Avalanche Forecast
Regions: Olympics.
While it is still possible to trigger a large avalanche in larger terrain, there are still excellent travel conditions in more protected lower angle terrain of lower consequence. The ongoing Persistent Slabs problem is uncommon and still requires terrain choices with a wide margin of safety.
Detailed Forecast
Little change in the overall weather pattern is expected Wednesday. Continued cold temperatures with relatively light winds Wednesday will not significantly improve the current avalanche danger in this zone. A slab avalanche was triggered that was big enough to kill a person on Monday and the same conditions should persist Wednesday, especially on non-solar facing steep slopes greater than 35 degrees.Â
Persistent Slabs can surprise even very experienced travelers and can break over terrain features and run long distances. Avoid large terrain, especially areas exposed to avalanches that could come from above. Avalanches may be surprisingly large and run farther than you expect. Make conservative terrain choices until we know more about these avalanches. A small triggered Loose-Dry avalanche or a small Wind Slab may be enough to trigger a Persistent Slab.
Cold temperatures will allow recent slabs to persist longer than usual. Avoid steep wind loaded terrain especially near and above treeline on all aspects. Wind sculpted features, cracks in the snow, pillows, cornices, and variable height of recent snow are all indicators that you could trigger a new or old Wind Slab avalanche. A Persistent Slab could be more easily triggered in an area where the slab is thinner, such as near rocks or trees.Â
The combination of Wind Slab and Persistent Slab requires very cautious decision-making and terrain selection. Persistent Slab avalanches are not common in the Olympics and require additional safety margins.
Small loose dry avalanches are likely on very steep slopes and could be problematic around terrain traps. They will not be listed due to larger and more dangerous avalanche problems forecast Wednesday. Â
Snowpack Discussion
Cold temperatures and refreshing snow showers over the past few days have added light amounts of low density snow to the snowpack with light winds.Â
About 18 inches of new snow has fallen on weak old surfaces in the Hurricane Ridge area since 2/15. Near surface facets and surface hoar were observed on all aspects near treeline prior to the weekend storm cycle. These weak sugary facets were formed over a hard crust, creating both a smooth sliding surface and a very weak layer above. Where a slab exists above, that is maintaining the possibility of deeper slab releases. The cold temperatures are helping preserve the weakness at this interface.
There have been many reports of similar Persistent Slab structures throughout the WA Cascades, with a few widely propagating avalanches observed in the Snoqualmie Pass and Stevens Pass areas Sunday and Monday.  Â
In exposed terrain at Hurricane Ridge, winds have formed drifts on lee slopes. N-NE winds Sunday likely redistributed snow onto unusual aspects. Below the weak facets in the upper snowpack there are no other layers of concern. Warm wet weather from the first week of February has created a relatively uniform and consolidated lower snowpack.
Observations
Monday 2/19, snowboarder triggered slab, East aspect of Maggies, est. 2 ft crown, no injuries. Suspected release on 2/13 facet-crust.
No observations since Monday.
NPS Rangers on Monday saw crown from recent Slab Avalanche release on Mt Angeles, SE aspect, size difficult to determine.
Additional relevant obs from Monday.Â
Up slope snow showers added a few inches of low density snowfall to the Hurricane Ridge area Sunday, but heavier snowfall at lower elevations closed the Hurricane Ridge road so new new observations were received.Â
Avalanche Problems
Persistent Slabs
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: North, North East, East, South East, West, North West.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1
Wind Slabs
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