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Avalanche Forecast

Archived

Dec 10th, 2015–Dec 11th, 2015

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

Regions

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Persistent weak layers are likely to linger in the northeast zone with a transition to the north central and southeast zone. Persistent weak layers are dangerous and cause large, rapidly propagating avalanches.

Detailed Forecast

Cascades East Slopes Forecast

An occluded front should move south to north over the Northwest Wednesday night and Thursday. South to southeast alpine winds Thursday morning should generally shift to southwest Thursday afternoon. Look for mostly light to moderate amounts of snow by later Thursday.

Along the east slopes watch for possible new wind slab on lee slopes and new storm slab in calmer less exposed areas. The light to moderate amounts of snow and the cooling should somewhat limit the extent of new wind and storm slab. Remember that the avalanche danger will be greater if you encounter stronger winds and heavier snowfall. Evaluate snow and terrain carefully Thursday. Change your plans if you find yourself in unexpectedly prolonged stormy weather.

Note that persistent weak layers are likely to linger in the northeast zone with a transition to the central east and southeast zone. Do not assume that these layers have stabilized especially in the northeast zone.

Less snow below tree line should limit the avalanche danger there.

 

Snowpack Discussion

Cascades East Slopes Weather:

Snowfall over the east slopes the first 7 days of December was about 1-4 feet with an overall warming trend. Then an atmospheric river arrived the past couple days (Main Fork of the Pineapple Express). This looks like it caused about another 14 inches of snowfall in the northeast zone, rain in the southeast zone, with a transition in between in the central east zone.

But snowfall has resumed on Wednesday with a cooling trend.

Cascade East Slopes Snowpack and Reports:

The snow and warming trend the first week of December followed by the heavy rain, building on crust layers and on faceted snow and surface hoar caused  instability and avalanches mainly Saturday to Tuesday such as seen at Mission Ridge.

It seems we now likely have varied conditions along the Cascade east slopes.

In the southeast zone there should be a shallower more consolidated and more stabilized snowpack.

Beware that persistent weak crusts and faceted snow and buried surface hoar layers are likely lingering in the northeast zone. These potentially dangerous layers will have been loaded by the additional snowfall in the northeast zone the past couple days! This layer may still be reactive and it should still be best to proceed with very great caution until there are more definitive observations.

A transition is likely in the central east zone between the conditions of the northeast and southeast zones.

NWAC observer Tom Curtis was at Blewett Pass in the central east zone on Wednesday and found sudden collapse test results in faceted snow at the November crust 15 cm from the ground. Persistent weak layer presence and distribution there is still uncertain.

 

 

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.

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.