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Fri, March 22nd, 2013 - 7:00AM
Sat, March 23rd, 2013 - 7:00AM
John Fitzgerald
Avalanche risk The Bottom Line

The avalanche hazard will increase to MODERATE this afternoon as new snow and wind build slabs above treeline.   These slabs will be sensitive to human triggers, especially in steep terrain. Below treeline the hazard will remain LOW where snowmachine and skier triggered avalanches are unlikely today.

Fri, March 22nd, 2013
Above 2,500'
2 - Moderate
Avalanche risk
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

New Wind Slabs

New snow and wind will work together to form wind slabs in the mountains today.  Do you remember what the snow surface looked like yesterday?  I do.  On south aspects (southern half of the compass) the surface was very firm and crusty.  Some areas on south facing slopes were holding a thin layer of loose snow. On north aspects (northern half of the compass) the surface snow was a wild mix of loose snow, firm crusts and slabs.  The overall surface pattern is what we call a potentially poor interface; faceted snow & crusts will behave as weak layers & bed surfaces respectively for avalanches today.  The question you will need to answer is: how big is this new slab?  The bigger the slab, the bigger your problem will be.  If precip totals trend toward the higher end, expect to see pockets of wind slab in the 12″ range today.

Old Wind Slabs
Yesterday my partners and I encountered older wind slabs that were formed by high winds earlier this week (Tuesday night).  Several parties have also reported triggering wind slabs up to 18″ in depth in the past two days.  These older slabs will be harder to trigger today but are worth avoiding, especially in steep terrain.

Avalanche Problem 2
  • 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

The most recent cornice fall observed in the region occurred in the last 2 days.  These backountry bombs have been dropping sporadically on their own over the past several weeks and have been large enough to do significant damage to a group of people.  New snow and wind will help to increase the sensitivity of aging cornices.  Avoidance is the only way to truly “manage” this type of problem.  Approach ridge crests with caution and don’t move out onto a ridge until you can see the snow below you.  Limiting your time below cornices will also help in avoiding this problem.

Fri, March 22nd, 2013

In the past 24 hours the mountains around Eastern Turnagain Arm have seen temperatures average 11 degrees F at the Seattle Ridge weather station (2,400′).   Winds at this site have averaged 12 mph out of the SE with gusts to 31 mph.   Precipitation has just begun this morning and stations are just beginning to report snow falling.

Today expect snowfall amounts to be in the 4″ to 8″ range, with greater amounts in the Girdwood, 20 mile and Placer Valleys.   Temperatures at 1,000′ will be in the mid twenties F and ridgetop winds will average 25mph out of the SW.

The extended outlook calls for an active weather pattern to bring more snowfall to the area through the weekend.


Wendy will issue the next advisory tomorrow morning, March 23rd.

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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.