Avalanche Forecast
Regions: Olympics.
Significant uncertainty exists in the Olympics right now so it is a good time to make conservative terrain choices. You may trigger fresh, reactive wind slabs at all elevations on Thursday. You might also trigger a persistent slab yourself or by initiating a wind slab avalanche. Reduce your hazard by traveling in terrain less than 35 degrees, avoiding wind loaded open slopes, and avoiding terrain exposed to overhead cornice hazard.
Detailed Forecast
Moderate to strong southerly winds will continue to build wind slabs at all elevations on exposed lee slopes. The storm will move new snow in addition to the older low cohesion snow observed Monday. Given the strength of the winds, these slabs may be large and are likely to be reactive.Â
The persistent slab problem remains a concern, though reports from the WA Cascades indicate it is very gradually healing, but still showing propagation potential in snow tests. Weak sugar-like facets can be found just above a firm crust layer formed in early February on all but south aspects. This layer is down about 2-3 feet, shallow enough to be human triggered or triggered by a wind slab stepping down to the weak layer. Observations on the distribution and sensitivity of this layer can be difficult to gather. When uncertainty increases, safe terrain choices decreases. Stay safe by avoiding locations where wind slabs might trigger, steep open slopes where a persistent slab avalanche may occur, and terrain threatened by cornice fall. Another potential persistent weak layer may have developed on W-SW-S-SE-E aspects with facets on a crust and will be something to watch as we gather more information following this storm.
Due to a lack of recent observations and a complicated snowpack, this is a good time to make conservative terrain choices.
Snowpack Discussion
On Wednesday afternoon, light snow fell along with moderate southerly winds with strong gusts were that were more than capable of moving significant snow and forming fresh and reactive wind slabs.
Cool and dry conditions Monday and Tuesday with sun breaks allowed the storm layers from last weekend to mostly settle and stabilize.Â
An active storm cycle began Friday evening. By Monday morning about 2 ft of new snow had accumulated in the Hurricane Ridge area. The most recent snow Sunday fell with slightly warmer temperatures and was easily transported by winds depositing fresh wind and storm slabs on weak low density snow deposited by early Saturday.Â
About 3 or more ft of of settled snow sits on top of the weak sugary facets that formed on a strong crust earlier in February. Snowpack tests continue to suggest that these facets can fail and produce avalanches. This layer has not been reported on South aspects in the Hurricane Ridge area.
There are no other significant layers of concern in the mid and lower snowpack.
Observations
NPS ranger Monday morning reported the most recent 9 inches of new snow at Hurricane Ridge had surprisingly little wind effects and was low cohesion.
Sunday, NPS rangers indicated the additional new 8 inches of snow was stiffer and more cohesive, being deposited over the previous days weak snow.
On Thursday Matt Schonwald and NPS rangers observed 20-30 inches of generally right-side-up surface snow over weaker snow (facets). Snowpack tests indicated the weak facet layer could still fail and produce avalanches. Northeast winds during the day Thursday redistributed snow onto SW-W-NW aspects.
Avalanche Problems
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: Likely
Expected Size: 1 - 1
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