ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Wednesday December 5th, 2018
Posted by Wendy Wagner on 12/05/18 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.

The avalanche danger remains  MODERATE  above 2500′ in the Alpine. As easterly winds continue, triggering a wind slab in steep terrain will be possible.  Additionally, triggering a slab avalanche 1-3′ thick, releasing on buried surface hoar, is still possible though trending to a lower likelihood.

2. Moderate
Alpine
/ Above 2,500'
Avalanche risk
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
1. Low
Treeline
/ 1,000'-2,500'
Avalanche risk
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
0. No Rating
Below Treeline
/ Below 1,000'
Avalanche risk

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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

Sustained moderate winds from the east have been blowing for four days and will remain, to varying degrees, over the region again today. These winds have redistributed the snow and formed wind slabs and not-so-fun crusts is many areas; see Monday’s field report from Turnagain HERE. Wind slabs could be found in steep terrain and could be hard enough, and stubborn enough, to allow a person on to them before popping loose. Look for stiff, pillowed snow, cracking and listen for hollow, drum-like sounds. 

Wind effect along Sunburst Ridge looking north toward Tincan Ridge.

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

At the upper elevations we are keeping tabs on a thin layer of buried surface hoar sitting 1-3′ below the surfaceAn observer Sunday found this layer to still be reactive in a snowpit on Sunburst right around 2500′. Monday at 3100′ on Sunburst there were no results testing this layer but it was very easy to spot laid over in the snowpack. Side note, this layer was unreactive last week at 3,150′ on Tincan as well. The concern is finding a slope with buried surface hoar that is still intact, upright and reactive enough that it propagates into an avalanche. At this point obvious signs of instability may not be seen but some lingering suspicion is advised even as the likelihood decreases. As always use safe travel protocol and choose terrain with consequences in mind. For example, where is the avalanche path and where would I end up if the slope slides? 

 

Pastoral Peak, looking east from Sunburst through Taylor Pass. Note the crown of a large avalanche under the rock band on Pastoral – earthquake triggered slab on Nov 30th. This avalanche likely released on the Nov. 23 buried surface hoar. 

 

Surface conditions below 1,500′ at Turnagain Pass.

Mountain Weather

Yesterday:   Mostly cloudy to obscured skies were over the area. Ridgetop winds over the past 24 hours have been moderate (10-20mph) from the east with gusts up to 40mph. Temperatures have climbed overnight and sit in the mid 30’sF at 1,000′ and the upper 20’sF along ridgetops this morning.

Today:    Partly cloudy to cloudy skies are expected with a chance for a few raindrops below 1,500′ and snowflakes above 1,500′. No measureable precip is forecast. Ridgetops winds will remain easterly in the 15-20mph range with gusts to the 40’s. Temperatures should remain warm, with daytime highs up to 38F at 1,000′ and 30F along ridgetops.  

Tomorrow:    Unsettled warm weather is forecast to continue with rain and snow showers picking up Thursday afternoon and intensifying on Friday. Our friends at the NWS said this about the upcoming system  The track of the storm favors western Prince William Sound/eastern  Kenai Peninsula for some of the heaviest precipitation with strong  east to southeast upslope flow. Warm air accompanying the low  means most of this will be in the form of rain”. The rain/snow like for Friday looks to push into the 2,000′ plus range.  

 *Seattle Ridge wind sensor is rimed over. Alyeska Mid Wx Station and Summit Lake Snotel snow depth sensor are not functioning.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 33   1   0.1   13  
Summit Lake (1400′) 29   0   0   5 (estimate)  
Alyeska Mid (1700′) 32   trace   0.23   0.2  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 25   NE   15   42  
Seattle Ridge (2400′) 28   *no data   *no data     *no data