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ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, March 21st, 2018 - 7:00AM
Expires
Thu, March 22nd, 2018 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger is MODERATE  above 1000′. Triggering a large, destructive slab avalanche 2-4+ feet thick is possible on all aspects above 1000′ and may be remotely triggered. Watch for wind slabs along ridgelines and avoid cornices. Pay attention to afternoon warming. Evaluate snow and terrain carefully.  

Below 1000′ the danger is  LOW.    

Check out the most recent  Summit snowpack and avalanche summary  if you are headed South of Turnagain Pass.

 

Special Announcements

An avalanche crossed the Hatcher Pass road Monday morning and the road remains closed as of this morning. A Professional observation from Hatcher Pass Avalanche Center is available HERE.   Read the ADN article  HERE. Check out our observations page for information about the avalanche activity that has occurred over the past week region-wide, from  Hatcher Pass all the way South to Lost Lake near Seward  HERE.  

 

 

Wed, March 21st, 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
  • Deep Persistent Slabs
    Deep Persistent Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
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.

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

A large avalanche was remotely triggered by a helicopter landing at a pick-up spot in Winner Creek near Girdwood yesterday. This avalanche occured at 2500′ and released on the layer of weak faceted snow on a slick crust that has been the culprit in most of the avalanches over the past week. It was triggered from 1/4 of a mile away, wrapped 750′ around a ridge releasing on multiple aspects and the crown was 3-6′ deep. The pick-up spot had been used 10 times the day before. Why did it go the 11th time? The likelihood of natural avalanches has decreased as winds have hammered many areas and stripped away all the soft snow but a skier or snowmachiner triggering an deep slab avalanche remains a very real and scary possibility. Knowing where in the terrain that avalanche could be triggered and what it will take to trigger it is the hard part. This is the unfortunate reality of this type of avalanche problem. With a deep slab problem it is important to remember no signs of instability may be present before a slope releases. It may be the 10th person onto the slope that finds the trigger point and slopes may be triggered remotely. It is crucial to visualize the consequences if the slope does slide. Are there terrain traps below?  Bigger slope = Bigger avalanche. Thin spots near rocks and along ridgelines are likely areas to find the trigger point. Widespread buried surface hoar and facets have been well documented at all elevations under a thick, connected slab and finding the wrong spot could be deadly.  

Crown in Winner Creek, avalanche wrapped around the ridge to the north.

 

Arrows point to crowns on both sides of the ridge

Snowpack structure on Tincan. Weak snow under a hard slab

 

 

Avalanche Problem 2
  • Wind Slabs
    Wind Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
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.

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

The Northwest winds continued for a second day yesterday blowing 25-35 mph and gusting into the 60s on Seattle Ridge. Winds are forecast to remain elevated today into tonight and then mellow out by tomorrow. Most of the soft snow is now either blown into the atmosphere or been pounded into hard slabs and sastrugi. However, while traveling along ridgelines, be aware of the potential for wind slabs on a variety of aspects due to unusual wind loading patterns and cross loading. Smooth supportable surfaces where the snow is hollow sounding are suspect, especially if the slope is unsupported. Look for cracking and identify terrain features with a pillow-shaped look where triggering a wind slab could break above you. A wind slab could step down to older snow in the snowpack and create a much deeper and more dangerous avalanche.

Seattle Ridge has been getting top-loaded by this wind direction. 

Additional Concern
  • 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

Cornices are large and looming and wind loading can add stress. Give these an extra wide berth and limit exposure underneath them.  A cornice fall could trigger an avalanche on a slope below.

Weather
Wed, March 21st, 2018

Yesterday was partly cloudy with temperatures in the teens at upper elevations and low 30Fs at sea level. Winds continued from the NW building mid day blowing 20-35 mph and gusting into the 60s. Overnight skies were clear, temperatures were in the teens to mid 20s and winds remained strong.  

Today is forecast to be sunny and clear with temperatures in the teens again at upper elevations and 20Fs to low 30Fs at lower elevations. Winds will be from the NW 15-30 mph gusting into the 40s and 50s. Temperatures overnight will be in the teens and single digits and winds are forecast to die down by the next morning.  

Thursday and Friday look to be clear, sunny and calm. The next weather system moves in over the weekend and the pattern looks active into mid-week. Stay tuned for details.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′)  24  0 0    82
Summit Lake (1400′)   27     0    0      32
Alyeska Mid (1700′)  24   0    0      77

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  15  NW  13 41  
Seattle Ridge (2400′)  22  NW    35 67  
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.