Avalanche Forecast
Regions: Cascades - North West.
While the Avalanche Danger is decreasing, dangerous conditions remain above treeline. You can trigger Wind Slabs near and above treeline and Persistent Slab avalanches on sunny slopes in the upper snowpack. Avoid wind loaded areas and large open sun-exposed slopes 35 degrees and steeper.
Detailed Forecast
You are most likely to trigger Wind Slab avalanches above treeline, today. You can avoid these avalanches by staying off of recent snow drifts, deeply pillowed features, and fresh cornices on slopes 35 degrees and steeper. These areas may exist far below ridge-lines and on mid-slope cross-loaded features. Soft non-wind-effected snow may cover new wind slabs making them harder to identify. You may see some small loose avalanches release on the surface of steep slopes today.Â
You may be able to trigger Persistent Slab avalanches in the upper snowpack on sun-exposed slopes greater than 35 degrees. If you dig about 2-3 feet below the surface you will find a series of thin sun crusts surrounded by very small facets. If you find this layer, avoid steep, open, sunny slopes as well as large avalanche paths to reduce your risk of these difficult to manage avalanches. In areas with a relatively shallower snowpack, avalanches triggered in the surface snow could step down to release a large and dangerous avalanche. These avalanches are difficult to trigger but the consequences are grave. The best way to avoid this low likelihood-high consequence problem is by avoiding triggering smaller avalanches in the surface snow.
Snowpack Discussion
About six inches of snow has accumulated in the last two days with up to four feet this week. New snow combined with moderate to strong winds transported snow in exposed terrain formed fresh wind slabs.
On E-S-W aspects, a thin breakable sun crust was formed early last week and buried on 2/23. Very small weak facets have been reported surrounding the crust. This layer has not yet had significant time to heal. It is found 2-3 feet below the surface on steeper slopes that have received direct sun during the past week.
Some observations suggest the presence of other persistent grains at this same interface on shaded slopes. Buried surface hoar and large preserved stellars have been reported in recent avalanches and snowpack tests at this interface.
Observers continue to report potential for a Deep Persistent Slab avalanche on a layer of weak sugary facets buried on 2/13. This weak layer is generally 3 to 5 feet below the snow surface just above a very firm melt-freeze crust (2/8). It's worth paying attention to this layer if your venturing into shallower areas. While these avalanches are unlikely, the consequences could be grave.
There are no significant layers of concern below the 2/8 crust.
Observations
On Wednesday NWAC professional observer Lee Lazzara traveled in the Mt Baker backcountry. Lee reported wind slabs forming on a variety of aspects near treeline. Snowpack observations showed a highly variable snowpack, but the facet/crust combination was found on steep sunny aspects 2 feet below the snow surface.
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.
Elevations: Treeline, Below Treeline.
Likelihood: Possible
Expected Size: 1 - 1