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Tue, December 19th, 2017 - 7:00AM
Wed, December 20th, 2017 - 7:00AM
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger is  MODERATE  at all elevations for storm slabs with new snow and wind forecasted for today.  Additionally, a MODERATE danger exists on the high elevations slopes (above 3,000′) for the possibility of a slab avalanche breaking near the ground. The most concerning areas for triggering a large avalanche are where the snowpack is shallower, such as on the South side of Turnagain Pass and toward Summit Lake as well as the Crow Pass region.  

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

Special Announcements

Two  CNFAIC Fireside Chats coming up this week!    Both will discuss lessons learned from past avalanche incidents. We hope to see you there!

Tue, December 19th, 2017
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
  • Storm Slabs
    Storm Slabs
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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

New snow falling today will land on weak surface snow and could easily become a storm slab. Surface hoar was observed to ridge-tops and the snow just below that has also begun to facet. Sluffing in the surface snow was also observed in steep terrain yesterday. Temperatures are on a rising trend today and the snow falling may be a bit upside down as warmer snow falls on colder snow. Winds are forecasted to blow into the 30s from the SE and may gust higher as there is some uncertainty about winds in the weather forecast discussion today. This could contribute to creating slabs especially in leeward terrain along ridgelines. Look for drifting and loading patterns. Give cornices a wide berth and pay attention to changing conditions. Hand pits will be a good way to assess how well the new snow is bonding to the old snow surfaces. As always practice safe travel protocols and watch for shooting cracks and whumpfing. 

Surface hoar observed along Seattle Ridge yesterday. 



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

Observations continue to point to a stabilizing snowpack however the big question is, above 3,000′ where the crusts from the warm wet storms disappear and facets under a deep hard slab may remain. Stability tests so far have been done up to 3,300′ and all signs point to a strong snowpack and no reactivity in the facets. This is promising. But, we must be vigilant and conservative this week as we explore the higher elevations and gather more data. Things to keep in mind:

    – Triggering a dangerous deep slab avalanche is still possible above 3,000′
    – Shallow snowpack areas are most concerning (more trigger points and possibly more reactive facets). For example: the South end of Turnagain Pass, Crow Pass and Summit Lake
    – No red flags are likely to be present to indicate an unstable snowpack

Photo: Snow pit from 2820′ in 1st Bowl/Main Bowl on the backside of Seattle Ridge. Stout melt-freeze crust is present in the upper snowpack from the warm/wet storms at this elevation. However, the basal facets in this pit were dry and loose. This is the type of set-up we are concerned in the upper elevations that don’t have the melt-freeze crusts in the upper snowpack. 



Tue, December 19th, 2017

Yesterday was clear with some patches of valley fog. There was an inversion with temperatures in the valley bottoms in the teens and temperatures at upper elevations in the 20Fs. Winds were very light. Overnight temperatures have been rising and warmer air is moving into the valley bottoms. Portage went from 20F yesteday to 33F this morning.  

Today is forecasted to be mostly cloudy with snow showers, 2-6″ of snow possible. The rain/snow line may rise in the afternoon to around 500′. Temperatures will be in the low 30Fs to mid 20Fs depending on elevation. SE winds will be 15-25 mph with higher gusts into the 30s and 40s. Temperatures cool down a bit overnight and winds shift to the SW, 10-20 mph.  

Tomorrow will be partly cloudy with temperatures in the 20Fs and light winds.  Thursday looks to be clear and sunny. Another storm system is potentially on tap for the weekend but timing, precipitation and temperatures are still TBD.  

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

**Seattle Ridge has been recording intermittently. Data today is not from a full 24 hr period.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 19   0    0 33  
Summit Lake (1400′)  9 0    0 10
Alyeska Mid (1700′)  21 0    0  26

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′) **18   **SE   **12   **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.