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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sun, December 21st, 2014 - 7:00AM
Expires
Mon, December 22nd, 2014 - 7:00AM
Forecaster
John Fitzgerald
Avalanche risk The Bottom Line

The avalanche danger in the alpine (2,500-5,000′) is CONSIDERABLE.   The chances of triggering an avalanche above 2,500′ are trending towards the low end of the scale but the consequences of being caught remain high.   Slabs up to 3′ in depth could be triggered in steep terrain and propagate across entire slopes today.

The avalanche danger is MODERATE in the upper reaches of the treeline (2,000′-2,500′) elevations.   It will be possible to trigger slabs 1-2′ in depth in steep terrain over 40 degrees.   Avalanches triggered in the alpine have the potential to run down into this elevation band today.   Triggering an avalanche in the lower portion (1,000′-2,000′) of this elevation band is unlikely today and the avalanche danger is LOW.

Below 1,000′ there is not enough snow cover to issue a danger rating.

Sun, December 21st, 2014
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
0 - No Rating
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

It has now been 5 days since the last storm abated.  Slabs up to 3’ deep were dumped on top of a widespread layer of buried surface hoar.  This layer has remained intact and is very slowly adjusting to this most recent load.  This combination of slab over weak layer is keeping the danger elevated mainly because the consequences remain high if one were to trigger an avalanche today.

On the plus side, the likelihood of triggering an avalanche has been on the decline since this last loading event.  As it becomes more difficult to trigger avalanches, it is important to pay attention to and avoid areas where that likelihood is higher.  These areas are what we commonly refer to as trigger points. 

Here are some examples of trigger points:

Steep rollovers and convex slopes.  These features have gravity working against the snowpack making it easier for slabs to release.

Thin spots, where the snow is shallow and weak.  In these areas it is easier to impact the weak layer because it is closer to the surface.  Exposed rocks and partially wind scoured slopes are where you will typically find these spots.

These are the main areas to avoid today, especially when they connect directly to steeper and bigger terrain.  Choosing to recreate on large, open slopes with a variety of slope angles and potential trigger points is a roll of the dice today.  You might get away with not triggering a slide or you might not.  The best tactic is to hedge your bets by sticking to lower angle non committing terrain and give this slab/weak layer combo more time to heal.

A quick video describing these concepts:

 

Weather
Sun, December 21st, 2014

Yesterday was another day of relatively calm weather.   Temperatures were in the 20s F at ridge top level, winds were calm and no new precipitation fell during the day.

Light snowfall has begun this morning with stations reporting 1-2 € of new snow.   Expect to see continued light snowfall as a weak low pressure center spins around the Kenai peninsula today.   Accumulation of an additional 1-3″ of snow is possible by the end of the day.   Winds will be light out of the East at 5 mph.   Temperatures at 1,000′ will be right around 32 degrees F.

The extended outlook is calling for a continuation of light snow showers into the early part of the week.

PRECIPITATION 24-hour data (6am – 6am)

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 29  1 .1 27
Summit Lake (1400′) 25  trace .1  4
Alyeska Mid (1700′) 28  2.5 .2 17.3

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  27 E 10  18
Seattle Ridge (2400′)  25  N/A  N/A  N/A
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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.