being found from 1.5 to 3 feet down from the surface in the region. Fresh snow and wind drifting may stress it again. 

On the 5th, an observer near Gallagher Head Lake found this layer down 18" at 6,500ft, but a stout rain crust was found over the top of it. At the uppermost elevations and further north in the zone, this rain crust may not exist, so the layer may still be in play. At elevations below about 6,000ft, water has percolated through much of the upper snowpack and refrozen, so this layer is less of a concern. Be observant of shooting cracks or collapsing, or the absence of a stout rain crust about a foot down.

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.