Avalanche Forecast
Regions: Snoqualmie Pass.
Identify and avoid steep wind-loaded slopes Thursday where you will be able to trigger a wind slab avalanche. We continue to deal with a Persistent Slab avalanche problem. You are most likely to trigger these avalanches on slopes greater than 35 degrees that have received direct sun in the past week. Avoid open slopes where this difficult to manage problem may exist.
Detailed Forecast
Use visual clues such as fresh cornices, snow blown off trees, and snow drifts to identify and avoid terrain where new wind slabs have formed. In some location soft non-wind-effected snow may cover new wind slabs making them harder to identify. Stay off any slope you expect has received wind loading. These areas may exist far below ridge-line and on mid-slope cross-loaded features.
We continue to have a persistent slab problem in the Cascades. A thin sun crust surrounded by very small facets has been observed in the Cascade Passes. It has been found in or thought to have caused several avalanches earlier this week. You are most likely to trigger a Persistent Slab on slopes greater than 35 degrees that have received direct sun. Snow profiles and snowpack test can confirm the presence of this layer; however they cannot prove its absences. Avoid any steep open large slopes where you may trigger these difficult to manage avalanches.
While it is not listed as a problem, there is still a potential for a Deep Persistent Slab avalanche to occur. These avalanches are becoming increasingly difficult to trigger, but may be caused by a smaller avalanche or cornice fall stepping down to older deeper weak layers. The best way to avoid this low likelihood-high consequence problem is by avoiding triggering smaller avalanches in the surface snow.
Snowpack Discussion
Winds Wednesday night formed new wind slabs on a variety of aspects near and above treeline. In sheltered areas generally soft unconsolidated surface snow exists.
On E-S-W aspect, a thin breakable sun crust formed early last week and was buried on 2/23. Very small weak facets have been reported surrounding the crust. This was the weak layer found or suspected in several avalanches 1.5-3 feet deep. 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 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.
Avalanche and snowpack observations continue to indicate that avalanches are possible on a layer of weak sugary facets buried on 2/13. This weak layer is generally 3 to five feet below the snow surface just above a very firm melt-freeze crust (2/8).
There are no significant layers of concern below the 2/8 crust.
Observations
NWAC forecaster Josh Hirshberg was on Nason Ridge Wednesday. He found the 2/23 crust 1.5 feet below the snow surface. While snowpack test were variable, he did report several avalanches on sunny aspects which occurred earlier this week.
Stevens DOT reported two avalanches Wednesday morning. One failed on the 2/23 interface on a NE aspect. The weaklayer appeared to be buried surface hoar. The other was a larger avalanche failing earlier in the weak on the 2/13 facet-crust combination. This is the most recent avalanche report we have on this layer.
NWAC observer Jeremy Allyn was in the Snoquamlie Pass area Wednesday. He reported wind transported snow near treeline. He found the 2/23 interface about 2 feet below the snow surface.
Avalanche and snowpack observations from around the western regions of the Cascades and Passes continue to indicate that propagation of an avalanche on the 2/13 facets is possible. This layer has been found 2.5 to 4 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