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Archives
ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, February 12th, 2013 - 7:00AM
Expires
Wed, February 13th, 2013 - 7:00AM
Forecaster
Graham Predeger
Avalanche risk The Bottom Line

A MODERATE avalanche hazard remains above treeline today where a skier or rider can still find reactive pockets of denser wind slab.   These slabs may be covered by 4-6 € of new, light density snow that is easy to sluff in steeper terrain.   Below treeline the hazard is generally LOW where wind affected snow is more difficult to find.   Today’s weather is unlikely to be a big contributor to our avalanche concerns, keeping the trend steady.

Tue, February 12th, 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

It will be unlikely that we’ll see any natural avalanches today though it will be possible for a skier or snowmachiner to trigger a wind slab formed during our last storm day (Saturday).  Pay attention to any red flags (recent avalanches, shooting cracks or collapsing in the snowpack) and continue to avoid cross-loaded gullies or steep wind-loaded terrain below ridges, as these are likely areas to trigger a wind slab. Stability tests from yesterday gave us some confidence in the Turnagain pass area that the new storm snow from over the weekend is bonding well to the older snow surface.

However, in areas such as the Girdwood Valley that received significantly more snow than Turnagain this past weekend, wind slabs will be deeper and easier to trigger.  A report of a class 3 avalanche yesterday in the Girdwood valley showed significant propagation, likely on facets forming near a crust around 2700’. 

One can sluice out several of these different crusts (rain, rime and melt-freeze) in our mid to upper pack with a hasty snow pit or probe poke.  These distinct layers can act as a good bed surface or interface that a relatively shallow avalanche may step down to.  Crusts are widespread throughout the forecast area, proving reactive in the mid elevation band at and just above treeline.

 

 

 

 

 

 

 

 

 

 

 

 

 

CNFAIC forecaster John Fitzgerald “feeling out the crusts” (~2600’) in addition to measuring spatial variability (depth) across a slope.

Avalanche Problem 2
  • Dry Loose
    Dry Loose
Dry Loose
Dry Loose avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.
More info at Avalanche.org

Loose, low-density surface snow will not be gaining any strength today as temps stay cool.  Sluffs initiated in steep terrain greater than 35 degrees will have the tendency to run fast and far.  Expect it, manage your terrain accordingly (avoiding terrain traps) and this should not prove a significant issue. 

Cornice fall also warrants a brief discussion as we did find evidence of cornice failure on Seattle ridge yesterday.  Cornices have a very nasty habit of breaking farther back than you expect so it’s a good idea to travel well away from the face of a cornice as these monsters continue to ripen into mid-February.

Weather
Tue, February 12th, 2013

Snow tapered off quickly yesterday morning giving way to mostly clear skies, calm winds and dropping temperatures.  

2-5 € of snow and southeast winds in the 10-20 mph range are expected today under cloudy skies.   Temperatures have rebounded somewhat this morning from a low of 16 degrees at 1800′ yesterday and look to land in the comfortable range of the high 20’s at 1000′ today.   Snowfall is expected to become heavier this evening and overnight as a series of weak surface lows are lining up this week, keeping the weather active in south central Alaska for the near-term.

________________________________________________________

Kevin will issue the next advisory tomorrow morning February 13th.

Observations
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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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