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Archived

Avalanche Forecast

Dec 10th, 2016–Dec 11th, 2016
Alpine
4: High
The avalanche danger rating in the alpine will be high
Treeline
4: High
The avalanche danger rating at treeline will be high
Below Treeline
3: Considerable
The avalanche danger rating below treeline will be considerable
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
3: Considerable
The avalanche danger rating below treeline will be considerable

Regions: Olympics.

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.

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 for the Olympics along the west slopes through Sunday 4 pm should average 6-12 inches with snow levels remaining relatively stable. Moderate W-SW transport winds should ease Sunday afternoon.   

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 in the Hurricane Ridge area, potentially leading to storm slabs failing on weak persistent grain types formed prior to this storm cycle. Storm slabs that initially fail within the new storm snow may 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 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 the Hurricane Ridge NWAC station recording over 4 feet of snow, including 10 inches of new snow through Saturday morning. 

Cold and fair weather was in place midweek likely 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. 

Beginning Thursday night and continuing on Friday, a switch to south-southwesterly winds occurred along with a moderate warming trend and light snowfall. 

Prior to this new snow, the snowpack in non-wind affected terrain was generally right side up with low density snow at the surface and no problematic layers in the lower snowpack. 

Recent Observations

Moderate easterly winds midweek likely built new wind slabs on lee aspects near and below treeline in the Hurricane Ridge area with plenty of low density of snow available for transport. A switch to light S-SW winds late this week may have begun to redistribute snow to more traditional aspects Friday. 

This redistribution of recent and new snow likely continued on Saturday. We don't have any snowpack observations identifying persistent weak layers in the Hurricane Ridge area so we have a large amount of uncertainty if new wind and storm slab avalanches will step down to deeper layers. 

Avalanche 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: North, North East, East, South East, North West.

Elevations: Alpine, Treeline.

Likelihood: Very Likely

Expected Size: 1 - 1

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