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Tue, December 23rd, 2014 - 7:00AM
Wed, December 24th, 2014 - 7:00AM
Graham Predeger
Avalanche risk The Bottom Line

There is a MODERATE avalanche danger today in alpine areas above 2,500′.   The odds of triggering an avalanche are on the decline as we are now a week out from the end of our last storm.   Given the persistent nature of the weak layer/ slab combo (1 €“ 3′ slab) that we know exists in much of the advisory area, this is a €œscary € MODERATE situation and if an avalanche initiates it has the potential to propagate far and wide across entire slopes.   Keep this last point in mind if travelling in the backcountry today and exercise safe travel protocol both on the skin up and the ski down.

Below 2,500′ the danger is LOW for initiating an avalanche though anything triggered from higher elevations does have the potential to run into this elevation band.   Again, safe travel protocol and situational awareness with other groups will be key today.

We all need to collectively ask Santa for snow for Xmas because below 1,000′ there is unfortunately still not enough coverage to issue a danger rating.

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Tue, December 23rd, 2014
Above 2,500'
2 - Moderate
Avalanche risk
1 - Low
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

Our current problem can be described as low probability/ high consequence.  The likelihood of waking up this documented layer of buried surface hoar that we’ve been talking about the last few days is waning as time ticks by and temperatures drop. However if one were to find the right trigger point (shallow areas in the slab or slope convexities are the likely areas) on a slope the resulting avalanche could be quite large.  We have a hard time classifying this type of problem using the North American Avalanche Danger Scale because it doesn’t totally capture this low likelihood/ high consequence scenario with any one color. 

Colors aside, it’ll be important today to travel through the backcountry with heightened awareness.  Ensure good spacing between parties and individuals when ascending steep terrain; one person at a time on the slope; plan out and communicate escape routes and safety zones; ensure safe zones are safe.  Keep in mind that with this avalanche problem, potential exists for a slope to avalanche wall-to-wall or even connect slopes through shallow bands of rock.

 As mentioned in the bottom line, this is a “scary” MODERATE avalanche problem and though our snowpack is gaining strength, surface hoar once buried accounts for more avalanche accidents than any other weak layer.

Tue, December 23rd, 2014

Calm weather dominated the eastern Turnagain arm zone yesterday.   After a light morning drizzle deposited a very thin crust on the snow surface up to about 1,500′ the skies began to break and visibility improve.   Ridgetop winds were in the single digits from the east with temperatures hovering around 32 degrees at 1,000′.

We may continue to see a few snow showers through the day as instability in our region weakens with an advancing ridge moving in from the west.   This will shift winds to a northwesterly direction where we may see gusts into the mid-teens or low 20’s mph today.   Temperatures at 1,000′ will linger right around the freezing mark.   We may also see fog development today and into tomorrow-in low-lying areas around the highway.

Looking out toward Xmas it appears that south-central AK will be influenced by westerly flow as a dominant low-pressure system in the western Aleutians tracks east toward the mainland.   Expect temperatures to slowly decline and periods of light snow thru the holiday.    

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 29    1  .1  28
Summit Lake (1400′) 24    0 0    4
Alyeska Mid (1700′)  32  1  .1  21

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 24  E  6  14
Seattle Ridge (2400′)  26 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.