ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Tuesday January 22nd, 2019
Posted by Aleph Johnston-Bloom on 01/22/19 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.

A generally  LOW  avalanche danger exists across all elevations bands for the Turnagain area today. Triggering a slab avalanche is unlikely but not impossible. As winds increase and snow falls the danger may rise to MODERATE in the Alpine late in the day. Pay attention to changing conditions and expect the avalanche danger to increase overnight. Avoid/limit your exposure time under glide cracks, give cornices a wide berth and watch your sluff.

SUMMIT LAKE / JOHNSON PASS:    A poor snowpack structure exists in this area, which is very different than Turnagain Pass. Triggering a slab avalanche is trending toward unlikely, yet may not be out of the question. Evaluate the snowpack and terrain.  

 

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.
1. Low
Below Treeline
/ Below 1,000'
Avalanche risk
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

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

The bulk of the approaching storm is not forecast to impact the area until late tonight into tomorrow. Yesterday we received a trace of snow which fell on top of surface hoar and near surface facets that formed over the past weak. This barely buried weak surface snow could become a concern as snow accumulates, especially in terrain that has a melt-freeze crust or wind hardened snow below the weak snow. Today it will be important pay attention to the weather and changing conditions. Shallow storm slabs may quickly form in the Alpine as snow falls and the wind increases.  Watch for blowing snow, cracking below your skis or machine and use small test slopes to assess whether the new snow is bonding to the old snow. 

Finding an old isolated slab especially in upper elevation terrain in Girdwood, Portage or Placer is not completely out of the question. This part of the advisory area received more snow in the last storm. There has been one known human triggered avalanche that failed in the facets below the new snow, which was last Thursday in the Placer ValleyLow danger before the storm does not mean no danger. Practice good travel habits, such as exposing one person at a time, watching your partners and grouping up in safe zones are key ways to minimize risk. Ease into steep terrain and factor in the consequences should you encounter one of the following:

  • An outlier slab avalanche: 
    • Triggering a slab avalanche would most likely occur on an exposed ‘unsupported slope’ that sits above a cliff or steep rocky terrain. An old wind slab or a pocket of buried near surface facets and/or surface hoar 5″-2′ below the surface may be lurking in an isolated area.
  • Glide avalanche:  
    • Identify glide cracks and avoid spending any time under these features. Glide avalanches are completely unpredictable and not human triggered.
  • Cornice fall: 
    • Remember cornices often break farther back from ridges than expected. Give them a wide berth.
  • Loose Snow avalanche (sluff):
    • Be aware of fast moving surface snow in steep terrain. 


Treeline surface hoar on Tincan, 1-20-19. Photo: Troy Tempel

Additional Concern
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

South of Turnagain – Johnson Pass/Summit Lake zone: poor snowpack structure exists in these areas.  Multiple mid-pack weak layers of facets and buried surface hoar have been found as well as a facet/crust combination in the bottom of the snowpack. No recent avalanche activity and calm weather has allowed the pack to slowly adjust. However, it still important to evaluate the terrain and snowpack. Upper elevation terrain with hard, wind-affected snow over the buried weak layers is the most suspect. 

Mountain Weather

Yesterday:  Skies were mostly cloudy skies with very light showers during the day. Easterly winds were 5-15 mph with gusts into the 20s. Temperatures were in the 20Fs to high teens. Overnight temperatures increased slightly.  

Today: Mostly cloudy skies and snow showers likely in the afternoon. Easterly winds will increase gusting into 30s during the day and 40s overnight. Temperatures will be in the 20Fs and low 30Fs. Rain/snow line forecast to be around 800′.  Precipitation will increase overnight.  

Tomorrow:  Snow with rain at lower elevations likely. Easterly winds increasing throughout day with gusts into the 70s and temperatures in the 20Fs and 30Fs. As this series of fronts impact the region expect temperatures to rise and periods of heavy precipitation especially near the coast through Thursday. Another storm system is on track for Friday.  

*Seattle Ridge weather station was heavily rimed and the anemometer (wind sensor) was destroyed. We are currently working to replace it.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 28   trace    0 50  
Summit Lake (1400′)  23     trace     0   21  
Alyeska Mid (1700′) 27   trace    0.07 35  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 19   NE    7 24  
Seattle Ridge (2400′)  24     *N/A   *N/A   *N/A