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Issued
Thu, January 17th, 2013 - 7:00AM
Expires
Fri, January 18th, 2013 - 7:00AM
Forecaster
Graham Predeger
Avalanche risk The Bottom Line

Above tree line elevations the avalanche danger remains MODERATE today.   The more time we can put between now and this most recent bout of wet storms the less likely it will be to trigger an avalanche.   However, large avalanches are still possible in isolated areas given the widespread buried persistent weak layers present in our pack.   Below tree line the danger is LOW.

Thu, January 17th, 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
  • 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 issue continues to be our primary concern today in the backcountry and won’t be going away anytime soon. Terrain management becomes particularly important when dealing with persistent weak layers and also happens to be the toughest skill to master for mountain professionals the World over.  Today and for the rest of the season it will be prudent to expose only one person at a time to avalanche terrain and don’t park or hang out in run out zones.

Though the likelihood of triggering a deep slab avalanche is decreasing with time, overlying slabs are still showing the potential to propagate if a fracture is initiated.  Likely areas to initiate a fracture may be near trigger points such as rocks or trees mid-slope where the slab is shallow or interrupted.  We’ve seen numerous examples since the New Year of the destructive potential these large avalanches can harbor, and in the case of Repeat Offender last week, that particular avalanche was triggered remotely from a shallow trigger point.

The best surface conditions for skiing and riding today will be found above ~1500’ in areas where the snow is deep and consistent.  Below 1500’ the surface consists of a few inches of new snow over a stiff crust capable of supporting a snowmachiner.  This crust, formed by Sunday’s wet storm and subsequent cold temperatures could prove a future weak layer at lower elevations once overlain by a slab.

Weather
Thu, January 17th, 2013

Looking at snowfall totals around south central, Anchorage and Hatcher Pass (~12″) both appear to be eeking out a bit more snow than the Turnagain pass/ Girdwood region due to the up-sloping nature of this latest front.

Today we can expect light winds from the north and west with snow tapering off this afternoon (24 hour accumulation of less than 6″).   Temperatures will stay below freezing at all elevations as skies clear throughout the day. Looking out toward the weekend it appears we may be back into an active pattern of weather.

Fitz will issue the next advisory tomorrow morning, January 18th.

Observations
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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.