Avalanche Forecast
Regions: Cascades - North West.
New shallow wind slabs will form during the day Wednesday especially above treeline as snow and wind impact the Cascades. Use visual clues to identify and avoid slopes where the wind is depositing blowing snow. Persistent slab conditions have not changed. Stay away from large steep open slopes where these large dangerous avalanches may occur.
Detailed Forecast
New shallow wind slabs will develop throughout the day Friday on wind loaded slopes especially above treeline. These avalanches will grow larger and easier to trigger as the day goes on. Use visual clues such as blowing snow, plumes, snow drifts, and fresh cornices to identify and avoid slopes where wind is depositing snow. With plenty of snow available for transportation, wind slabs may grow large in the afternoon.
Conditions will not allow for much, if any change in the persistent slab problem. You are most likely to trigger this weak faceted layer on large open slopes greater than 35 degrees. Other avalanches may also step-down and trigger this weak layer. Stay away from large open terrain where these large persistent slabs may occur. This avalanche problem is not going away.
Due to the special variability of the facet layer in the Baker backcountry, you may be less likely to trigger this layer in this zone. However, any resulting avalanche would still be large. Continue to give yourself a wide safety buffer to handle the uncertainty of this difficult problem.
Excellent travel conditions can still be found in many locations. Lower angled slopes, treed terrain, and areas away from larger avalanche paths can provide more conservative terrain options.
Snowpack Discussion
Following last weekend's storm cycle, the upper snowpack consists of 2-4 feet of soft settled snow. In many locations this snow sits on top of weak sugary facets. These facets can be located just above the most recent (2/8) crust layer. Several avalanches, collapses, and whumphs have been triggered on this persistent weak layer in other areas of the west slopes of the Cascades. Snowpack tests and observations from around the west slopes of the Cascades continue to demonstrate that this layer can fail and produce avalanches.
A high degree of spatial variability exists in the Mt Baker area where the facet layer has been found but is more localized.
Below the 2/8 crust there are no significant layers of concern.
Observations
North
Professional observations from the Mt Baker area report a high degree of spatial variability in the location of the weak facet layer. Five avalanches were triggered in this region last weekend.
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: North, North East, East, West, North West.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1