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ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, March 22nd, 2017 - 7:00AM
Expires
Thu, March 23rd, 2017 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger is MODERATE on slopes 35 degrees and steeper at all elevations. Human triggered slab avalanches 1-2+’ thick are possible in areas that saw a foot or more of new snow from over the weekend. In the steeper terrain, expect to trigger loose snow sluffs that could run far. There are a few glide cracks out there, limit time underneath them and give cornices a wide berth.

Summit Lake:  Read the  Saturday Summit Summary  HERE.  

Wed, March 22nd, 2017
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. 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
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

Cold weather and light winds have helped Sunday’s new snow stay mostly loose and unconsolidated while it slowly settles and adjusts. If you are just tuning in now, the ‘sleeper storm’ over the weekend dropped highly varying amounts of snow to the area; quick recap: Placer Valley 2-3+’, Turnagain Pass 10-14″, Girdwood Valley 7-10″ and Summit Lake 4-6″. The new snow fell on a weak old surface consisting of near surface facets and surface hoar. This is keeping it from bonding quickly, but the good news is, the slab (new snow) is so weak and loose in places only sluffing is occurring (no slab). Determining whether the new snow will act as a slab is the current question and will determine what type of avalanche could be triggered. Furthermore, the deeper the new snow the larger the avalanche will be if a slab is triggered – such as in the Placer Valley.

Questions to ask:  What is the character of the new snow? Is it loose and unconsolidated or is it becoming ‘slabby’ with daytime warming on Southerly facing slopes? A slope around noon could be stable only to heat up at 3pm and become unstable, taking out your prior tracks. Was it affected by the winds during the storm and stiffer, aka a wind slab? Any slope where the new snow is acting like a slab will be suspect for triggering an avalanche. How well the slab is bonding with the old surface should be suspect as well – expect poor bonding until proven otherwise. 

As for avalanche activity yesterday, there were a handful of snowmachine and skier triggered slides. We had a report of several soft slabs releasing near the toe to of the Skookum glacier in the Placer Valley. In the heart of Turnagain Pass on Cornbiscuit ridge, a skier triggered a shallow soft slab on a Southerly facing slope that was warming during the day. There were also a few small pockets skier triggered on Magnum.

 

Photo: Snowmachine triggered soft slab avalanche in small terrain around 18″ thick. It was taken near the toe of the Skookum glacier. Photo: Rusty Allen

Photo: Shallow 12″ soft slab triggered by a skier on a South facing slope during the heat of the day. Photo: Adam Baxter

 

Photo: The annotation says it all. Winds during the storm have likely created a stiffer ‘wind’ slab resting on weak faceted snow on this higher elevation slope. Photo: Peter Wadsworth

 

Avalanche Problem 2
  • Dry Loose
    Dry Loose
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.
More info at Avalanche.org

Watch your sluffs. The new snow sat under cold skies last night and this will only encourage it to become looser today – meaning sluffs could be larger and run further than expected. However, southerly aspects may sport a thin sun crust, if this is the case, sluffs will be limited. 

Photo: Skier triggered long running sluff on the North side of Cornbiscuit. Photo: Peter Wadsworth

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

As always, give cornices a wide berth and minimize exposure under them. 

GLIDE AVALANCHES?  Limit time under glide cracks. There is one still opening on Seattle Ridge, left of the up-track, and some in the Skookum Valley along with others scattered about.

Weather
Wed, March 22nd, 2017

Sunny skies, light Northerly winds and cool temperatures covered the region yesterday. Ridgetop winds during the past 24-hours have been 5-10mph with gusts into the teens. Temperatures warmed up to the upper 20’s to 30F with direct sunshine but have cooled dramatically overnight to the single digits in valley bottoms.  

Today, another beautiful sunny day is on tap. Temperatures should be cooler however, warming up from the single digits to the low 20’sF in most locations. Ridgetop winds are expected to be light again from the North and West, 5-10mph, with some areas seeing stronger gusts into the 20’s mph.

The remainder of the week should be similar. We will be watching the winds, yet at this time they look to remain light to moderate.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 15   0   0   66  
Summit Lake (1400′) 14   0   0   30  
Alyeska Mid (1700′) 17   0   0   60  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 14   W   5   11  
Seattle Ridge (2400′) 16   N   4   15  
Observations
Recent Observations for Turnagain Pass
Date Region Location
02/19/24 Turnagain Avalanche: Base of Seattle Ridge
02/18/24 Turnagain Observation: Lynx creek
02/18/24 Turnagain Observation: Tincan Trees
02/17/24 Turnagain Observation: Turnagain (below the uptrack)
02/15/24 Turnagain Observation: Sunburst
02/13/24 Turnagain Observation: Tincan Backdoor, Center Ridge
02/12/24 Turnagain Avalanche: Tincan Trees
02/11/24 Turnagain Observation: Cornbiscuit
02/10/24 Turnagain Observation: Tincan
02/04/24 Turnagain Observation: Eddie’s
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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.