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ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, January 15th, 2013 - 7:00AM
Expires
Wed, January 16th, 2013 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

We have a CONSIDERABLE avalanche danger above treeline today where deep and deadly avalanches can be triggered by a person on slopes steeper than 35 degrees. Several large avalanches were triggered yesterday and Sunday with one very large slide in the Turnagain Pass region. This snowpack is still adjusting to the load placed on it over the weekend and demands respect. Expert level travel skills and very conservative terrain management is required for a safe day in the backcountry.

Special Announcements

Alaska DOT will be conducting avalanche hazard reduction work today Tuesday Jan. 15 2013 9:00am-10:00am between Girdwood and Portage.   There will be traffic delays. Updates posted at  511.alaska.gov.

 

Tue, January 15th, 2013
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. 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
  • 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

The deep slab instability reared its head the past couple days in the Eastern Turnagain Arm area. This was at the tail end of a two day storm that brought warm temperatures and 2-5″ of rain below treeline with 20-40+” of snow above treeline. This was a ripe set up for big slides as it all fell on top of a snowpack harboring persistent weak layers near the ground. There are several large avalanches to report – both natural and artillery/explosive triggered. Most of these occurred along the Seward Highway between Bird and Girdwood as the Alaska DOT conducted avalanche hazard reduction work (another video here). Also, Alyeska Ski Patrol was able to pull out one very large deep slab on the North Face yesterday. For natural activity, a large natural avalanche deposited a powder cloud and a small amount of debris on the Seward Highway near MP 94 while the road was open to traffic. The majority of the debris stopped above the highway elevation.  Dot crews responded immediately to reduce the avalanche hazard near this location. There were two additional large natural avalanches – one on Mount Alpenglow and another in the Lynx Creek drainage (Turnagain Pass zone).

The very large Lynx Creek slide (pictured below) is most concerning for backcounty recreationalists as it points to unsurvivable deep slab avalanches near the tipping point in our core forecast zone. 

The message for today is simple: Patience while the snowpack adjusts to the rapid changes it received over the weekend is vital. Keeping off slopes steeper than 35 degrees, especially in the larger terrain, and out of runnout zones will be key. The powder will likely be enticing at the higher elevations but the consequences will be high for anyone pushing the steeper slope angles.

Temperatures have cooled off overnight and this will help to stabilize the snowpack, more so below treeline where the pack was mostly wet yesterday. Crusty surface conditions can be expected in these below treeline areas with dryer snow above. The weak faceted snow near the ground, responsible for our deep slab problem, will be adjusting at a much slower rate however and our recommended travel on lower angle slopes will persist.

Avalanche Problem 2
  • 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

Both fresh wind slabs forming today as well as some lingering slabs from the storm will be good to watch for. These will be above treeline in the upper elevations where dry snow fell despite the warm storm. The new and older slabs will likely be on different aspects as the wind has shifted from the east to the west overnight.  Watch for fresh slabs today in areas that did not load during the storm. These west winds could also overload the deeper weak layers in areas that were not stressed with wind loading prior. Add to that, any wind slab triggered has the potential to step down and trigger a large deeper slab with unfavorable results. These are just a couple more reasons to keep terrain choices conservative out there..

Weather
Tue, January 15th, 2013

The warm river of moisture over us the past few days has moved east overnight. Skies have cleared, temperatures dropped and winds have shifted around to the west where they are bringing in colder air. There was only a trace of rain on Turnagain Pass yesterday morning but the day remained quite soggy. Girdwood Valley picked up .2 € water and ~2 € of snow above treeline in the morning. Storm totals between Saturday morning and Monday morning were 2″ rain (~14″ snow) on Turnagain Pass and 5″ rain (35+” snow) in the Girdwood Valley. Upper elevations had higher snow amounts.

Today, expect winds to pick up from the west to the 20-30mph range on the ridgetops and temperatures should continue to drop to the mid- teens on the ridges and low 20’s on the Pass at 1,000′. Skies should be partly cloudy with some high clouds and fog in areas.

Tomorrow a weak system moves through bringing some cloud cover, warming temperatures and slight chance for a few inches. Thursday and Friday it looks like cold and mostly clear conditions set in for a change from our onslaught of storms since Christmas.


Kevin will issue the next advisory tomorrow morning, January 16th.

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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.