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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, March 22nd, 2014 - 7:00AM
Expires
Sun, March 23rd, 2014 - 7:00AM
Forecaster
Kevin Wright
Avalanche risk The Bottom Line

Confidence in the overall stability of our snowpack is increasing each day.  It has been a full week since the end of the last storm.  Since then we saw a typical rash of avalanches immediately following the storm.  The last few days people have really started skiing on steep terrain, without triggering the deep slab that has concerned us.      

Overall, we are starting to feel pretty good about our stability, but a twinge of doubt for the possibiltity of triggering a deep slab in steep complex terrain is preventing a low danger rating for now.  Our current stability is good, but it’s not as good as it can get.
 
A MODERATE danger can be found in some areas above treeline in steep terrain >35 degrees for deep persistent slab and wind slab.  Everywhere else the danger is  LOW.
Special Announcements

Come join CNFAIC forecasters this Sunday for a  FREE AVALANCHE RESCUE WORKSHOP  at Turnagain Pass!   Focus will be on beacon practice through avalanche rescue scenarios.   We’ll meet in the motorized lot at 10:30am and wrap up by 1:30pm.   Skiers and sledders welcome!   Please drop a note to  kevin@chugachavalanche.org  to reserve a spot for you and your crew, as space is limited.

Sat, March 22nd, 2014
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

This problem is increasingly becoming unlikely, but the hazard associated with it could be dangerous if triggered.  The storm snow from last week has now settled to about 3+ feet of hard consolidated snow.  This slab is strong and difficult to trigger, but it sits at an interface that sometimes shows poor bonding to the older layer below it.  

The deep slab is most likely to be triggered from a shallow area where the slab tapers down to 1 foot deep or less.  Alternatively a large trigger like multiple skiers or a snowmachine could initiate through a deeper portion of the slab.  

Photo – Typical steep terrain with deep slab avalanches evident from the storm and large cornice features.

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

We got a report of a small wind slab triggered in Pastoral a couple days ago.  Click here for observation.  An east wind was loading up westerly aspects.  Today it is possible to find stiff aging wind slab that could pop when traveled on.  Keep this in mind, especially in steeper terrain.

Additional Concern
  • Cornice
    Cornice
Cornice
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
More info at Avalanche.org

Cornices are big and mature this time of year.  As people get onto steeper terrain the cornice factor becomes more significant.  In some steep areas they are a mandatory nuisance to be negotiated.

Remember to scope out a safe entrance that avoids a blind approach onto overhanging cornice features.  Take a rope for a safe belay if you think your chosen line can’t avoid cornice exposure.  

Weather
Sat, March 22nd, 2014

The last storm ended more than a week ago.  High pressure and sunny skies have dominated the weather pattern since then.  We had some moderate wind on Thursday that blew some snow around.

We are looking at another calm sunny day ahead of us.  Temperatures will start out cold this morning, ~10 degrees at valley floor and ~20 degrees at the ridgetops.  The sun will quickly warm things up to the high 30s in the afternoon.  East wind to 10mph.

Sun will stay in the forecast into mid week.  No major storms are on the horizon as the Rex block continues to dominate weather over Alaska.

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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.