ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Wednesday December 26th, 2018
Posted by Wendy Wagner on 12/26/18 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.

A  MODERATE  avalanche danger remains for cornice falls and wind slab avalanches in terrain above 1,000′. Several people have accidentally triggered cornice falls in the past few days and today cornices are likely to be just as unstable – give cornices a wide berth. Additionally, watch for fresh wind slabs formed by yesterday’s increase in wind. Glide cracks are still opening and releasing, limit exposure 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.
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

Cornice falls have been the most prevalent avalanche problem during the past several days. Wind slabs on the other hand have been relatively small and from what we know, triggered predominantly by cornice falls. If you are just tuning in now, take a look at the past few days of observations (several close calls and one person who did go off with a cornice but is alright). A few inches of new snow in the past 24-hours, sustained moderate to strong winds and relatively mild temperatures all contributed to growing cornices yesterday. If skies open for travel into the Alpine and along ridgelines, take stock of the cornices around you and give them an extra wide berth, why not? Also, watch for who’s above you, could they inadvertently trigger a cornice that sends debris over you? 

WIND SLABS:   Expect to see some fresh wind slabs from yesterday’s winds, even if new snow amounts are low, there is plenty of snow to transport left over from the Solstice Storm. Wind slabs are likely to be shallow, up to a foot thick, and could be touchy as the underlying surface could be loose snow or surface hoar in cases. Watch for shooting cracks, hollow sounding snow and be especially aware of the terrain – if a wind slab does release where will you go?

 

Recent cornice fall triggered small windslab and loose snow below on SW face of Magnum on Monday – also note glide cracks below rocks.

 

Surface hoar that formed on Sunday and Monday is now covered by 2-5″ on new snow, maybe more. 

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

Slab avalanches breaking in weak layers deeper in the snowpack? If you are headed to areas south of Turnagain, keep in mind triggering a large slab avalanche is possible. Buried weak layers, roughly 2′ below the snow surface, have been found in the Summit Lake zone and possibly 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. 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. 

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

On Monday two glide avalanches (actual releases, not just cracks) were observed in the Lynx Creek drainage. One of these clearly visible in the photo below as a brown streak. Observers have reported the glide cracks on Sunburst and in Warm-up (-1) Bowl are opening and people continue to see new cracks throughout the region. It is important to remember glide cracks can release into full-blown avalanches at any time and are not associated with human triggers. Keep your eyes peeled for cracks and limit travel underneath them.

Glide avalanches in Lynx Creek drainage. The slide in the middle of the photo is located in a very common zone to snowmachine.

Mountain Weather

Yesterday:   Mostly cloudy skies were over the region with light rain up to 500′ and light snow above this. Turnagain Pass saw 2-4″ of new snow, while Girdwood looks to have picked up around 5″ (mid-elevation snow stations). Ridgetop winds averaged 20-30mph for the past 24-hours from the east. Winds have quieted down to ~10mph as of 6am. Temperatures are steady in the 20’sF along ridgetops and 32F at 1,000′

Today:   Instability showers will be over the region with mostly cloudy skies and a chance for a trace of snow above 1,000′ and rain below this. There is also a chance for clearing skies and some blue holes if the front parked over the Kenai and Girdwood area heads north during the daylight hours (keep track on the satellite). Ridgetop winds are expected to be generally light, in the 5-15mph range from the east. Temperatures should remain in the 20’sF along ridgetops and 32F at 1,000′.

Tomorrow:    Another low-pressure system will move in for Thursday, this one wrapping in some warm air. Timing and temperature with this storm is still uncertain, however it looks like snow might fall to 1,000′ but not to make it to sea level. Precip amounts at his point are in the 0.5″ of water range. 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′) 30   3   0.2   58  
Summit Lake (1400′) 30   0   0   12
Alyeska Mid (1700′) 31   5   0.33   36  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 22   ENE   23   49  
Seattle Ridge (2400′) 27   *N/A   *N/A     *N/A