Avalanche Forecast
Issued: Dec 13th, 2016 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
Overall the avalanche danger will be less on Wednesday than the past few days. But small areas of new wind slab might be possible depending on the local strength of the east to southeast winds.
Summary
Detailed Forecast
A low pressure and frontal system should pass by to the south across Oregon and north California on Wednesday. It looks like this will system will cause light to moderate southeast winds in the Olympics and Cascades and mostly light snow over Mt Hood and the south Cascades on Wednesday. East winds will pick up in the Cascade passes and temperatures should stay cool on Wednesday.
Overall the avalanche danger will be less on Wednesday than the past few days. But small areas of new wind slab might be possible depending on the strength of the east to southeast winds.
New wind slab should not be extensive as east to southeast winds are not expected to be strong on Wednesday. But significant local transport is possible in places such as around Snoqualmie Pass. Since older wind slab is still possible on mainly north to southeast aspects and new local wind slab is possible on northwest to west aspects, wind slab will be indicated on all aspects. Watch for firmer wind transported snow which would be be more extensive if winds are stronger than expected.
Storm slabs from the recent storm cycle will further settle on Tuesday.
There is still uncertainty regarding the regional extent, spatial variability and the stability of the potential December 8th persistent weak layer which would be buried at about 2-4 feet along the west slopes. Ratchet back your plans if you experience collapsing or whoomping. Further time and snowpack tests from many areas will be needed to determine is this layer is still a regional problem.
Loose dry avalanches will not be listed as an avalanche problem on Tuesday. But in steep terrain lacking a slab structure continue to watch for small loose dry avalanches near terrain traps like cliff bands where even small avalanches can have big consequences.
With deep unconsolidated snow in many wind protected areas there remains a risk for tree well and snow immersion suffocation at this time. Ride or ski with a partner and keep them in sight at all times!
Even though the lower part of the below treeline band is filling in, watch for early season travel hazards such as barely covered rocks and open creeks.
The regional avalanche danger is expected to further slightly decrease on Thursday.
Snowpack Discussion
Weather and Snowpack
Cold and fair weather early last week allowed for a sun crust to form on many solar aspects and for cold low density snow, near surface faceting and/or surface hoar to become fairly widespread.
These layers began to get buried in most areas on about December 8th when the latest storm cycle began starting with a couple day warming trend as southwest flow eroded an Arctic air mass over the Northwest. This initially buried the cold low density snow, near surface faceted snow and/or surface hoar with denser snow in most areas. The warming trend leveled off with temperatures staying well below freezing December 10th and 11th and snowfall tapered off by December 12th. NWAC stations along the west slopes had about 2-4 feet of snowfall for the 4 days ending Monday morning!
Recent Observations
A regional storm and avalanche cycle was seen about Thursday to Sunday especially along the west slopes with some avalanches releasing on the December 8th layer.
The Mt Baker pro-patrol Sunday reported widespread 18-24 inch natural storm slab avalanches on most NW-NE facing slopes along the Shuksan Arm though it remained uncertain how sensitive slabs were to triggering on more deeply buried persistent layers if present.
Professional observations at Snoqualmie and Stevens Passes Sunday found the December 8th layer buried at about 3 feet or more. Several field personnel experienced collapsing of this layer although the snow above lacked the significant slab character to produce avalanches at the test locations.
There was a snow immersion close call snow immersion fatality at Snoqualmie on Sunday and apparently at Stevens on Saturday. There was also apparently a full avalanche burial on Mt Herman near the Mt Baker ski area on Sunday but the party has not yet been contacted and we will see if any details become available.
Several reports are also available for the west slopes Sunday via the NWAC Observations tab.
Settlement and stabilizing occurred by Monday and Tuesday but backcountry reports are not available.
The Mt Baker pro-patrol reported more limited avalanche control results on Monday with only local new 7 inch storm slab on sheltered slopes.
The Crystal pro-patrol skied extensively in the backcountry on Monday while searching for a missing skier and noted recent settlement and no avalanche activity. The skier was later found by Highway 410.
By Tuesday the Mt Baker pro-patrol reported no signs of instability.
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
Expected Size
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. 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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
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.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Dec 14th, 2016 10:00AM
