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Archives
ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, December 19th, 2012 - 7:00AM
Expires
Thu, December 20th, 2012 - 7:00AM
Forecaster
Kevin Wright
Avalanche risk The Bottom Line

ANNOUNCEMENTS
Join Wild Alpine and the Friends of the CNFAIC for the Girdwood Premier of TGR’s new film “The Dream Factory” – with scenes shot at Alyeska and the surrounding Chugach Mountains.  This is a fundraiser for the CNFAIC  and takes place at the Sitzmark Bar & Grill – Alyeska Resort. More details  HERE.

BOTTOM LINE

A Moderate avalanche danger continues in areas above treeline due to poor snow structure.   The old weak layers from November are buried under 1-2 feet of stronger and heavier snow from December, making for a relatively unstable combination.   Below treeline, Low avalanche danger dominates areas that have little influence from wind.  

Wed, December 19th, 2012
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
  • Persistent Slabs
    Persistent Slabs
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.
More info at Avalanche.org

Well developed facets, buried surface hoar, even depth hoar is lingering in the snowpack across our region.  Only major precipitation or melting is going to make this problem disappear…  In other words, we will be talking about this problem for some time to come.  Signs of this problem are still easily found, including – collapsing, shooting cracks, and propagation in snowpit tests.  

In practice, the best way to deal with persistent weak layers is to tone down your terrain choices.  As time passes since the last snowfall, the likelihood of triggering avalanches diminishes.  Steeper areas may never become fully trustworthy without a big change in the weather pattern.  For now, choosing moderate terrain is the only sure way to stay away from the avalanche problem. 

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

Watch for wind slab in areas above treeline.  Recently deposited, stiffer snow is going to be the most likely to trigger and step down to the older weak layers.  Pay attention to the direction of recent wind loading (likely from the east, loading the west aspects.)

Weather
Wed, December 19th, 2012

No major changes are expected in the near future.   Today, decreasing temperatures and slightly increased wind is predicted.   Clear and cold weather will continue over the next several days.   A strong pressure gradient will produce gap winds across some of the major passes.   The next hope for snow is listed in the extended forecast sometime next week.

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).  

Our newest forecaster, John Fitzgerald will issue the next advisory Thursday morning, December 20th.   Welcome Fitz!

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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.
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