Avalanche Forecast

Issued: Dec 10th, 2016 11:15AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

The latest round of snowfall should push our snowpack to the breaking point and natural and human triggered avalanche activity is expected to become widespread Sunday. Wind loading of lee slopes will push the avalanche danger to HIGH near and above treeline, but expect sensitive and dangerous conditions in all bands. Storm slabs that initially fail within the new storm snow may step down to recently buried persistent weak layers. Remote triggering of avalanches is possible with this snowpack structure. Choose low angled terrain and avoid slopes connected to avalanche paths with wind loaded start zones.

Summary

Detailed Forecast

Here we go again! With another round of light to moderate snow forecast Saturday night, followed by perhaps a brief break Sunday morning and then with renewed snowfall Sunday afternoon, 24 hr snowfall totals along the west slopes through Sunday 4 pm should average 12-18 inches with snow levels remaining relatively stable. W-SW transport winds should stay moderate especially for the central and south Washington Cascades.  

With this latest round of snowfall, we are betting that we have arrived at the breaking point where avalanche activity becomes widespread, with natural avalanches likely and human triggered very likely. Wind loading of lee slopes will push the avalanche danger to High near and above treeline, but expect sensitive conditions in all bands. 

Additional snowfall at more moderate temperatures will aid in a more uniform slab structure throughout the west slopes, leading to storm slabs potentially failing on weak persistent grain types formed prior to this storm cycle. Storm slabs that initially fail within the new storm snow can step down to these increasingly deeper layers. Remote triggering of avalanches is possible with this snowpack structure. Choose low angled terrain and avoid slopes connected to avalanche paths with wind loaded start zones.

Expect new wind slab near and above treeline in all zones especially on lee easterly aspects. Avoid all wind loaded terrain Sunday. 

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 is 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 3-5 feet of snow.

Cold and fair weather was in place midweek allowing near surface faceting and/or surface hoar to become widespread. Beginning Wednesday night and continuing through Thursday, considerable wind transport from easterly winds occurred near and above treeline from Stevens Pass and south to the Crystal and Paradise environs. Less wind effect occurred in the Mt. Baker area. 

A switch to westerly flow occurred outside the Cascade Passes on Friday with moderate warming while the Passes stayed easterly and cold. Beginning Friday night and continuing through Saturday, easterly flow eased and the Passes finally moderated temperature-wise. Post-frontal showers Friday night delivered 10-15 inches of snow through early Saturday morning. 

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 releasing down to either near surface facets or a surface hoar layer buried Dec. 8th. Debris from two natural storm slabs was observed in Highland Bowl of the Stevens Pass area that likely occurred during the wind shift and warming Friday night.  

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. 

Problems

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

Very 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

Very Likely

Expected Size

1 - 1

Valid until: Dec 11th, 2016 11:15AM