ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Monday December 24th, 2018
Posted by Aleph Johnston-Bloom on 12/24/18 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 cornice fall or  a wind slab avalanche will be possible in areas exposed to Saturday’s winds. Glide cracks are opening and could release at anytime, avoid travel under them.

JOHNSON PASS / LYNX DRAINAGE / SUMMIT LAKE:    South of Turnagain Pass 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 information from these areas. Please consider  submitting an observation  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

Heading to Hatcher Pass? Check out the Hatcher Pass Avalanche Center Saturday morning forecast  HERE  and recent observations HERE.

Avalanche Problem 1
Cornice
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
  • TYPE
    Cornice
  • Chance
    Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size
    Historic
    Very Large
    Large
    Small

We received multiple reports of cornice falls being triggered yesterday. One was triggered by two snowmachiners walking out too far onto the Warm-up Bowl corniced ridge. From what we know one person went down with the cornice chunks but was ok. Another cornice fall was triggered skinning along the Tincan ridge towards Tincan Proper and one was dog triggered along the Sunburst ridge. Luckily no one was injured in these and the cornice falls did not trigger avalanches on the slopes below.  Remember to give cornices a wide berth on the ridge as they can break off further back than expected and potentially take you for a nasty ride. They could also trigger a wind slab or larger avalanche below, creating an even bigger problem. It is also important to limit exposure time below them especially with so many people out enjoying the snow this holiday week. 

WIND SLABS:  Saturday’s sustained NE winds that were strong enough to move snow around and form cornices also potentially created wind slabs on steep, unsupported slopes. There were reports of observers yesterday finding small pockets of wind slab. Continue to be on the lookout for these today. At this point they may be stubborn and could allow a person well out onto them before releasing. Watch for shooting cracks, hollow sounding snow and be especially aware of the terrain – if a wind slab or cornice does release where will you go?

Cornice fall triggered skinning along Tincan Ridge towards Tican Proper, 12-23-18. Photos: Owen Smith

 

Cornice diagram from the Avalanche.org encyclopedia. Note the potential fracture zone on the 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

Buried weak layers roughly 2′ below the snow surface have been found in areas south of Turnagain Pass in both the Summit Lake zone and as far south as Lost Lake.  We suspect the snowpack may be similar around Johnson Pass, Lynx drainage and Twin Peaks/Silver Tip. These weak 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.  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. 

Slab avalanches just south of Turnagain Pass that occurred during the Solstice Sleeper Storm believed to have released on the crust/facet set-up. 12-20-18

 

 

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

Watch 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. There are glide cracks are opening up on the SW faces of Tincan and Sunburst as well as the Johnson Pass area and Gold Pan in upper Bertha Creek. 

Glide cracks and cornices in Gold Pan, 12-23-18. Photo: Peter Biskind. 

Mountain Weather

Yesterday: Skies were clear with some valley fog. Temperatures were in the 20Fs and winds were light. Overnight temperatures were in the teens and low 20Fs. Winds remained light.  

Today: Skies will be partly sunny with temperatures staying in the teens to low 20Fs and winds continue to be light. Temperatures drop into the low teens overnight.  

Tomorrow: Skies will be cloudy, temperatures will be in the high 20Fs and there is a chance of Christmas snow showers as a front moves into the area. We are paying attention to the next series of lows developing that may impact the region later in the week! Stay tuned.  

*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′)  24    0  0 61  
Summit Lake (1400′)  18       0    0

13    

Alyeska Mid (1700′) 24  0   0   33  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 24    SE 4   14  
Seattle Ridge (2400′) *N/A   *N/A   *N/A   *N/A