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Issued
Fri, February 8th, 2013 - 7:00AM
Expires
Sat, February 9th, 2013 - 7:00AM
Forecaster
Kevin Wright
Avalanche risk The Bottom Line

6-12 inches of new snow yesterday combined with strong wind created wind slabs and lingering pockets of CONSIDERABLE danger above treeline.   The danger is on a decreasing trend throughout the day, but tender slabs will be likley in wind loaded areas at higher elevations.   Below treeline you may find occasional small and shallow pockets of wind slabs and a LOW to MODERATE danger rating.  

Fri, February 8th, 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. Careful snowpack evaluation, cautious route-finding, and conservative decision-making essential. 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
  • 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

By early afternoon yesterday we could get small soft slabs to pop easily with the influence of a skier just above treeline elevation.  If we had ventured higher we would have found larger triggerable pockets with enough mass to cause injuries or burial by the end of the day.  A couple of natural avalanches were observed farther south on the Kenai peninsula despite generally poor visibility.  The main concern today is these same wind loaded pockets which may still be triggerable by a person.  The snowfall stopped around 1000pm last night and wind has diminished significantly, allowing some time for yesterday’s new snow to bond and gain strength.  

 Today there will be plenty of safe places to enjoy in the backcountry.  Below treeline and areas with moderate slope angles below 35 degrees are a good bet for safe travel.  Travel above 2500 feet will require careful consideration of wind loading patterns and possibly avoidance of wind loaded areas.  At Turnagain Pass the primary wind direction was from the east, but other weather stations showed different patterns.  Determining which slopes have wind loaded pockets will require eyes on the ground.

Avalanche Problem 2
  • 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

With some areas receiving more than an inch of new water to the snowpack in the last 48 hours, I’m going to bring back the deep slab discussion.  We have not seen a deep slab avalanche in several weeks, however it remains a low probability high consequence concern.  The problem seems to have gone dormant but a couple factors could bring it back to life. 

1.  New snow adding stress to the snowpack will make it more likely to trigger the deep weak layers until the snowpack has adjusted to that new stress. 

2.  Finding a trigger point in shallow areas of the snowpack could be the influence that initiates a collapse that propagates to deeper areas. 

This problem is still in the back of my mind and is still having a subtle influence on my travel decisions in the backcountry.  Despite a period of good behavior, the deep slab is still guilty until proven innocent.

Weather
Fri, February 8th, 2013

Snowfall and wind totals from yesterday

Alyeska top –             10-12 inches     gusts to 63mph

Turnagain Pass – 6-8 inches     gusts to 78mph

Summit Lake –       4-6 inches   gusts to 33mph

Snowfall from Wednesday and Thursday was low density powder, making for good skiing.   You can still feel the hard crust at low elevations underneath the new snow, but snow quality has improved.

Today looks like a break of calm and partly sunny weather before another storm on Saturday.   Isolated snow showers are possible today.   Temperatures in the high 20s to low 30s, and little to no snow is forecasted until tonight.

For the weekend a high wind watch is in effect starting Saturday morning.   More snowfall is also expected.  


Fitz will issue the next advisory tomorrow morning, February 9th.

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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.