ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Sunday December 23rd, 2018
Posted by Wendy Wagner on 12/23/18 at 7:00 am.
Avalanche risk
The Bottom Line
Moderate Avalanche Danger
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

There is a  MODERATE avalanche danger for slopes above 1,000′.   Triggering a wind slab avalanche or breaking off a cornice will be possible in areas exposed by yesterday’s winds. Wind slabs could be 1-3′ thick and composed of hard snow, allowing a person onto them before releasing. Glide cracks are opening and could release at anytime, watch for cracks and avoid being under them.

JOHNSON PASS / LYNX DRAINAGE / SUMMIT LAKE:    South of Turnagain Pass proper weak layers exists under 2-3′ of snow. Human triggered slab avalanches over 2′ thick are possible on slopes over 35 degrees. We are currently gathering as much information from these areas and please consider submitting a report if you head there – thank you!

LOST LAKE:    This zone is out of the advisory area, but is also suspect for harboring weak layers 2-3′ below the snow surface due to recent reports. Triggering large slab avalanche should be on the radar here as well. Pay attention for signs of instability like collapsing and recent avalanches.

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

Hatcher Pass avalanche conditions:  Make sure and check out Hatcher Pass Avalanche Center Saturday morning forecast  HERE  if headed this direction.

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

With so much loose powder snow from the Solstice Sleeper Storm blanketing the mountains, yesterday’s winds were like an unwanted Christmas guest. The Sunburst weather station reported just over 24 hours of sustained NE winds in the 20-30+ mph range with the strongest gust at 60mph at 11am. Although winds quieted down overnight, we can expect to see a bit of damage today. Wind slabs, wind crusts, scoured zones, and hopefully many areas that were saved from the wind. Temperatures have dropped to the upper 20’sF at sea level, freezing the wet snow at lower elevations. 

Limited visibility yesterday made it difficult to assess the extent of natural wind induced avalanche activity. We can expect there were some cornice falls and wind slabs that released. We did see one of these in the Summit area on Tenderfoot (photo below) – a cornice fall that triggered a wind slab. Today, wind slabs are likely to be hard (do to the warm temperatures) and could allow a person onto them before releasing. Watch for shooting cracks and be especially aware of the terrain – where will someone go if a wind slab knocks us off our feet? 

CORNICE FALLS:  Cornices have grown and could be teetering on the brink of failure, give them a wide berth as they can break off further back than expected. They could also trigger a wind slab or larger avalanche below, creating an even bigger problem. 

 

Difficult photo, but this is a small cornice fall that triggered a small wind slab below in the Summit Lake area on Tenderfoot Ridge (Dec 22)

Avalanche Problem 2
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

Buried weak layers roughly 2′ below the snow surface have been found in areas south of Turnagain Pass, such as Johnson Pass, Lynx drainage, Twin Peaks/Silver Tip, the Summit Lake zone and Lost Lake. These layers are composed of facets associated with crusts and have been showing signs they could be reactive enough a person could trigger a large avalanche. For the first time yesterday, we found these layers not to be reactive, which is a good sign but this is only one data point and not something to hang our hats on. If you are headed to areas south of Turnagain, keep in mind triggering a large slab avalanche is possible. Listen and feel for whumpfing (collapsing of the snowpack) and look for avalanche activity from the storm that may have steeped down into the deeper layers.

This is a snowpit from the Summit Lake area on Tenderfoot yesterday. Snow height was 4′. The top 1′ is the recent storm snow that had significant wind effect above treeline. This is the first pit we’ve had that did not show failure in the facet/crust combinaiton. 

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

As visibility improves, keep a close eye out for opening glide cracks. These are likely to be oozing down the slopes with all the new snow weighing the snowpack down. Glide avalanches can release at any time and are not associated with human triggers. It’s a case of not wanting to be in the wrong place at the wrong time. We knew of many cracks opening up on the SW faces of Tincan and Sunburst as well as the Johnson Pass area and Gold Pan in upper Bertha Creek. 

Location of known glide cracks on Sunburst – under the weather station near Taylor Pass. Photo from Dec 11th.

Mountain Weather

Yesterday:   Mostly obscured skies and intermittent snow showers were over the region yesterday. Favored areas saw up to 4″ of new snow (Girdwood Valley and the north end of Turnagain Pass), while many areas only saw a trace to and inch. Ridgetop winds blew in the 30’s mph with a gust to 60 midday before quieting down significantly overnight to less than 10mph. Temperatures were in the mid 20’sF along ridgelines and 32F at 1,000.

Today:   Partly cloudy to obscured skies will be over the area. Valley fog could set in when skies begin to clear today or tomorrow. Ridgetop winds are forecast to remain light and variable and temperatures mild, mid 20’sF along ridgetops and 30F at 1,000′.  

Tomorrow:   Partly cloudy to sunny skies are on tap Christmas Eve as we continue with a break between storms. Temperatures look to cool slightly (around 32F at sea level) and models are showing another chance for a few inches of snow for Christmas Day.  

*Seattle Ridge weather station is rimed over and not recording any data.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 31   2   0.2   64  
Summit Lake (1400′) 30   0   0   16  
Alyeska Mid (1700′) 31   1   0.2   37  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 23   NE   23   60  
Seattle Ridge (2400′) *N/A   *N/A     *N/A     *N/A