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Issued
Mon, January 29th, 2018 - 7:00AM
Expires
Tue, January 30th, 2018 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger is MODERATE above 1,000′ for triggering a slab avalanche 1-2′ thick in the Turnagain Pass zone. In the Girdwood Valley, slabs could be up to 3′ thick and due to stronger overnight winds, the danger could be trending to CONSIDERABLE. Slopes with recent wind loading will be the most suspect for triggering an avalanche. Additionally, watch for easily initiated  loose snow avalanches (sluffs) on steep slopes and at high elevations, above 3,000′, there is still a chance someone could trigger a deep persistent slab avalanche. The danger is LOW below 1,000′.

Mon, January 29th, 2018
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
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
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
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.

Likelihood of Avalanches
Terms such as "unlikely", "likely", and "certain" are used to define the scale, with the chance of triggering or observing avalanches increasing as we move up the scale. For our purposes, "Unlikely" means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. "Certain" means that humans will be able to trigger avalanches on many slopes, and natural avalanches are expected.

Size of Avalanches
Avalanche size is defined by the largest potential avalanche, or expected range of sizes related to the problem in question. Assigned size is a qualitative estimate based on the destructive classification system and requires specialists to estimate the harm avalanches may cause to hypothetical objects located in the avalanche track (AAA 2016, CAA 2014). Under this schema, "Small" avalanches are not large enough to bury humans and are relatively harmless unless they carry people over cliffs or through trees or rocks. Moving up the scale, avalanches become "Large" enough to bury, injure, or kill people. "Very Large" avalanches may bury or destroy vehicles or houses, and "Historic" avalanches are massive events capable of altering the landscape.

Signal Word Size (D scale) Simple Descriptor
Small 1 Unlikely to bury a person
Large 2 Can bury a person
Very Large 3 Can destroy a house
Historic 4 & 5 Can destroy part or all of a village
More info at Avalanche.org

After an exciting day of several human triggered avalanches Saturday (with no one caught), yesterday we had no reports of avalanche activity, nor did we see any. If you know of an avalanche yesterday, please let us know HERE. Avalanches on Saturday were a function of 1-2′ of new snow from Jan 26 that fell on buried surface hoar. The snow was so light that it was only in scattered areas where it was cohesive enough to act like slab, and hence folks were able to trigger slab avalanches. The cold and clear weather is taking the light snow and continuing to loosen it though the faceting process. This is slowly eating away at remaining slabs out there. Despite this, there is still the possibility of finding and triggering one of these lingering slabs. 

With cold and clear weather on tap this week, the big question for increasing avalanche danger will be what are the winds doing? There is plenty of loose snow available for transport and wind slabs could form quickly. So far the Turnagain Pass area has escaped any moderate/strong winds, but it does not seem that way in the Girdwood Valley. Ridgetop winds here increased overnight for a period into the 20’smph with gusts to 40mph, enough to transport snow. If you are headed to the Girdwood Valley area, be on guard for recent wind loading and potential for more dangerous avalanche conditons. Fresh wind slabs are likely to release on, or step down to, buried surface hoar under the Jan 26 snow and could be very touchy and run further than expected. 

Quick hand pits to check how the top 1-2′ of snow is bonding is a good way to assess the conditions along your route. Additionally, watching for any shooting cracks, whumpfing, recent avalanche activity and any wind affect/loading patterns will be key for avoiding unstable slopes. 

Surface conditions at mid elevations at Turnagain Pass – loose faceting snow with a new crop of surface hoar on top. (photo: Ray Koleser)



Avalanche Problem 2
  • Dry Loose
    Dry Loose
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Dry Loose
Dry Loose avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.

Likelihood of Avalanches
Terms such as "unlikely", "likely", and "certain" are used to define the scale, with the chance of triggering or observing avalanches increasing as we move up the scale. For our purposes, "Unlikely" means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. "Certain" means that humans will be able to trigger avalanches on many slopes, and natural avalanches are expected.

Size of Avalanches
Avalanche size is defined by the largest potential avalanche, or expected range of sizes related to the problem in question. Assigned size is a qualitative estimate based on the destructive classification system and requires specialists to estimate the harm avalanches may cause to hypothetical objects located in the avalanche track (AAA 2016, CAA 2014). Under this schema, "Small" avalanches are not large enough to bury humans and are relatively harmless unless they carry people over cliffs or through trees or rocks. Moving up the scale, avalanches become "Large" enough to bury, injure, or kill people. "Very Large" avalanches may bury or destroy vehicles or houses, and "Historic" avalanches are massive events capable of altering the landscape.

Signal Word Size (D scale) Simple Descriptor
Small 1 Unlikely to bury a person
Large 2 Can bury a person
Very Large 3 Can destroy a house
Historic 4 & 5 Can destroy part or all of a village
More info at Avalanche.org

Watch your sluff – loose snow avalanches ‘sluffing’ is possible on steep terrain features protected from wind. As the cold weather continues to loosen and facet the top foot of the snow, we are expecting sluffs to become larger and faster by the day.

Additional Concern
  • 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

Triggering a deep slab is becoming unlikely, but is still not out of the question above 3000′. At these high elevation zones there are a variety of weak layers in the mid pack and near the ground. The most likely trigger spots are in thin areas in the snow cover, often near rocks, or where the slope rolls over. The Southern end of Turnagain Pass to Johnson Pass is more suspect for triggering a deep slab due to a thinner snowpack. 

Weather
Mon, January 29th, 2018

Yesterday’s weather saw valley fog up to 2,000′ with brilliant sunny skies above. Ridgetop winds were Northerly in the 5mph range and bumped up to 10mph overnight briefly with gusts to 20mph – the exception is Girdwood Valley where ridgetop winds from the NE overnight were in the 20’smph with gusts to 40mph. The inversion remains in place as temperatures stayed in the single digits in valley bottoms and in the teens F at mid and upper elevations.

Cold and clear weather (with valley fog) is again expected for today. The coldest station reporting this morning is the Granite Snotel near Johnson Pass trailhead at -13F, burr. The inversion will keep temperatures in the -10 – +5F range in valley bottoms and in the teens at the mid and upper elevations. Ridgetop winds are expected to be Northerly between 5-15mph.

Looking into the crystal ball, weather models have the cold air mass over Alaska in place for the remainder of this week and through the weekend – this means  cold/clear weather with an eye for what the outflow winds are doing.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 15   0   0   68  
Summit Lake (1400′) 1   0   0   19  
Alyeska Mid (1700′) 10   0   0   54  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 16   NE   7   21  
Seattle Ridge (2400′) 16   E   3   17  
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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.