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ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, April 5th, 2013 - 7:00AM
Expires
Sat, April 6th, 2013 - 7:00AM
Forecaster
Kevin Wright
Avalanche risk The Bottom Line

Sunny skies and warm temperatures this afternoon will again bring a late day increase in avalanche danger.  This morning will start with a  LOW avalanche danger, increasing to  MODERATE by the late afternoon.  

South facing slopes with the most solar exposure during the day will be the greatest concern.  Shaded north aspects are a better bet for finding quality snow.

Fri, April 5th, 2013
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
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. Dangerous avalanche conditions. 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
  • Wet Loose
    Wet Loose
Wet Loose
Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
More info at Avalanche.org

The wet avalanche problem was worse just a few days ago.  At this point the cold freezing overnight is creating a stiff crust on the snow surface that takes time for the solar radiation to melt and penetrate into deeper layers.  Roller balls and other signs of instability at the surface show up more quickly in powder snow than in our current crust.  That being said, sunny skies and warm temperatures can be the trigger for avalanches this afternoon.  Travel decisions should be adjusted accordingly to limit exposure to south facing slopes late in the day.  

With freezing overnight, the temperatures in the deeper snowpack remain relatively cold.  We are not yet reaching the isothermal status where larger and deeper avalanches are expected.

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

These are pictures of the Goat couloir avalanche from Tuesday, a great example of the worst case scenario right now.  This appears to be a very large cornice failure, which triggered a slab below and to the side of the cornice.  The resulting avalanche traveled far for this path.  

Cornices will become less stable in the afternoon as the sun heats them up and weakens their overhanging structure.

Weather
Fri, April 5th, 2013

Several days of clear skies and strong temperature swings between day and night are creating a melt/freeze cycle on the snow surface.  Temperatures last night dipped to the mid teens at 4000 feet elevation after reaching above freezing during the day.  At sea level peak daytime temperatures hit the low 40s in some areas.  Wind has been light, and we haven’t had any precipitation in several days.

Today’s weather looks like more of the same, but a change is expected tonight.  Snowfall is in the forecast for the weekend and into next week.


Fitz will issue the next advisory Saturday, April 6th.

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.