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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, November 22nd, 2014 - 7:00AM
Expires
Sun, November 23rd, 2014 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

There is a MODERATE avalanche danger for wind slab avalanches at elevations near treeline and in the upper elevation alpine zones. Below treeline a lack of snow cover results in No Rating. Wind slabs and cornices have been forming over the past two days and may continue to grow today. These are 4-24″ thick (variable) and will be possible to find and trigger on predominantly West and South aspects, although all aspects are suspect. The potential to trigger a wind slab will increase with elevation as the snow becomes drier and slabs need more time to bond.  

The next advisory will be Tuesday morning at 7am.  
Outlook for Sunday and Monday: The weather and resulting avalanche conditions are expected to be very similar for Sunday and into Monday.

Special Announcements

Good Morning CNFAIC users!! This is our first advisory for the 2014/15 winter season and we have a couple new items to introduce:

First, we will be issuing danger ratings for three elevations bands – Alpine (above 2,500-3,000′), Treeline (1,500′ to 2,500-3,000′) and Below Treeline (Below 1,500′). These elevation numbers are not exact since treeline elevations are variable within the region. This expansion will help forecasters, as well as hopefully the public, to better describe and understand the differences in danger and avalanche concerns with respect to elevation.

Second, we have added a new table to the Mountain Weather section, see below.

Sat, November 22nd, 2014
Alpine
Above 2,500'
2 - Moderate
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
  • 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

It is a pleasure to report we have had new snow during the past two days! After an onslaught of rain up to 3,500′ on Tuesday, temperatures have dropped and Thursday we picked up around 6″ above 1,800′ with another 2″ yesterday – and add to that, another 1-3″ possible today. Numbers may be modest and the rain/snow line high, but it’s snow nonetheless.

Very strong winds have redistributed the new snow into variable drifts and slabs from a couple inches thick to 3′ in some locations. These sit on a uniform rain crust from Tuesday’s rain that extends up to 3,000′ and likely higher. Hence, the snow surface yesterday was a combination of new snow drifts and slabs with scoured and VERY SLICK rain crust in between. 

For anyone venturing into the backcountry today, watch for current wind loading to increase the sensitivity of wind slabs. Yesterday, slabs were stubborn and showing signs of bonding to the preexisting surface (ice) well, yet this was in one location and is not a certainty. Steeper slopes approaching 40degrees should be approached with extra caution. Any ride, in even a small avalanche, could send you down an icy slope littered with rocks. Beware of the ice under the new snow. Performing hand pits and digging in the new snow to assess the new/old snow interface are good ways to suss out how well the slabs are bonding. Also, keep an eye out for any recent avalanches and cracking in the snow around you.

If you get out this weekend please let us know what you are seeing – pass on a photo or two and/or an observation!!

Below is a classic small cornice break during a period of wind loading (Sunburst 11/21 2,300′ SW facing).

A look at the snow line (~1,700′)

*High elevations over 4,000′: There is much uncertainty as to how much snow has fallen at these high elevations (Raggedtop Mtn for example) and if the faceted weak layer that was reported earlier in November remains under the bulk of the November snow. Under these circumstances, large avalanches are possible. Digging in the snow to assess the snowpack structure is a must if considering travel in these areas.

Avalanche Problem 2
  • Cornice
    Cornice
Cornice
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
More info at Avalanche.org

Watch for cornices to break under the weight of a person. Cornice growth has been significant during the past couple days at the high elevation ridgelines. 

Additional Concern
  • Glide Avalanches
    Glide Avalanches
Glide Avalanches
Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.
More info at Avalanche.org

There are still a fair number of glide cracks littered about the mountains at elevations between 2,000-3,500′. Many of these are obscured by the new snow so keep an eye out and steer clear from being under cracks. Although we have seen no new glide avalanche activity for close to a week, these unpredictable avalanches are something to avoid as a general rule.

Weather
Sat, November 22nd, 2014

During the past 48-hours we have seen 6-8+ inches of new medium density snow at the mid and upper elevations with rain at the lower elevations. Strong Easterly winds averaging 20-30 mph with gusts in the 50’s accompanied the new snow. The rain/snow line has been 1600-1800′ after dropping substantially from 3000+’ earlier in the week.

Today, an additional 1-3″ of snow is possible above 2,000′ and rain below. Ridgetop winds have decreased overnight where they are expected to remain, in the 15-25mph with stronger gusts from the East. Temperatures will stay near the mid 30’sF at 1,000′ and the mid 20’sF on the ridgelines. Skies should remain mostly cloudy with a few breaks in the clouds through the day.

Sunday and Monday conditions look remarkably similar. A large low pressure system spinning in the Gulf will keep the Eastern Turnagain Arm area under off-and-on snow showers above 1,500-2,000′ with 2-5″ of snow accumulation each day (.2-.7 rain below 1,500′).

Below are our new tables for this season that will spell out the past 24-hour weather data for key weather stations in Turnagain Pass, Summit Lake and Girdwood Valley.
Also, for a better look at November’s weather see our monthy chart HERE.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′)  33  1  0.2  15
Summit Lake (1400′)  34  0  0 0  
Alyeska Mid (1700′) 35    0 0.1    6

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  25 E  20 48  
Seattle Ridge (2400′) 28  NE  12 32
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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.
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