Avalanche Forecast
Regions: Olympics.
Avalanche conditions will remain dangerous Tuesday. You can still trigger dangerous Persistent Slab avalanches that are big enough to kill you on slopes steeper than about 35 degrees. The combination of Wind Slabs and uncommon Persistent Slabs requires conservative decision-making and cautious terrain choices.
Detailed Forecast
Cold temperatures with a chance of a few light snow showers Tuesday will not significantly improve the current avalanche danger in this zone Tuesday. A slab avalanche was triggered that was big enough to kill a person on Monday and the same can be expected on Tuesday, especially on steep slopes greater than 35 degrees. Persistent Slabs can surprise even very experienced travelers and can break over terrain features. Put a wide buffer of terrain between where you travel and any steep slopes. Use extra caution in areas where avalanches could come down 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 the usual playbook of waiting a day or two for storm instabilities to settle out does not apply.
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 Tuesday. Â
Snowpack Discussion
About 16 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. There have been many reports of similar Persistent Slab structure throughout the WA Cascades, with surprising and widely propagating avalanches observed in the Snoqualmie Pass area over the weekend. 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, N aspect of Maggies, est. 2 ft crown, no injuries.
NPS Rangers on Monday saw crown from recent Slab Avalanche release on Mt Angeles, SE aspect, size difficult to determine.Â
Upslope 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
On Friday, National Park Rangers triggered a large avalanche that crossed from a northeast to east aspect near treeline. The avalanche was 70 feet wide and cracks propagated beyond the crown. On Thursday, NWAC observer Matt Schonwald and NPS Rangers were on Mt Angeles. They observed 4 inches of weak sugar-like snow on the surface (near surface facets) on all aspects and elevations traveled. They were able to trigger a small isolated wind slab on an E aspect near treeline.
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: All aspects.
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