ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Thursday January 3rd, 2019
Posted by Heather Thamm on 01/03/19 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 is  MODERATE in the Alpine and Treeline zones for triggering a large slab 2+’ thick in steep terrain. Give cornices a wide berth and avoid being under glide cracks. Natural glide avalanches are possible today and could release without warning.

There is LOW avalanche danger below 1000′ where a surface crust has strengthened the snowpack.

SUMMIT LAKE / JOHNSON PASS / LYNX DRAINAGE:    South of Turnagain Pass, keep in mind old buried weak layers exist and there is potential for triggering a large slab avalanche that breaks near the ground.

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

In Chugach State Park and Hatcher Pass dangerous avalanche conditions have been reported this week. Check out the Hatcher Pass Avalanche Information Center mid-week summary click  HERE and recent observations from the Front Range HERE.

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

UPDATE: We just recieved a report of a large remote triggered avalanche in the Seattle Creek drainage that occured yesterday. We don’t have much info at this time and its unknown what layer this avalanche failed on. We do know that remote triggered avalanches are a sign of a persistent slab problem and it may be buried surface hoar. Keep this new info on your mind if you’re heading to Turnagain Pass.  

Cooling temps and light winds are helping to improve stability of wind slabs since the New Year’s storm dumped 2-3’ of snow and blasted our snowpack with hurricane force winds. Clear and sunny weather today will make it easy to identify smooth pillowed convexities, cross-loaded gullies, and hollow sounding snow – wind slab habitat. What makes this avalanche problem challenging is it’s transition into becoming a persistent slab. As this storm snow problem strengthens, a layer of buried surface hoar from Christmas remains on our minds. Hand pits yesterday were challenging to find this layer due to how deeply buried (2+’) it is in places. Many observations over the last week have been documenting the location of buried surface hoar and its presence and reactivity have been variable. With that said – triggering a wind slab on a mid-storm density change or on buried surface hoar are both possible today.

If you’re headed out, ease into steeper terrain with a conservative mindset. Evaluate terrain and snow as you travel and remember ‘whumpfing’ is an obvious clue of instability.

CORNICES:  There are some very large cornices along many ridgelines across our region. Triggering a cornice with the weight of a person or snowmachine is possible today. Remember these can break further back than expected. Give cornice features lots of space and avoid being directly under them.

 

A storm triggered slab from the New Years storm below Hippy Bowl on SW aspect of Tincan. Also note the wind sculpted snow in the alpine and cornice along the ridgeline.  

 

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

Glide cracks exist in popular ski and snowmachine terrain and a may be tricky to identify with new snow covering them. Several glide cracks have avalanched this week and it’s possible more will release in the coming days. The best way to manage this problem is to avoid being under slopes with cracks opening up. They can release at any time and are not typically associated with human triggers. Glide avalanche have occurred in Lynx Creek, on Lipps, and Seattle Ridge this week.

 

 

A glide avalanche on Lipps SW face that released just before the New Years Storm is not covered by new snow and looks very different

Photo of Lipps glide crack taken yesterday (1-2-19). Although part of this crack has released the additional portion can still avalanche without warning.  

 

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:  A shallow and poor snowpack structure exists in the Summit Lake zone. Buried weak layers of facets associated with crusts sit near the base of the snowpack. An observation from Lynx Creek on Friday also found a reactive layer of facets mid-pack. Keep in mind Summit Lake has received additional loading from the New Years storm – strong winds and a foot of new snow. Recent avalanche activity and ‘whumpfing’ will be good reminders to keep terrain choices conservative in these zones. Triggering a larger avalanche that releases near the ground is not out of the question. Check out the Summit observations HERE for the more snowpack details.

Mountain Weather

Yesterday: Snow showers were observed in the morning with a trace of new snow at Turnagain Pass. Rain/snow line was near 300′. Light to moderate ridgetop winds from the East shifted to a NW direction by mid-day. Temperatures cooled in the upper elevations to low 20F’s/upper teens (F) as skies cleared in the afternoon.  

Today: A cooling trend will continue today as a high-pressure system establishes itself over Southcentral, Alaska. Expect temps at 3000′ to reach low teens/upper single digits today. Temperatures near sea level will drop into low 20F’s to teens by this evening. Northwest ridgetop winds will be in 5-15mph range. No precipitation is expected.

Tomorrow: A similar pattern of clear skies, cold temps and light NW winds will continue into the weekend.

*Seattle Ridge weather station was heavily rimed and the anemometer (wind sensor) was destroyed.    

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 25   trace   0   63  
Summit Lake (1400′) 23   0   0   22  
Alyeska Mid (1700′) 25 1   0.12   50  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 16   NW   5   26  
Seattle Ridge (2400′) 21   *N/A   *N/A     *N/A