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ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, December 12th, 2012 - 7:00AM
Expires
Thu, December 13th, 2012 - 7:00AM
Forecaster
Kevin Wright
Avalanche risk The Bottom Line

A modest amount of new snow today will raise the danger rating to CONSIDERABLE for storm snow over the old persistent weak layers.   Snowfall amounts and wind through the day will determine how much the avalanche danger rises.   The tender wind slabs we saw over the weekend are likely to become active again today during the storm as more stress gets added to the snowpack.  

Wed, December 12th, 2012
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

Old wind slabs built last Saturday by redistribution of the snow during high wind are probably the most dangerous specific concern today.  A fresh blanket of snow will make it more difficult to identify those areas that are stiffer and deeper.  New storm snow today will add to the stress and make them more likely to be triggered by a skier.  This problem is generally confined to above treeline on specific wind loaded features steeper than 35 degrees.  Fresh wind slabs from today’s event are also likely on north and west facing slopes.  Watch the snow build today and make decisions accordingly.

Avalanche Problem 2
  • Storm Slabs
    Storm Slabs
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). 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.
More info at Avalanche.org

This storm doesn’t look very intense by our standards.  However, any new weight on the weak layers from November will be cause for concern.  Above and below treeline, watch for standard red flags (fresh avalanche activity, shooting cracks, whoomphing sounds, wind loading, new snowfall…).  In less wind affected areas we may be dealing with more of a sluff issue than slabs, but due to bad persistent weak layers it’s still worth keeping your guard up.

Weather
Wed, December 12th, 2012

NWS predictions are for 5-7 inches of snow today with SE wind gusting to 30mph at the ridges. This is similar to the expected snow amount in Anchorage today.  

This storm is hitting us with a Western flow (source of the storm is in the Bering Sea), which usually doesn’t give Girdwood and Turnagain Pass as much snow as Anchorage and Hatcher Pass.   We should expect some snow, but not a lot.   As of this morning snow has begun, with a few inches in Girdwood already.   Intensity should drop by noon today with snow showers continuing through this evening.   Temperatures are expected to keep precipitation below freezing in the mountains through this storm.


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).  

Graham will issue the next advisory Thursday morning, December 13th.

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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.