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Avalanche Forecast

Archived

Mar 12th, 2018–Mar 13th, 2018

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

Regions

Stevens Pass.

Complex avalanche conditions still exist in the Stevens Pass area. Continue to avoid all open slopes greater than 30 degrees. A Cornice failure could trigger a large and destructive Persistent Slab avalanche. Limit your exposure to overhead hazard Tuesday, staying away from large avalanche paths. 

Detailed Forecast

Several overlapping avalanche problems exist in the Stevens Pass area with a high degree of uncertainty.  The warm weather over over the past few days has likely helped round and settle older persistent layers. However, it will be best to continue to travel with a large degree of safety and avoid avalanche start zones.  

New persistent slabs formed Friday (3/8) and have been observed in several recent avalanches around the Stevens Pass area. Continue to stay off of all open slopes greater than 30 degrees; this includes areas low on the slope and well away from higher start zones. This weak and potentially dangerous layer has been observed on most aspects near and below treeline. Persistent slabs are difficult to manage and have proven deadly in the Cascades over the last several weeks. The recent warm weather over the past few days should help, but continue to travel cautiously.

Several older deeper persistent weak layers exist in the snowpack. This is a classic low likelihood-high consequence scenario. Just because you see tracks on a slope does not mean it is safe. While it may be very difficult to trigger these deeper layers, any avalanche failing on them will be large and destructive. The avalanche that resulted in a fatality Saturday on Park Butte near Mt. Baker is believed to have released down to the 2/8 crust - 2/13 facet interface. This is a recent example of the low likelihood-high consequence structure that remains in our snowpack.

Limit your exposure to overhead cornices as you travel. They may fail without warning due to daytime heating and the sun.

Snowpack Discussion

Sunshine and temperatures in the 40's to 50's over the past few days has allowed for significant snowpack settlement and for a variety of surface conditions to develop. Wet surface snow in the daytime and firm melt-freeze crusts are forming overnight on all but true north facing slopes which are barely hanging on to some settled old snow that fell Thursday.

Clear weather over the past few nights has allowed for surface hoar or near surface faceted snow to develop on many slopes. These surfaces are easily destroyed by wind or sun and warming, so may end up being widely variable when future snowfall occurs. 

Less Loose-Wet avalanche activity was reported Monday with more widespread activity reported Sunday across the west slopes of the Cascades on steep sunny slopes.

In the Stevens Pass area, several slab avalanches were reported Saturday. Most of them are thought to have occurred on the new persistent weak layer (3/8). Avalanches have occurred on most aspects near and below treeline. Many of them occurred noticeably low on the slope. Observations have 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 open slopes 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 including a widespread layer of weak sugary facets (2/13) that 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 3-4 feet below the snow surface.

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

Observation

Stevens Pass Ski Patrol found buried surface hoar (3/8) below the recent storm snow on both NE and SW aspects at 5500’ Saturday.

An avalanche professional in the Stevens Pass area on Saturday reported new observed slab avalanches on both SE and N aspects. He also experienced one whumph on the 3/8 persistent layer while traveling through lower angled terrain.

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 (3/8) buried below the recent storm snow. 

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.

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.

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.

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.