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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, March 28th, 2013 - 7:00AM
Expires
Fri, March 29th, 2013 - 7:00AM
Forecaster
Graham Predeger
Avalanche risk The Bottom Line

A MODERATE danger rating exists again today above treeline where shallow slabs yesterday proved quite easy to trigger on a variety of bed surfaces.   Expect these slabs to build in thickness today, as light density snow from Monday’s storm will be easily transportable under SE winds forecasted to be in the 30 to 45mph range.   If snowfall amounts exceed 6 € during the day, with forecasted winds the avalanche danger will very likely increase to CONSIDERABLE by days end.  

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Thu, March 28th, 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
  • 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

In the Turnagain pass zone yesterday my partner and I found shallow soft wind slabs (6-10” deep) that were easy to trigger in terrain greater than ~38 degrees.  This problem was shown to be quite manageable yesterday as these slabs were all small and relatively predictable.  Today is going to be an entirely different ball game as SE winds and new snow will build these slabs to ‘unmanageable’.  Slabs are sitting on a variety of crusts and stiff wind board on south, east and west aspects; all exhibit attributes of a slick bed surfaces.  To top it off, there do still exist pockets of facets acting as a weak layer from our prolonged mid-March sunny spell.  It’ll be wise to pay particular attention to leeward facing slopes and cross-loaded gullies, as this is where you’ll find deeper, wind loaded pockets ripe for a trigger today.  Pay attention to indicator slopes and/ or try and test the snow on smaller test slopes prior to committing to bigger or steeper terrain.

                              Shallow, easily triggerable wind slab yesterday.

 

Avalanche Problem 2
  • 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

With plenty of snow available for transport, moderate winds today and increasing temperatures on the horizon I think cornices warrant a brief discussion.  It’s been several weeks since we’ve seen a cornice fall so what this tells me is that they are continuing to grow and ripen as we near April 1st.  Cornices are nearly impossible to forecast for so what we can do is mitigate this hazard by reducing time spent below a cornices and travel well back from the roof of a cornice.  There are some enormous cornices in the backcountry right now and inevitably many of them will fail in the last month or so of our season.  Play it safe and avoid these backcountry bombs!

Weather
Thu, March 28th, 2013

Temperatures warmed substantially yesterday from single digits to the low 20’s at 1000′ as we watched the approach of high clouds stream into south-central.   Southeast winds in the 30-45 mph range look to be the biggest game changer today as a warm front advances through the Gulf of Alaska and into our region.    Snowfall amounts throughout the day are expected to be in the 2-6 € range with temperatures in the low 30’s, meaning mostly snow at sea level.   Overnight, winds and precip are expected to increase with the potential for another foot of snow by Friday morning.  


Fitz will issue the next advisory tomorrow morning, March 29th.

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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.