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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, March 17th, 2014 - 7:00AM
Expires
Tue, March 18th, 2014 - 7:00AM
Forecaster
John Fitzgerald
Avalanche risk The Bottom Line

The avalanche danger is MODERATE on slopes under 35 degrees, and most terrain below treeline.  

Above treeline, in wind loaded areas and on steep upper elevation slopes the avalanche danger is CONSIDERABLE.

The likelihood of triggering fresh wind slabs 12-18 € in depth is on the high end of the scale today.

The likelihood of triggering a dangerous deep slab avalanche up to 5′ in depth is much lower.   However, the consequences of triggering a deep slab are high due to their potential size and volume.

An enjoyable day can be had on slopes under 35 degrees, away from the runout of large paths and by avoiding likely trigger points.

Mon, March 17th, 2014
Alpine
Above 2,500'
3 - Considerable
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. 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

A modest amount of new snow overnight and continuing into today will be easily blown around by the wind.  The most likely avalanche problem to encounter today will be freshly formed wind slabs on leeward slopes, in starting zones and in cross loaded gullies.  Slab depths will be in the 12-18” range.  

General wind direction will be out of the East and in the 30 to 40 mph range on ridge tops.  Keep in mind that wind direction will vary from place to place.  Rather than focus on the compass, it is more important to be able to recognize wind slabs by their look and feel.  They will appear rounded, smooth and pillowy.  They will feel “upside down” and hollow.  Shooting cracks, an obvious sign of unstable snow, commonly occur on newly formed wind slabs.

Avoiding terrain where wind slabs are present will be the best way to manage this problem today.

Avalanche Problem 2
  • Deep Persistent Slabs
    Deep Persistent Slabs
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.
More info at Avalanche.org

A layer of weak snow buried 2-5 feet deep is still taking time to adjust to the load it received last week (5’ snow/5” water).  This layer was the likely culprit in numerous large natural and human triggered avalanches over the weekend.  While time is helping to heal this wound, the destructive potential of an avalanche at this layer is significant.  This newest slab, a combination of the March 10th and March 14th storms, is thick enough in most places that it is difficult to impact the layer in question.  Because of this it will be possible to travel on steep terrain without any of the obvious signs, including avalanches, to show themselves.  It will also be possible for a slope to have a plethora of tracks on it before releasing.  Add in the fact that we have had avalanches triggered remotely and this problem becomes even more complex.

Check our observations page for recent avalanche activity and a powerful VIDEO of a snowmachine triggered deep slab avalanche just outside of the forecast area.

Rather than trying to outsmart this instability, it is better to negotiate this problem by avoiding steep slopes and likely trigger points altogether.  Likely trigger points include shallow spots in the snowpack and steep rollovers.

Weather
Mon, March 17th, 2014

In the past 24 hours the mountains around Eastern Turnagain Arm have picked up 4-6 € of new snow with .3 € of water.   Ridgetop winds have averaged 15mph out of the East with a max gust of 47mph.   Temperatures have averaged 10 degrees F and are currently in the teens F.

Today expect lingering snow showers with another 2-3 € of snow possible.   Temperatures at 1,000′ will warm into the high 20s/low 30s F.   Ridgetop winds will be out of the East in the 30 to 40 mph range.

The extended outlook is calling for an unsettled pattern to bring light snow amounts through Wednesday.   A clearing trend looks to develop later in the week.

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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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