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Avalanche Forecast

Archived

Mar 9th, 2018–Mar 10th, 2018

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

Stevens Pass.

A new and potentially dangerous persistent weak layer has formed in the Stevens Pass area. Several avalanche problems exist in the terrain necessitating conservative travel choices. Avoid all steep slopes where avalanches may start, including steep open slopes well below treeline.

Detailed Forecast

You will be able to trigger newly formed persistent slab avalanches on opens slopes near and below treeline. Avoid open areas greater than 30 degrees. This includes rollovers lower on a slope and open areas below treeline. Recent snow has buried surface hoar and near surface facets on a variety of aspects near and below treeline resulting in several avalanches. You will be more likely to trigger these layers in areas where storm snow settlement or wind has created a more cohesive slab. If you experience sudden collapses, shooting cracks, and/or whumphing sounds avoid all nearby avalanche terrain. Persistent slab avalanches can fail over large areas, across terrain features, and onto lower angled slopes.

Wind slabs have formed on lee slopes and cross-loaded features. Use visual clues such as snow drifts and fresh cornices to identify and avoid steep slopes where the wind has deposited snow. Wind slabs can be difficult to assess. Give wind loaded terrain a wide berth.

Wet surface snow will develop Saturday on steep sunny slopes. Expect roller balls, pinwheels, and small loose wet avalanches to occur. These conditions will develop first on steep rocky slopes receiving direct sunshine. Stay off of any slope where you see signs of wet surface snow developing.

Several older weak persistent layers exist within the snowpack creating a low likelihood - high consequence scenario. While it is unlikely for you to trigger these deeper weak layers, smaller avalanches have the potential to step down creating very large avalanches.  Continue to stay out of large avalanche paths where these larger avalanches could occur.

Snowpack Discussion

Several avalanches were reported in the Stevens Pass area Friday. These avalanches occurred on a variety of aspects (E, SE, S, and NW) between 4400-5400 feet. Observations nearby found buried surface hoar (4-6mm) and small (0.5mm) facets. These weak persistent layers were found below the 12-15 inches of recent storm snow.

On Thursday night and Friday 12-15 inches of new snow fell in the Stevens area. Moderate to strong SW winds transported the new snow including on more exposed below treeline slopes. This new snow fell on a variety of snow surfaces including surface hoar, near surface facets, and melt-freeze crusts. 

Several older persistent weak layers can still be found in the snowpack. On E-S-W aspects near and below treeline a facet-crust combination buried on 2/23 has been observed for several weeks. This layer appears to be gaining strength, but has been found to be reactive in some snowpack tests. A more widespread layer of weak sugary facets (2/23) can be found just above a very firm and thick crust (2/8). Snowpack tests continue to show that if a failure is initiated on this layer it can propagate and cause an avalanche. This layer is generally found 4-5 feet below the snow surface.

There are no other layers of concern below the 2/8 crust.

Observations

Stevens

NWAC professional observer Matt Primomo traveled near Tye Peak Friday. Matt reported 12-15 inch slab avalanches on Moonlight Bowl (E, 4500), Skyline Ridge (NW, 5200’) and Tye Peak (SE, 4800’). He also experienced several large whumphs while traveling a ridgeline above Tye Lake. Observations in the area found surface hoar and facets buried below the recent storm snow. Snowpack tests on the 2/13 facet layer continue to demonstrate propagation of a potential avalanche.

An avalanche professional near Vahalla Lake Friday also found buried surface hoar 15 inches below the snow surface. He observed a rain crust up to 4300 feet below the most recent snow.

Numerous snowpack tests from the Stevens Pass area gathered over the last several weeks by multiple avalanche professionals continue to suggest that the 2/13 facet layer can produce avalanches. The most consistent test has been the Propagation Saw Test. 

Problems

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of 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 Slab.

Wind Slabs

Wind Slab avalanches are the 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.

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.