ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Thursday February 7th, 2019
Posted by Heather Thamm on 02/07/19 at 7:00 am.
The Bottom Line
Moderate Avalanche Danger
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

The avalanche danger remains a ‘scary’ MODERATE on slopes above 2,000′. Triggering a large unsurvivable slab avalanche 2-3′ thick is possible on steep slopes. This kind of slab can be triggered remotely from a ridge, similar to an avalanche triggered yesterday on Eddies. In addition look for signs of windloading and roller balls on solar aspects. Avoid cornices and glide cracks and watch your sluff. Identify high consequence terrain and practice safe travel protocols. A lot of uncertainty exists in our current snowpack.

SUMMIT LAKE / JOHNSON PASS: A variety of weak layers exist in the snowpack and human triggered slab avalanches 1-3′ thick remain possible. Choose terrain wisely.

LOST LAKE: We received a report of a human triggered avalanche yesterday, which occurred on Saturday on Exit Glacier trail. New snow and winds over the weekend have elevated the avalanche danger near Seward and Lost Lake. This area has very little snowpack info and extra caution and conservative decision making is necessary.

2. Moderate
Alpine
/ Above 2,500'
Avalanche risk
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
2. Moderate
Treeline
/ 1,000'-2,500'
Avalanche risk
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
1. Low
Below Treeline
/ Below 1,000'
Avalanche risk
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

Special Announcements

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Avalanche Problem 1
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.
  • TYPE
    Persistent Slabs
  • Chance
    Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size
    Historic
    Very Large
    Large
    Small

If you are headed to Turnagain Pass you get to start your day with a red flag warning, a recent avalanche. Yesterday two skiers remotely triggered an avalanche on the Southwest face of Eddies from 30’ away on a flat ridge. They had just dug a pit and found the culprit, buried surface hoar, one foot below the surface. Multiple tracks were present along the ridge before the avalanche released no obvious signs like whumpfing were observered.

This is the same buried surface hoar “MLK BSH” we’ve been talking about for two weeks since it was originally buried on Matin Luther King Day. This weak layer is widespread across our forecast zone and has been responsible for numerous large human triggered avalanches over the last 10 days, all in Seattle Creek drainage, and not visibile from the road. So far no one has been caught or carried and most of these avalanches have been triggered remotely from an adjacent ridgeline. Yesterday was the first human triggered avalanche on the non-motorized side of the road. Today’s sunshine and soft snow could make it tempting to push into steeper terrain. Keep in mind incremental loading over the last week and period of stronger winds on Monday may have added more stress to this weak layer. Don’t forget this is a challenging avalanche problem to evaluate and manage, and warrents extra caution.

Keep in mind as you travel today:

  1. The weak layer is around 2-3 feet under your feet or snowmachine, but triggering this weak layer will be more likely in a thinner part of the slab.
  2. No obvious signs of instability like “Whumpfing” are expected until a slope releases.
  3. Previous tracks on a slope do not mean its safe. The 2nd, 3rd or 10th person could find a thin spot and trigger a large slab that propagates across an entire slope.
  4. Remotely triggering a slab from the ridge, sides or below is possible.
  5. Use safe travel protocols: One at a time in avalanche terrain, Stop in safe zones, Watch your partners and Avoid terrain traps.
  6. Assess the consequences if the slope slides. The larger the terrain = the larger the potential avalanche!

South of Turnagain – Johnson Pass/Summit Lake zone: Areas south of Turnagain Pass harbor a thinner, weaker snowpack with multiple weak layers present, including the MLK buried surface hoar. This area also received additional snow on Sunday and Monday and elevated Easterly winds. Similar to Turnagain Pass an avalanche triggered in this zones could propagate an entire slope and be large enough to bury or kill a person.

Remote triggered avalanche on Eddies occured yesterday afternoon after mulitiple people had skied the ridge.   

 

Avalanche Problem 2
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.
  • TYPE
    Cornice
  • Chance
    Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size
    Historic
    Very Large
    Large
    Small

CORNICES: We had a report of a large cornice fall in upper Seattle Creek drainage over the weekend. Give cornices an extra wide berth as they often break farther back than expected.

LOOSE SNOW AVALANCHES (dry and wet): Daily warming and radiation from the sun could warm surface snow on steep Southerly aspects. Keep your eye’s out for roller balls or small wet-loose avalanches under rocks. On shaded aspects dry loose “sluff” is possible in steep terrain.

WIND SLABS: There is a lot of snow available for transport. Ridge top winds should remain light from the Southeast, but winds could puff up along some localized upper elevation zones. Should you see drifting snow, expect any recently wind loaded feature to be tender. Any wind transport will also be adding stress to a larger more dangerous avalanche problem, where triggering a small wind slab could step down into a deeper more dangerous layer.

Additional Concern
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. The 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.
  • TYPE
    Glide Avalanches

Glide cracks are opening again. We know of one glide avalanche that has released recently in the Summit zone just north of Manitoba. Look out for glide cracks and limit exposure under them!

Mountain Weather

Yesterday: Thick valley fog was observed below 2000′. Above this elevation skies were partly cloudy. Temperatures remained around 30F near sea level and mid 20Fs near ridgetops. Ridgetop winds were light and variable. No new precipitation.

Today: Valley fog will continue through the morning and may burn off in some areas by the afternoon. Above the fog skies will be partly to mostly sunny. Daily warming will cause temps at sea level to increase from mid 20F’s to low-30Fs. Upper elevation temps will be in the mid to upper 20F’s. Ridgetop winds from the Southeast will range from 5-15mph with gusts in the low 20’s mph. No precipitation is expected.

Tomorrow: A similar day is in the forecast with partly sunny skies. Daily warming and mild temperatures in the mid to upper 20F’s is expected. Valley fog is possible. No new precipitation is expected and ridgetop winds should remain light.

*The Seattle Ridge anemometer (wind sensor) was destroyed. We have a replacement on the way and it should be operational by mid February.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 26   0   0   58  
Summit Lake (1400′) 20   0   0   27  
Alyeska Mid (1700′) 25   0   0   52  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 24   variable   2   12  
Seattle Ridge (2400′) 26   *N/A     *N/A   *N/A