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Archives
ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, December 27th, 2012 - 7:00AM
Expires
Fri, December 28th, 2012 - 7:00AM
Forecaster
Graham Predeger
Avalanche risk The Bottom Line

The backcountry avalanche danger remains HIGH today.   The Turnagain pass region has seen over four feet of snow (4.3 € water equivalent) accumulate since Monday on what we know is a very weak foundation of faceted snow.   Based on pit results and yesterday’s field observations the snowpack is proving very unstable right now and an avalanche triggered today in steep terrain will likely be unmanageable for a skier or snowmachiner.   Backcountry travel is not recommended today.  

ANNOUNCEMENTS

There will be intermittent closures for avalanche hazard reduction work on Thursday December 27, 2012 on the Seward Highway between mileposts 44-20 (Summit Lake, Sterling Wye, and Kenai Lake).   Motorists should expect delays of up to 45 minutes between 8:30 AM and 5:00 PM.

Updates will be posted on the 511 system.   http://511.alaska.gov/

Thu, December 27th, 2012
Alpine
Above 2,500'
4 - High
Avalanche risk
Treeline
1,000'-2,500'
4 - High
Avalanche risk
Below Treeline
Below 1,000'
4 - High
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
  • Storm Slabs
    Storm Slabs
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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

New storm snow proves to be a valid concern today after several observations this week of human triggered avalanches and some downright spooky test results from our snow pits yesterday, all from below tree line.  As we have watched our seasonal snowpack more than double in a few short days, the weak layers we’ve been talking about for the last month finally have a 2-5 foot slab sitting on top of them.  This is the piece of the avalanche puzzle that has been missing until this week!  The new slab sits on several different weak layers, any of which may fail with the weight of a person causing an avalanche that will prove unmanageable in steep terrain.  We are in a direct-action avalanche situation and triggering an avalanche should be of no surprise today based on what we know about our weak foundation and overlying storm slab.  The old mantra to stay out of the mountains for at least 24 hours during and after a storm rings loud and true this week!

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

Easterly winds looked to have picked up overnight with the approach of this latest low-pressure system.  Even though we have limited data from upper elevations (due to poor visibility and dangerous avalanche conditions) we can safely say that tender wind slabs are forming and most likely avalanching naturally in our core advisory area.  Human triggered avalanches will be very likely in terrain above tree line today.  AKRR and DOT crews were able to affect wind slabs with artillery above Turnagain arm yesterday.  Shots were yielding consistent results producing avalanches big enough to destroy a small building or break trees.  We can expect wind slabs to continue to build today in our upper elevations.  If these don’t avalanche naturally, only time will help to heal these slabs. 

Weather
Thu, December 27th, 2012

Well the snow continues to pile up this week as the pattern change we were all hoping for did not disappoint!   Turnagain pass saw an additional 8-10 € of snow yesterday, turning to a rain-snow mix at 1000′ overnight.   Winds looked to have peaked from the east in the low 80’s last night.  

Today looks to be more of the same with another 3-6 € of snow expected and moderate winds from the east.   The rain/ snow line will be hovering somewhere between sea level and 1000 feet as this latest low begins to dissipate today.

Looking out toward the weekend there appears to be more unsettled weather in our future!

Total snow accumulation beginning early morning on the 24th:

Girdwood Valley €“ ~50 € snow with 5 € of water equivalent
Turnagain Pass €“ ~45 € snow with 4.3 € of water equivalent
Summit lake €“ ~12 € snow with 1.1 € water equivalent


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).

Fitz will issue the next advisory Friday morning December 28th.

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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.