Avalanche Forecast

Issued: Dec 11th, 2016 10:04AM

The alpine rating is considerable, the treeline rating is considerable, and the below treeline rating is considerable. Known problems include Persistent Slabs, Storm Slabs and Wind Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Recent deep storm and wind slab deposits should be given ample time to settle and stabilize, especially due to the slow stabilization rates given the expected cold temperatures. Human triggered avalanches should remain likely on Monday. Choose low angled terrain and avoid slopes connected to avalanche paths with wind loaded start zones. More deeply buried persistent layers are now a factor!

Summary

Detailed Forecast

Continued storm snow deposits overnight Sunday should give way to partial clearing Monday as high pressure affects the area. Winds should be light Sunday night and Monday with further cooling.   

While the ending showers with light winds and cold temperatures should lead to an overall improving avalanche danger Monday, there has been such significant storm snow, along with periods of moderate winds over the past three days, that another day or more of very cautious travel in backcountry terrain is urged.

Storm and wind slabs should begin to slowly settle and stabilize beginning Monday. However, careful snowpack evaluations, cautious routefinding and conservative decision-making will remain essential, as these dangerous avalanche conditions slowly improve.

Be wary of the potential buried persistent layer of surface hoar or near surface facets, potentially in combination with a thin crust layer. A high degree of uncertainty exists as to the spatial variability of this layer and the sensitivity to human trigger as it is now buried 3-4 feet or possibly more. 

Choose low angled terrain and avoid slopes connected to avalanche paths with wind loaded start zones. 

Loose dry will not be listed as an avalanche problem, 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 an increased 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.

Snowpack Discussion

Weather and Snowpack

An active and cool weather pattern has resulted in a snowy December thus far with NWAC stations along the west slopes recording about 4-7 feet of snow thus far.

Cold and fair weather early last week allowed for a sun crust to form on many solar aspects as well as for near surface faceting and/or surface hoar to become widespread. This layer was buried late Thursday night or early Friday in many areas, notably the Stevens and Snoqualmie Pass areas where it is showing up in test pits.

A storm cycle began in earnest Friday through Sunday depositing 2-3 feet of storm snow with some periods of fluctuating temperatures reversing the previous favorable layering and depositing denser warmer snow over weaker colder layers.  

Recent Observations

Widespread near-surface faceting and surface hoar formation was observed throughout the west slopes, including the Passes during the middle of the week along with some excellent ski conditions. Strong easterly winds built sensitive, but shallow wind slabs roughly from Stevens Pass and south to White Pass on Thursday. 

Light snowfall and/or localized shifting winds and warming Friday did not tip the scales and cause an uptick in reported avalanche activity. Most of the ski areas checked in Friday and generally noted shallow storm or wind slabs in their areas that were neither especially sensitive or widespread during control. 

Additional loading Friday night pressed on the avalanche scale for more varied results. In the Stevens Pass area, pro-patrol described touchy results with ski cuts and explosives triggering soft storm slabs. Debris from two natural storm slabs was observed in Highland Bowl in the Stevens Pass area that likely occurred during the wind shift and warming Friday night, midstorm. See NWAC instagram post and a description.  

Observations in the Snoqualmie Pass area were less clear cut. Soft storm slabs or dry loose below treeline and isolated pockets of wind slab near treeline were not widespread according to Alpental pro-patrol. Pro-observer Ian Nicholson was in the Alpental Valley and saw varied results in snowpit tests with some column tests failing easily and propagating across the column on a layer of near surface facets. However, Ian observed no direct signs of instability and noted the overlying slab generally lacked cohesion.   

In snowpack tests pro-observer Simon Trautman found sensitive results from facets resting above a sun crust in the Mt. Baker area on solar slopes near and above treeline and providing a weak layer/bed surface interface. Also from the Mt. Baker backcountry, a private party sent an email to NWAC forecasters Saturday afternoon reporting easily triggered slabs on south facing and lee slopes, with crowns ranging from a few inches to 3 feet. One slide was remotely triggered and caught and at least partially buried a skier who fortunately was not injured. 

NWAC observers also in the Crystal backcountry observed stiff westerly winds Saturday afternoon quickly building wind slab below ridges.

An additional 12-20 inches of storm snow by Sunday produced widespread natural avalanches on most steep NW-NE facing slopes along the Shuksan Arm visible by mid-morning Sunday. Control work at Mt Baker produced widespread soft slab avalanches on all aspects from 18-24 inches within the latest storm snow. However, true backcountry layering will differ and it remains uncertain how sensitive slabs are to triggering on more deeply buried persistent layers, where present.

Professional observations both at Snoqualmie and Stevens Passes Sunday found evidence of the buried surface hoar and/or near surface faceted or intact stellar crystals buried near 3 feet or more.  On solar aspects these layers are in combination with a thin sun crust. Several field personnel experienced rapid settlement on this layer. In the areas tested, the snow above lacked the significant slab character to produce avalanches. That is not likely the case in all areas, however, especially wind exposed areas. 

Problems

Persistent Slabs

An icon showing 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

Possible

Expected Size

1 - 1

Storm Slabs

An icon showing 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

Likely

Expected Size

1 - 1

Wind Slabs

An icon showing 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: North, North East, East, South East, North West.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 1

Valid until: Dec 12th, 2016 10:04AM