ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Saturday December 29th, 2018
Posted by Wendy Wagner on 12/29/18 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 is  MODERATE  above treeline where increased winds last night may have formed sensitive wind slabs on lee aspects. Any fresh wind slab found is likely to be soft and shallow, yet could be easy to trigger. Cornices are still a concern and likely grew with last night’s wind. Glide cracks are moving! Please keep a close eye out for these brown cracks and limit travel underneath them.  

SUMMIT LAKE / JOHNSON PASS / LYNX DRAINAGE:    South of Turnagain Pass, weak layers of facets have been found 1-3′ below the surface. These layers are suspect to be difficult to trigger but not out of the question. If someone hits the right spot, a large avalanche may result. Heads up in these thinner snowpack zones.

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.

Special Announcements

Hatcher Pass:   Problems remain in the snowpack at Hatcher Pass, please check this recent observation  of a remote triggered avalanche yesterday and HPAC’s Saturday morning forecast HERE if headed this way.

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

A few inches of new snow plus some strong wind overnight and we can expect to see pockets of wind slabs today. These are likely to be soft, shallow and found above treeline where the easterly winds have been blowing. Over the past few days, several inches of new snow has accumulated with an additional 2-4″ overnight at the higher elevations. Winds have decreased this morning and another 2-4″ of snow is forecast for today. This new layer of snow could obscure the wind effect from last night – so we’ll need to be on our guard for hidden wind slabs if skies clear enough for travel into the Alpine.

Quick hand pits and feeling for stiffer snow over softer snow are good ways to check for wind slabs or any other surface instability. A couple thin layers of buried surface hoar from the past several days intermixed in the soft surface snow may make any wind slab that forms more touchy. 

CORNICE FALLS:  There could still be large cornices that are close to breaking off and only need a person’s weight to tip the balance. Many cornice falls were reported last week and they continue to grow slowly with recent snow and wind. Continue to give them a wide berth and limit exposure under them. 

 

A new crop of small surface hoar seen yesterday (photo Trip Kinney). There is another layer of buried surface hoar 4-6″ deep in soft snow that was buried on Christmas. We are tracking these layers, as they could develop into future weak layers when the next real snow storm hits.

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 are moving – beware. These can release at anytime and are most suspect when we have proof to their movement, as seem below. Cracks are littering the slopes in Lynx Creek drainage and one released on Corn Biscuit in the last couple days. Others we know of are on the SW aspect of Sunburst, Corn Biscuit, Warm-up (-1) Bowl on Seattle Ridge and the SW face of Magnum. Check out this link for more information on glide avalanches. The best way to manage this problem is to keep your eyes peeled for cracks and limit travel underneath them.

 

Glide cracks moving on SW facing Magnum Ridge (Thamm, Moderow)


Glide avalanche on Corn Biscuit, seen and photographed on 12/28 by Andy Moderow.

 Glide crack in Lynx Cr. This crack is SW facing. (photo: Graham Predeger) 

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: keep in mind triggering a large slab avalanche is still possible. Buried weak layers, roughly 2-3′ below the snow surface, have been found in the Summit Lake zone. There is some uncertainty around Johnson Pass, Lynx Cr drainage and Twin Peaks/Silver Tip where the snowpack transitions to that with poorer structure as found at Summit Lake. A report from Lynx Ck yesterday found a buried layer of facets reactive (but stubborn) in the snowpack. Observations in Summit Lake over the last few weeks have been finding facets associated with crust reactive in stability tests. The most recent avalanche activity in this zone was observed last week (Dec.20) where some of these avalanches stepped down to older layers within the snowpack. Should you head into the periphery of the forecast zone, evaluate the snowpack and terrain as you travel. Listen for whumpfing — an obvious clue for triggering a slab avalanche.

Mountain Weather

Yesterday:   Clear skies gave way to overcast conditions yesterday as a small storm moved in during the evening hours. Around 2″ of new snow fell at the mid-elevations and likely up to 4-5″ and at the upper elevations. Ridgetops winds picked up last night from the east into the 20’s mph with gusts up to 40mph and have quieted down this morning. Temperatures warmed with the snowfall and have been in the 20’sF along ridgetops, 30F at 1,000′ and mid 30’s F at sea level.  

Today:   Snow showers are likely to continue through the morning, adding an additional 2-4″ of snow (possibly a rain/snow mix for sea level). Skies may clear up this afternoon and snowfall cease as the system heads north. Ridgetop winds are expected to remain relatively light, 5-15mph from the east. Temperatures should stay near 32F at sea level, 30F at 1,000′ and in the mid 20’F along ridgelines.  

Tomorrow:   Another more powerful system looks to impact the region Sunday. This will be the first in a series of storms that are expected to bring snow, wind and rain to sea level through Monday. Stay tuned!

*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′) 28   2   0.2   54  
Summit Lake (1400′) 25   1   0.1   13  
Alyeska Mid (1700′) 27   2   0.2   37  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 20    ENE 15   40  
Seattle Ridge (2400′) 25    *N/A  *N/A    *N/A