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Issued
Sat, January 12th, 2013 - 7:00AM
Expires
Sun, January 13th, 2013 - 7:00AM
Forecaster
Kevin Wright
Avalanche risk The Bottom Line

Persistent deep slab problems are lingering in the backcountry.   The CONSIDERABLE danger rating above treeline is for a low likelihood but very high consequence avalanche problem.   Recent avalanche activity, both natural and human triggered, gives us enough evidence to call the backcountry dangerous right now.   The danger will be increasing this weekend as a large weather system moves in this afternoon.

Sat, January 12th, 2013
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
Avalanche risk
0 - No Rating
1 - Low
2 - Moderate
3 - Considerable
4 - High
5 - Extreme
Avalanche risk Avalanche risk Avalanche risk Avalanche risk Avalanche risk
Travel Advice Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features. Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern. Dangerous avalanche conditions. Dangerous avalanche conditions. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended. Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain.
Likelihood of Avalanches Natural and human-triggered avalanches unlikely. Natural avalanches unlikely; human-triggered avalanches possible. Natural avalanches possible; human-triggered avalanches likely. Natural avalanches likely; human-triggered avalanches very likely. Natural and human-triggered avalanches certain.
Avalanche Size and Distribution Small avalanches in isolated areas or extreme terrain. Small avalanches in specific areas; or large avalanches in isolated areas. Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas. Large avalanches in many areas; or very large avalanches in specific areas. Very large avalanches in many areas.
Avalanche Problem 1
  • Deep Persistent Slabs
    Deep Persistent Slabs
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.
More info at Avalanche.org

The deep slab problem is still our primary concern in the backcountry.  Remember, besides being a deep slab, it’s also a persistent weak layer.  This means the problem is not going away quickly.  The trend for a problem like this is to take a long time for stability to improve in the absence of provoking weather.  Natural avalanches will become more likely as new precipitation starts today, which also makes human triggered avalanches more likely.  The size of expected avalanches remains very large and destructive.

Recent avalanche activity in our region includes the large snowmachine triggered slide on Seattle ridge, and the similar slide in Main Bowl on Tuesday.  Explosive triggers brought down large avalanches on Wednesday with crowns 10-15 feet deep.

Yesterday in snowpit testing we found the relatively deep and heavy snowpack is still reactive on the facets at the ground interface.  Pit tests show disconcerting results with easy failure on isolation and a significant collapsing of the weak layers.  This means that although it will be difficult for a person to initiate the collapse through the deep and strong slab layer, if a collapse begins the resulting avalanche will likely propagate, pull the entire depth of the snowpack, and become very large and destructive.

Careful routefinding will be important this weekend.  Steep slopes should be avoided.

Repeat Offender avalanche 1-8-13

Avalanche Problem 2
  • Wind Slabs
    Wind Slabs
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.
More info at Avalanche.org

Recent wind above 2000 feet showed scoured ridges in some areas of Turnagain Pass.  Isolated pockets of stiffer wind slabs should be expected if you travel up the ridges above treeline. 

Weather
Sat, January 12th, 2013

The last couple days have brought generally good weather to our region.   A change is coming this weekend with expected warming, rain, and snow.

We expect an increase in the avalanche danger correlated to this weather event, with the intensity expected to peak Sunday-Monday.   For today, the beginning of this storm system will move in, with a mild increase in the avalanche danger associated with the weather event by this evening.

NWS graphic

Observations
Recent Observations for Turnagain Pass
Date Region Location
11/16/23 Turnagain Observation: Tincan Common
11/14/23 Turnagain Observation: Sunburst/Magnum
11/14/23 Turnagain Observation: Seattle Ridge
11/13/23 Turnagain Avalanche: Tincan Trees
11/12/23 Turnagain Observation: Sunburst
11/12/23 Turnagain Avalanche: Goldpan – avalanche
11/11/23 Turnagain Observation: Tincan Common
11/11/23 Turnagain Observation: Taylor Pass – Sunburst
11/10/23 Turnagain Observation: Tincan
11/10/23 Turnagain Observation: Tincan Trees
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This is a general backcountry avalanche advisory issued for Turnagain Arm with Turnagain Pass as the core advisory area. This advisory does not apply to highways, railroads or operating ski areas.