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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, December 29th, 2018 - 7:00AM
Expires
Sun, December 30th, 2018 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger is  MODERATE  above treeline where increased winds last night may have formed sensitive wind slabs on lee aspects. Any fresh wind slab found is likely to be soft and shallow, yet could be easy to trigger. Cornices are still a concern and likely grew with last night’s wind. Glide cracks are moving! Please keep a close eye out for these brown cracks and limit travel underneath them.  

SUMMIT LAKE / JOHNSON PASS / LYNX DRAINAGE:    South of Turnagain Pass, weak layers of facets have been found 1-3′ below the surface. These layers are suspect to be difficult to trigger but not out of the question. If someone hits the right spot, a large avalanche may result. Heads up in these thinner snowpack zones.

Special Announcements

Hatcher Pass:   Problems remain in the snowpack at Hatcher Pass, please check this recent observation  of a remote triggered avalanche yesterday and HPAC’s Saturday morning forecast HERE if headed this way.

Sat, December 29th, 2018
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
1 - Low
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
  • 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

A few inches of new snow plus some strong wind overnight and we can expect to see pockets of wind slabs today. These are likely to be soft, shallow and found above treeline where the easterly winds have been blowing. Over the past few days, several inches of new snow has accumulated with an additional 2-4″ overnight at the higher elevations. Winds have decreased this morning and another 2-4″ of snow is forecast for today. This new layer of snow could obscure the wind effect from last night – so we’ll need to be on our guard for hidden wind slabs if skies clear enough for travel into the Alpine.

Quick hand pits and feeling for stiffer snow over softer snow are good ways to check for wind slabs or any other surface instability. A couple thin layers of buried surface hoar from the past several days intermixed in the soft surface snow may make any wind slab that forms more touchy. 

CORNICE FALLS:  There could still be large cornices that are close to breaking off and only need a person’s weight to tip the balance. Many cornice falls were reported last week and they continue to grow slowly with recent snow and wind. Continue to give them a wide berth and limit exposure under them. 

 

A new crop of small surface hoar seen yesterday (photo Trip Kinney). There is another layer of buried surface hoar 4-6″ deep in soft snow that was buried on Christmas. We are tracking these layers, as they could develop into future weak layers when the next real snow storm hits.

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

Glide cracks are moving – beware. These can release at anytime and are most suspect when we have proof to their movement, as seem below. Cracks are littering the slopes in Lynx Creek drainage and one released on Corn Biscuit in the last couple days. Others we know of are on the SW aspect of Sunburst, Corn Biscuit, Warm-up (-1) Bowl on Seattle Ridge and the SW face of Magnum. Check out this link for more information on glide avalanches. The best way to manage this problem is to keep your eyes peeled for cracks and limit travel underneath them.

 

Glide cracks moving on SW facing Magnum Ridge (Thamm, Moderow)


Glide avalanche on Corn Biscuit, seen and photographed on 12/28 by Andy Moderow.

 Glide crack in Lynx Cr. This crack is SW facing. (photo: Graham Predeger) 

Additional Concern
  • Persistent Slabs
    Persistent Slabs
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.
More info at Avalanche.org

South of Turnagain: keep in mind triggering a large slab avalanche is still possible. Buried weak layers, roughly 2-3′ below the snow surface, have been found in the Summit Lake zone. There is some uncertainty around Johnson Pass, Lynx Cr drainage and Twin Peaks/Silver Tip where the snowpack transitions to that with poorer structure as found at Summit Lake. A report from Lynx Ck yesterday found a buried layer of facets reactive (but stubborn) in the snowpack. Observations in Summit Lake over the last few weeks have been finding facets associated with crust reactive in stability tests. The most recent avalanche activity in this zone was observed last week (Dec.20) where some of these avalanches stepped down to older layers within the snowpack. Should you head into the periphery of the forecast zone, evaluate the snowpack and terrain as you travel. Listen for whumpfing — an obvious clue for triggering a slab avalanche.

Weather
Sat, December 29th, 2018

Yesterday:   Clear skies gave way to overcast conditions yesterday as a small storm moved in during the evening hours. Around 2″ of new snow fell at the mid-elevations and likely up to 4-5″ and at the upper elevations. Ridgetops winds picked up last night from the east into the 20’s mph with gusts up to 40mph and have quieted down this morning. Temperatures warmed with the snowfall and have been in the 20’sF along ridgetops, 30F at 1,000′ and mid 30’s F at sea level.  

Today:   Snow showers are likely to continue through the morning, adding an additional 2-4″ of snow (possibly a rain/snow mix for sea level). Skies may clear up this afternoon and snowfall cease as the system heads north. Ridgetop winds are expected to remain relatively light, 5-15mph from the east. Temperatures should stay near 32F at sea level, 30F at 1,000′ and in the mid 20’F along ridgelines.  

Tomorrow:   Another more powerful system looks to impact the region Sunday. This will be the first in a series of storms that are expected to bring snow, wind and rain to sea level through Monday. Stay tuned!

*Seattle Ridge weather station was heavily rimed and the anemometer (wind sensor) was destroyed.    

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 28   2   0.2   54  
Summit Lake (1400′) 25   1   0.1   13  
Alyeska Mid (1700′) 27   2   0.2   37  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 20    ENE 15   40  
Seattle Ridge (2400′) 25    *N/A  *N/A    *N/A  
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.
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