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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, January 30th, 2014 - 7:00AM
Expires
Fri, January 31st, 2014 - 7:00AM
Forecaster
Kevin Wright
Avalanche risk The Bottom Line

Colder temperatures at all elevations are freezing the snowpack surface into a hard supportable crust.  The crust is adding a component to the snowpack that we haven’t seen in a couple weeks – strength.  

Underneath the crust, especially below 3,000 feet and in areas where the total depth is shallow, there is still a lot of very weak moist snow.  We still must acknowledge the possibility of avalanches being triggered on these weak layers.  Just 2 days ago we saw explosives produce large avalanches.    

The danger rating is  MODERATE  for the possibility of human triggered slides that can break full depth.  Trigger points will be much more likely in shallow points on the snowpack.  The problem is affecting buried wet snow at low elevations and deep persistent weak layers at high elevations.  

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Thu, January 30th, 2014
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
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. Dangerous avalanche conditions. 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

We don’t have a lot of information about the high elevation deep slab because so few people have been traveling in the mountains over the last couple weeks.  Yesterday on Seattle ridge we dug in a deeper area at 2900 feet and found 5 feet of mostly very strong snow on top of the old Nov/Dec persistent weak layers.  (click on link for photos and pit profile)  We could not get an extended column test to fail, but 2 compression tests revealed sudden collapse Q1 failures at the early December drizzle crust (CT24, CT21 Q1).  The good news is that our recent wet storm has made some very strong snow – it’s difficult to cut through it with a sharp saw.  I think it will be unlikely for a person to trigger the deep weak layers where the snowpack is in fact deep.  It may be possible to trigger from shallow points where it could propagate to much deeper snow.  

Avalanche Problem 2
  • Wet Loose
    Wet Loose
Wet Loose
Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
More info at Avalanche.org

I’m still going to mention wet avalanches even though we now have a surface crust over everything.  You have to dig through the crust to find the wet snow.  

If someone were to trigger an avalanche below 3,000 feet, it will have some characteristics of a wet slab.  Think about heavy, slow moving, bulldozer-like avalanches.  

We dug a shallow pit just off the slope from the standard snowmachine up-track to Seattle ridge yesterday.  Underneath the 4 inch think supportable crust is unbelievable weak, moist snow.  Skis glide over the crust with no penetration, but trying to walk on it will sometimes punch a boot through the crust, which then drops through to the ground.  It’s a very abnormal setup, and I’m honestly not sure how to measure its stability.  

As more of the snowpack freezes up with the colder temperatures, strength will increase quickly and the possibility of triggering more avalanches in the wet snow will decrease dramatically.

Weather
Thu, January 30th, 2014

Sunny breaks over the last few days will continue today.  Southcentral Alaska is dominated by a blocking high pressure that is currently keeping clouds and storm from the west from entering our airspace.  The big weather change that is affecting our snowpack is a drop in temperature.  Ridgetop stations have been reading below freezing for over 48 hours.  Lower elevation sites are now also freezing up.  Wind is light and variable today.

Looking into the weekend – high pressure is expected to be the dominating feature for our region.  Mostly clear skies are forecasted, with valley fog in some areas.

Graphs show the temperature profile at Center Ridge (1880 ft) and Sunburst (3812 ft).  Blue line is 32 degrees (freezing).

Observations
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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.