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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, December 21st, 2017 - 7:00AM
Expires
Fri, December 22nd, 2017 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

A  MODERATE  avalanche danger exists in the Alpine.  On the high elevations slopes (above 3,000′) triggering a deep slab avalanche breaking near the ground is possible and may be triggered remotely.  Additionally, wind slabs are possible in leeward terrain.  Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully.  

The avalanche danger below 2500′ is  LOW  where the snowpack is predominately thick layers of melt-freeze crust and triggering an avalanche is unlikely.  

There is no hazard below 1,000′ due to lack of snow.

*Please remember your safe travel practices! This includes, exposing one person at a time in avalanche terrain, watching your partners, being rescue ready and having an escape route planned.

Special Announcements

TONIGHT! CNFAIC Fireside Chat: Avalanche Lessons Learned from Last Season (and a quick look at what’s happening in the snowpack this season).  December 21st at Powder Hound Ski Shop  in Girdwood, 6:30-8 pm with Aleph Johnston-Bloom.  We hope to see you there!

 

Thu, December 21st, 2017
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
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
  • Deep Persistent Slabs
    Deep Persistent Slabs
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.
More info at Avalanche.org

Yesterday a party on Pastoral found the trigger point for a large (D3: could bury and destroy a car, damage a truck, destroy a wood frame house, or break a few trees) deep slab avalanche. The crown depth was estimated to be around 10 ft. deep. This avalanche was triggered from below as the two were crossing the slope. They reported, “It sounded like a distant explosion when it went, then we heard and felt the snowpack below us drop several inches.” When this was triggered it also sympathetically released a slope approximately half a mile away.  Last Friday the forecast said, “This deep persistent slab problem and will be guilty until proven innocent.  The 1st person on the slope or the 10th might trigger this type of avalanche. It is a total roll of the dice or Russian roulette set-up. Finding the shallow spot could have devastating results. This type of avalanche would be unsurvivable. There may be no obvious clues to indicate instability and digging to find the weak layer could be challenging.”  The verdict is out. This snowpack set-up is guilty and could be a scary issue for quite a while. The combination of faceted snow on or under a crust near the ground with a stiff slab above makes for an instability that is contiguous over terrain. A collapse of the weak layer in one spot can propagate/communicate a failure across the area. Translation: I step here. It avalanches over there. The way to manage this is to avoid avalanche terrain specifically in the Alpine. Shallow areas are more likely to be trigger points to cause the failure. This avalanche emphasizes paying attention to what is above you, what the terrain you are on is connected to and where other groups are. Thanks to the party involved for sharing details and all the other observations submitted.

  • Triggering a dangerous deep slab avalanche is still possible above 3,000′. The avalanche yesterday was triggered around 3600′ and released at approximately 4200′.
  • Shallow snowpack areas are most concerning (more trigger points and possibly more reactive facets).
  • No red flags (recent avalanches, shooting cracks or whumpfs) are likely to be present to indicate an unstable snowpack. The party involved reported that there were tracks on the slope from the day before and no signs of instability (red flags) observed.
  •  It might not be the first person on the slope that triggers the avalanche and remote triggering is possible.

Photo: Mike Ausman

 Crown photo: Mike Ausman

Photo of debris: Stephen Ellison

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

Yesterday observers reported stiff wind affected snow in the Alpine. There is still snow to move around and winds are forecasted to be easterly, gusting into the 40s today. Wind slabs will be possible on steep, leeward, loaded slopes. Look for drifting, cracking and pay attention to stiff snow under foot.  Even a small wind slab can be very dangerous in high consequence terrain.  In addition, give cornices a wide berth. They have been growing with each new snow and loading event. 

 

 

Wind loading and cornice along  -1/Warm-up Bowl.

Weather
Thu, December 21st, 2017

Yesterday was partly cloudy. Winds were westerly 10-20 gusting into the 30s. Temperatures were in the 20Fs. Overnight skies cleared and temperatures cooled into the low 20Fs and teens as an inversion set-up.

Today will be clear and sunny with some valley fog. Temperatures will be in the teens and low 20s. Winds will be easterly 15-25 gusting into the 40s. They are forecasted to increase tonight into Friday.  

Friday will be partly cloudy with a chance of snow. Easterly winds will blow 30-40 mph with gusts into the 50s. Temperatures will be in the mid 20Fs to low 30s. Saturday looks to be partly sunny as the storm track for the weekend has shifted and there is less precipitation forecasted for the advisory area but we may get a few inches for the holiday.  

* Sunburst weather station is down due to loss of battery power.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 27    0  0 32  
Summit Lake (1400′) 16  0  0 12  
Alyeska Mid (1700′) 26 0    0  28

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) *n/a   *n/a   *n/a   *n/a  
Seattle Ridge (2400′)  22  W  8 25  
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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.