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Issued
Mon, March 12th, 2018 - 7:00AM
Expires
Tue, March 13th, 2018 - 7:00AM
Forecaster
Heather Thamm
Avalanche risk The Bottom Line

CONSIDERABLE  avalanche danger remains above 1000′ on all aspects. Large slab avalanches 2-4 feet thick are likely to be triggered on slopes steeper than 30 degrees.  These may be triggered from a distance or from the bottom of a slope.  Additionally, moderate winds today may be forming shallow wind slabs and adding stress to the snowpack. Cautious route finding and conservative decision-making will be essential for safe backcountry travel.

MODERATE avalanche danger exists below 1000′ where triggering a large slab avalanche is possible in areas with avalanche terrain like Portage Valley and Placer Valley.

Natural avalanche activity was observed in Summit Lake area over the weekend – see Saturday’s  snowpack and avalanche summary and observations from yesterday HERE.  

Special Announcements

**Heightened avalanche conditions are being seen region-wide. Human triggered avalanche activity was reported in the  South Fork of Eagle River on  Saturday.

Mon, March 12th, 2018
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
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. 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

Triggering a large and unmanageable slab avalanche 2-4′ thick remains our primary concern.  Evidence of natural avalanches were seen yesterday near Silvertip and in Summit Lake. These avalanches likely occurred during a storm that ended Friday with strong winds and 2-4 feet of new snow across our region.  On Saturday, a snowmachiner remotely triggered two slabs on Seattle Ridge from a meadow below the slope, and a skier triggered a slab in Girdwood in the Notch Mtn area. These human triggered avalanches were in the mid elevation bands and on small steep terrain features. Due to poor visibility this weekend little information exists from the Alpine. What we do know is various layers of old faceted snow and buried surface hoar sit under 2-4 feet of storm snow from Friday. These persistent weak layers have been well documented and are wide spread across our region. 

Much uncertainty exists as to how our snowpack is adjusting to its new load,  and today’s moderate winds will be transporting 3-4 inches of loose surface snow. This is not quite enough wind to anticipate natural activity, but just enough to be adding additional stress to the snowpack where a human could tip the balance. 

If you are headed out into the backcountry today things to keep in mind are:

  • The mountains are still adjusting to the several feet of new snow that fell three days ago
  • Avalanches can be triggered from the flats or remotely from an adjacent slope – be extra cautious to avoid being in a runout zone 
  • Stick to slopes less than 30 degrees and ease into steeper terrain slowly – after careful snowpack assessment. Evaluate the consequences if the slope releases; where will the debris go?
  • Listen for whumpfing (collapsing) in the snowpack, an obvious clue that an avalanche could be triggered in steeper terrain

Recent natural avalanche activity on a heavily cross-loaded SE aspect between Silvertip and Moose Mountain. Be aware that the Southern end of Turnagain Pass and Summit Lake have a thinner and weaker snowpack. 

 

Stability tests yesterday in the mid elevation zone of Corn Biscuit showed propagation potential on a layer of buried surface hoar mixed with facets.

 

A Skier triggered avalanche from Saturday (lookers right side) at 2000’ on a NW aspect of Notch Mountain in Girdwood. No one was caught and the party reported that the slide on lookers left was there before they skied the slope and may have occurred naturally or was triggered remotely from above. 

Additional Concern
  • 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

Wind slabs: Shallow wind slabs may be forming on leeward features today due to moderate Easterly winds in the Alpine. Triggering a wind slab will likely be shallow, but could step down to a deeper layer of the snowpack and create a much larger and more dangerous avalanche. 

Cornices: Cornices have grown and are suspect for breaking while traveling along ridgelines. Give these an extra wide berth and minimize any time below them. Cornice falls can trigger avalanches on slopes below.

Loose snow: Several inches of low-density snow fell this weekend, and moderate winds along ridges could initiate some loose-dry point releases (sluffs.) Radiation from the sun is not expected to be a factor today, but should we experience a period of clearing skies, small wet-loose sluffs are possible on Southerly aspects in steep rocky terrain. 

Weather
Mon, March 12th, 2018

Yesterday was overcast becoming obscured in the afternoon by snow showers that dumped 4 € of low density snow in Turnagain Pass and Girdwood. Light Northwest ridge top winds (5-15mph) increased overnight to 15-30mph from the Southeast. Temperatures ranged from the upper teens (F) to mid-20F’s.  

Ridgetop winds will remain moderate (10-30mph) from the East throughout the day and decreasing to Light by the evening. Scattered snow showers may bring an inch of new snow, but not a lot of accumulation. Skies are expected to be overcast, but patches of clearing skies may occur later in the day and overnight.   Daily temperatures may swing from the low-20F’s into the low-30F’s by mid-day.  

A showery and active pattern is expected to continue throughout the week with periods of snow flurries, but not much accumulation. Wednesday looks like the first chance for a partial clearing, but lots of uncertainty remains in our daily forecast on timing of several Lows passing through our region. Daily temperatures swings are expected to be in the low-20F’s to mid-30F’s and winds may range from Light to Moderate throughout the week.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 27   4   .2   93  
Summit Lake (1400′) 23   2   .1   35  
Alyeska Mid (1700′) 22   4   .2   83  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 18  E 7   32  
Seattle Ridge (2400′) 22   n/a   n/a     n/a    
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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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