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Sun, April 12th, 2015 - 7:00AM
Mon, April 13th, 2015 - 7:00AM
Heather Thamm
Avalanche risk The Bottom Line

Today the avalanche danger is MODERATE where triggering a large avalanche is possible in the Alpine and at Treeline. Below 2500′ a very €œupside down € snowpack persists where a slab 1-5′ thick could release. On Northern shaded aspects above 2500′ a slab 3-5′ thick is sitting on layer of weak faceted snow and if triggered could have very high consequences.  It will be important to avoid all steep open slopes at Treeline and on Northern aspects in the Alpine.  Newly forming isolated  windslabs will also be something to watch for if snow showers occur as forecasted today.

* The most desirable snow will be found on shaded aspects and this could make decision-making challenging. Be sure to practice safe travel rituals; identify islands of safety, only expose one person at time, and always have an escape zone.    

Special Announcements

Beginning tomorrow, April 13th, we will be issuing advisories 5 days a week.   Advisories will be posted at 7 am on all days of the week except Mondays and Wednesdays.   The final advisory of the season will be posted on Thursday April 30th.

Sun, April 12th, 2015
Above 2,500'
2 - Moderate
Avalanche risk
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

Below 2500’ a layer of wet poorly bonded snow is sitting beneath a slab 1-5’ thick, depending on the location. On the Northern side of the Turnagain pass this slab is closer to 5’ thick. Yesterday on Eddies’ South Face several large (D3) avalanches released naturally on this wet layer. Simultaneously several smaller avalanches released sympathetically on adjacent slopes. It was reported that this occurred in one single event during the warmest part of the day, just after 3pm. Several parties all in different locations described a very loud “Canon” sounding collapse and felt the snowpack drop ½ an inch.  

On the far Southern end of Turnagain Pass where this slab is only 1-2’ thick below 2500’, recent activity and collapsing were not observed yesterday. What this means is that there is more stress on this weak layer where the slab is thicker and thus more likelihood of it reaching its critical tipping point. This problem exists on all aspects below 2500’ and could become active even in places where the slab is thinner. At this elevation band it will be important to avoid being on or near large slopes steeper than 35°. Some examples of these slopes are the East Face of Seattle Ridge and the Library near Center Ridge.

*Even thought temperatures are dropping below freezing at night the wet snow below is unable to re-freeze due to the insulating properties of the slab. This problem is likely to persist for a while and will be most active during the warmest parts of the day. 

Close up photo of one of the many avalanches that occured yesterday on Eddies. This one was on a SW apsect. Photo by Laine Parish.


 View of Eddies West Face. Several larger (D3) avalanches occured at a similar elevation band on the South Face, but could not be seen in entirety from the roadside. 


Oblique view of the South aspect of Eddies. Several large (D3) avalanches are out of view from roadside perspective. 



Avalanche Problem 2
  • 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

Weak layers of faceted snow buried as deep as 5’ sit on some slopes on North facing terrain in the Alpine.  This set up will not be found everywhere.  Because of this variability, it is especially important to treat this terrain with suspicion.  Assessment of this issue is very difficult.  The best strategy for “managing” this problem is to avoid this terrain for the time being and give those underlying weak layers ample time to adjust to the large loads (3’+ snow/3”+ H20 over the last week) that have been placed on them.  This is a low likelihood/high consequence scenario that requires patience and time.

Additional Concern
  • 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

Moderate ridgetop winds from the Northeast and several inches of new snow could create small isolated wind slabs in the Apline. These wind slabs will likely be small (2-6”) but could be tender if we see 3+ inches of new snow today. Pay attention to the amount of new snow accumulation. Southern aspects have a sun crust that could make a good sliding surface if these wind slabs form today.  

Sun, April 12th, 2015

Yesterday skies were mostly clear with light winds out of the Northeast along ridgetops. Temperatures reached the low 40s F near 1000′. No precipitation occurred during the day.

Overnight light flurries produced an inch of new snow in Turnagain Pass.  No new snow was recorded in Girdwood. Temperatures dipped into the high 20s F at lower elevations and the low 20s F near ridgetops. Winds were 10-20mph from the NE along ridgetops.

Today will be mostly cloudy with scattered snow and rain showers throughout the day. Up to 3 inches of accumulation is expected. Ridgetop winds will be light to moderate (10-20 mph) from the Northeast.  

*Wind direction data not available at Seattle Ridge Wx Station. Wind amounts only available after 2pm on April 11th.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 33   1   0.1   70  
Summit Lake (1400′) 33   0   0   11  
Alyeska Mid (1700′) 33   0   0   40  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 23   NW   9   33  
Seattle Ridge (2400′) 26   *n/a   *7   *23  
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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.