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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, January 24th, 2013 - 7:00AM
Expires
Fri, January 25th, 2013 - 7:00AM
Forecaster
Graham Predeger
Avalanche risk The Bottom Line

Fresh wind slabs above tree line coupled with our deep slab potential will keep the avalanche danger at MODERATE today.   Below tree line the avalanche danger is trending toward LOW as we see temperatures and the rain/ snow line descend throughout the day.

Thu, January 24th, 2013
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
1 - Low
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
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

Snowfall and winds above tree line were not exceptionally impressive yesterday and overnight but consistent enough to build tender, shallow wind slabs throughout the day.  Snow falling at ridge top locations was dense, though easily transportable by the moderate southeasterly winds.  Expect wind slabs to form on north and west aspects above tree line where small human triggered avalanches will be possible today.  Below the rain/ snow line where wind slabs have not formed there is a generally low avalanche danger.  Yesterday that level was about 1700’ in the Turnagain pass area.  We should see that dropping to around 1000’ today.

Cornice fall-

Cornices are growing quite large in the forecast area with one natural failure noted on Magnum last week that resulted in an avalanche.  The moderate winds and dense snow will continue to add weight and mass to these backcountry bombs.  It is wise to recognize cornices and give them a wide berth when travelling through the mountains.

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

As we continue to add weight to our snowpack we cannot forget about the deep slab issue we are dealing with this winter.  It’s unlikely a person will trigger a deep slab avalanche today however; either of the previous mentioned issues (wind slabs and cornice fall) can act as the catalyst to affect deeper weak layers resulting in a cataclysmic deep slap avalanche.

Weather
Thu, January 24th, 2013

Below about 1800′ we have continued to lose depth in our snowpack due to warm temperatures and precipitation falling mostly as rain the past several days.   Overnight, temperatures have begun a downward slide and we will see our rain/ snow line descend back to sea level by Friday evening.

Today we can expect 1-2 € of new snow with winds shifting from east to southwesterly, calming by this evening.   For the weekend, temperatures appear to be more of what we come to expect for January in Alaska in the 10 €“ 20 degree range.

_____________________________________________________

Fitz will issue the next advisory tomorrow morning, January 25th.

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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.