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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, December 15th, 2012 - 7:00AM
Expires
Sun, December 16th, 2012 - 7:00AM
Forecaster
Kevin Wright
Avalanche risk The Bottom Line

ANNOUNCEMENT

Turnagain Pass, Johnson Pass and riding areas in the Seward District of Chugach National Forest will be OPEN to motorized use today.   Placer river, Skookum valley, and 20 mile remain CLOSED due to shallow snow cover.  

BOTTOM LINE

Most areas in the backcountry are showing a MODERATE danger rating, with wind loaded slopes above treeline on the dangerous end of the MODERATE spectrum.   Small avalanches will be possible in specific areas, large avalanches may be triggered in large terrain.

Natural avalanche activity as recently as 2 days ago combined with a low amount of backcountry skiing activity (slope testing) is giving us significant uncertainty in evaluating the danger rating.   What we do know – recent natural avalanches caused by wind, very poor snow structure with multiple weak layers, 2 snowfall events in the last week, a concensus in the avalanche community that our current snowpack is bad news.   Some areas are more likely to find avalanches including east facing slopes and north and south crossloaded gullys and anywhere with recent windslab on top of the facets and buried surface hoar.  

Sat, December 15th, 2012
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. 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

Recent wind slab from Thursday is the primary concern.  The worst areas will have a stiff layer of surface snow on top of very weak facets.  This means above treeline in wind loaded zones including east facing slopes or pockets of wind slab in specific cross loaded terrain.  The most obvious examples we have of this are the aspects easily visible from the highway of Seattle ridge in Turnagain Pass and Fresno ridge near Summit Lake.  Both of these areas had large natural avalanches caused by wind loading.  As time goes on, the likelihood of triggering similar slopes is diminishing, but the rate of stabilizing is slow due to the persistent nature of our weak layers. 

There is no doubt in my mind that avalanches can be triggered today in the right terrain.  Watch out for slopes steeper than 35 degrees and signs of recent wind loading.  Avalanche size and destructive potential will be directly correlated to the size and consequences of the terrain. 

Avalanche Problem 2
  • 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

The only change in the persistant weak layers in the last week is they are now buried by 1-2 feet of new snow.  Which means that we now have heavier snow sitting on the extremely weak snow from the early season.  When you add the wind loading component to the mix, it becomes a dangerous recipe.  Yesterday we found frequent whumphing (collapsing) of the weak layers, telling us that they are tenuously carrying the stress of recent snowfall.  That collapse would be the initiation of an avalanche if the slope was steep enough…

It’s good to remember that we don’t often deal with weak layers on the scale we currently see them.  Our typical strong maritime snowpack has been replaced by a weak and shallow continental snowpack you would more often find in Colorado.  This means that problems don’t diminish for days or weeks following a storm.  The only way to ensure safe mountain travel when you find abnormally dangerous conditions is to stay in mellower terrain.  We can’t go the same places we could in our normal stable snowpack and expect a positive outcome.

Check out this video from our colleagues in Utah for some good theory on terrain management.

Weather
Sat, December 15th, 2012

Mostly sunny this weekend with colder temperatures.   We may see some stiff northwest wind in the mountains today.   The next chance of snowfall is still a few days away.  

The temperature inversion that gave us comfortable temperatures above treeline yesterday seems to be breaking down this morning.   The coldest areas are showing lower single digit temperatures.  


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).  

Wendy will issue the next advisory Sunday morning, December 16th.

Observations
Recent Observations for Turnagain Pass
Date Region Location
02/19/24 Turnagain Avalanche: Base of Seattle Ridge
02/18/24 Turnagain Observation: Lynx creek
02/18/24 Turnagain Observation: Tincan Trees
02/17/24 Turnagain Observation: Turnagain (below the uptrack)
02/15/24 Turnagain Observation: Sunburst
02/13/24 Turnagain Observation: Tincan Backdoor, Center Ridge
02/12/24 Turnagain Avalanche: Tincan Trees
02/11/24 Turnagain Observation: Cornbiscuit
02/10/24 Turnagain Observation: Tincan
02/04/24 Turnagain Observation: Eddie’s
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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.