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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, March 23rd, 2015 - 7:00AM
Expires
Tue, March 24th, 2015 - 7:00AM
Forecaster
John Fitzgerald
Avalanche risk The Bottom Line

The danger is MODERATE in the Alpine today, where the possibility exists for pockets of slab 2-3′ thick to be triggered on steep slopes.   While the likelihood of triggering is trending towards the low end of the scale, the consequences are high enough to injure or bury a person.   Wet loose avalanches will be higher on the likelihood scale and lower on the consequence scale, as volume will be generally low but increase on steep sustained slopes.   Cornices will feel the heat of the sun today and could release naturally.

The danger is LOW this morning at Treeline.   Daytime heating will increase the chances for shallow wet loose avalanches and nudge the danger to MODERATE in steep terrain over 40 degrees.

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Mon, March 23rd, 2015
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
0 - No Rating
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
  • Persistent Slabs
    Persistent Slabs
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.
More info at Avalanche.org

It has now been 5 days since the last loading event occurred.  In that time we have seen a daily cycle of melting and freezing on sunlit aspects and generally mild temps.  This pattern has helped 2-3’ slabs that formed last week to become less reactive.  In some areas these slabs are resting on a thin layer of weak faceted snow.  In areas where we have been able to find this combo, there still exists the possibility of triggering one of these slabs.  This problem is variable across the landscape, as it is not something you will find on every slope.  Digging in the snow can help you to understand the problem better, but could also be misleading.  

How do we manage this uncertainty?  By using effective terrain management techniques.  Only expose one person at a time on suspect slopes, use islands of safety for spotting and re grouping, and identity escape routes.

Recent slab avalanche at 3,500′ W aspect.  Debris pile visible left of center coming out of the shadow. 

Slab at head of Ingram Cr

Avalanche Problem 2
  • 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 more likely avalanche concern today will be wet loose avalanches.  These will be generally low in volume and are easy to anticipate.  Volume will increase on steep sustained slopes.  As the days get longer, so does the amount of terrain that the sun affects increase.  Steep slopes being impacted by direct sunshine will continue to shed shallow loose snow avalanches.  Several natural wet loose avalanches that triggered shallow wet slabs were observed in the Girdwood Valley yesterday.  More of the same is possible today.

Pay attention to the snow surface on sunlit aspects today.  As your skis or board begin sinking more than several inches it will be important to begin dialing back slope angles.

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 grew significantly with last week’s storms.  Direct sun and a lack of wind will help to weaken these features.  As always give cornices a wide berth.  Minimize time spent underneath and make sure you can see the profile before approaching any corniced ridges.

Weather
Mon, March 23rd, 2015

Yesterday brought abundant sunshine under clear skies.   Temps were mild and winds were generally calm.   No new precipitation was recorded.

Today looks to be a carbon copy of yesterday, with slightly warmer temps.   Temperatures will reach into the low 40s F at 1,000′ and near 32 F along ridgetops.   Winds will be calm and skies will be clear.

High pressure stretching from Southeast Alaska through Southcentral and into the interior will prevent a low pressure system centered over Bristol Bay from impacting the area.   This pattern looks to break down by mid week as we return to warm and wet Southerly flow.

PRECIPITATION 24-hour data (6am – 6am)

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 33 0 0 55
Summit Lake (1400′) 30 0 0 12
Alyeska Mid (1700′) 35 0 0 32

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 27 var 4 14
Seattle Ridge (2400′) 29 var 4 15
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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.