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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, December 22nd, 2012 - 7:00AM
Expires
Sun, December 23rd, 2012 - 7:00AM
Forecaster
Kevin Wright
Avalanche risk The Bottom Line

A MODERATE danger continues above treeline, on upper mid slopes in areas with stronger surface snow.   A LOW danger rating describes the areas below treeline where nearly all the snowpack consists of loose weak layers.   We are unlikely to go to Low above treeline while we still find periodic collapsing and pit tests like the ones in the video here.

Sat, December 22nd, 2012
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
1 - Low
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
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

A shallow persistent slab is causing the greatest problems in the snowpack.  Looking through the observations photo gallery, a clear pattern emerges among the recent natural and human triggered avalanches over the last week.  We aren’t finding those avalanches all the way at the ridge tops, but rather on wind loaded mid slopes below the ridges.  This pattern directly helps us make some travel decisions by showing us the specific terrain to avoid this weekend. 

An observation from yesterday in Moose Pass showed clear red flags of collapsing (whumpfing).  This underscores the problem of a stronger slab overlying the persistent weak layers, and tells us that avalanches are still possible to trigger on steeper slopes that hold the right combination of strong snow over weak snow. 

Overall, we can assume that the likelihood of finding avalanches is decreasing slowly over time since the last snowfall.  The size potential may be decreasing as well.  Due to the persistence of the weak layers (they don’t get stronger easily or quickly) and the cold temperatures we’ve had recently, the trend is nearly stabilized.  This means that current problems aren’t getting better or worse, but are likely to stay at the same danger until we get a change in the weather.  Distribution is widespread on all aspects, but most common on upper mid slopes with wind loading. 

Weather
Sat, December 22nd, 2012

It’s been a cold week, but on the positive note we gain 11 seconds of daylight today!   A chance of snowshowers is possible for the eastern Turnagain Arm, but don’t get your hopes up for much snow.   A weakening high pressure across Alaska is still keeping the moisture stream to the south, giving snow to everybody in the lower 48 and Canada…

Expect cold temperatures to continue today.   A strong temperature inversion can be found on the Kenai, with some low spots reading in the -20s.   Ridge tops are balmy by comparison in the low teens.   Wind is a little breezy up high, but probably not strong enough to move our old snow or contribute to avalanche danger.


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 23rd.

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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.