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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, February 29th, 2016 - 7:00AM
Expires
Tue, March 1st, 2016 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger for Alpine and Treeline (above 1000′) is CONSIDERABLE. This means  human  triggered avalanches are likely on slopes 30 degrees and steeper and natural avalanches are still possible. Cornices are huge and could fall, glide cracks are still releasing and storm snow/wet snow avalanches remain concerns. All of these avalanche problems will be amplified if the sun shines at all today.  Elevated caution is recommended. Very cautious route finding and conservative decision-making are important if venturing into the mountains today.  

The danger is  MODERATE  below 1,000′ where debris from an avalanche above could run in steep terrain.

Elevated caution and a conservative mindset is recommended in the Summit Lake area where a variety of avalanche concerns exist. See Saturday’s  Summit Lake Summary  and click  HERE  for recent observations.

Mon, February 29th, 2016
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
  • Storm Slabs
    Storm Slabs
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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

In the last 24 hours an additional .8″ of moisture fell either as rain or above 1800′ as snow. This added on to the 10 inches of water we received this week. Yesterday there was a window to get out and see some of the natural avalanche activity from the past two days. There were some wet slab avalanches and recent glide avalanches in the Treeline elevation band and some storm slabs in the Alpine. An observer watched a storm slab avalanche occur as the sun shined onto the slope. Today all of our storm snow concerns will still be present and human triggering is likely. Avoid slopes steeper than 30 degrees and steer clear of cornices. The snowpack still really needs time to adjust to all the loading from the past week.

Storm Slabs: Fluctuating temperatures and snowfall may have combined to create storm slabs. These could still be triggered and if there are patches of sun today or a temperature spike the reactivity of the snowpack will increase.

Cornices: Natural cornice fall activity is possible and could trigger an avalanche on the slope below. This is something you want to avoid traveling on or underneath. Cornices will be tender and could break farther back than expected. Falling down the slope with a refrigerator to school bus sized chunk of snow on your snowmachine or skis would be terrible, if not deadly. Cornices will also be even more likely to be triggered if the temperatures increase or they receive direct sunlight. 

Wind Slabs: Winds today will continue to add stress to very large wind slabs in leeward terrain. Triggering one of these could be very dangerous due to the depth of slab. Avoid steep wind-loaded slopes. These will also be more hazardous with any rapidly warming temperatures or on sunny aspects. 

Natural sun induced storm slab yesterday.

 

Avalanche Problem 2
  • Glide Avalanches
    Glide Avalanches
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.
More info at Avalanche.org

The snowpack in the mid elevation band (1000′-2500′) has received a significant amount of loading in the form of mostly rain the past week and temperatures have been fairly warm. There is a deep snowpack gliding down the slopes. Over the last few days a handful of glide avalanches have occurred in steep terrain and it is possible more will release today or did with the cooling overnight or will with any sun today. They are still totally unpredictable. Who knows… The fact that is Leap Year today may cause glide avalanches?? Glides threaten a lot of well-travelled terrain on both the motorized and non-motorized side of the highway. Avoid travel below existing glide cracks. 

 

This glide avalanche on Seattle Ridge that occurred on Friday morning is a great reminder that they do release in popular recreating terrain.

 

Additional Concern
  • Wet Loose
    Wet Loose
Wet Loose
Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
More info at Avalanche.org

Temperatures have cooled overnight and are forecasted to remain cooler throughout the day. This will have locked up much of the wet surface snow in a crust. The concern today will be primarily any direct sunlight on the slopes and subsequent rapid warming. This could make triggering a wet slab or wet loose avalanche possible on steep slopes that still are mostly saturated. If you find yourself all of a sudden sinking into sloppy snow it’s time to get off the slope. Watch for natural and human trigger roller ball activity. This is an indication that the surface snow is loosing strength. 

 

 

Wet slide activity observed yesterday on Seattle Ridge. This occurred sometime in the last two days. 

Weather
Mon, February 29th, 2016

Yesterday was mostly cloudy in the morning with rain and snow falling. Skies cleared in the afternoon. Temperatures were in the 30Fs dropping into the 20Fs overnight. Winds were easterly,15-35 mph with some stronger gusts. The advisory area received an additional .5-.8″ of water, 5-10 inches of snow at upper elevations. Rain fell up to at least 1800′.

Today will be mostly cloudy with rain and snow showers forecasted throughout the day, 1-3 inches of snow possible.  Rain/snow line will be approximately 1100′.  Temperatures are forecasted to be a bit cooler with a high around 33 at 1000′ and staying in the mid 20Fs at higher elevations. Winds will be 15-25 mph from the East. There may be some periods of clearing during the day.  

Tonight and Tuesday will be similarly mostly cloudy with rain and snow showers. There is a drying trend and a pattern shift looking later into the week with some clear sunny days in the forecast.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 30   rain   .8   146  
Summit Lake (1400′) 30 2 and rain .5   42  
Alyeska Mid (1700′) 32  1.5 and rain  .7 105  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 23   ENE   30   66  
Seattle Ridge (2400′) 27    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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