ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Thursday December 27th, 2018
Posted by Heather Thamm on 12/27/18 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 triggering a cornice or a wind slab avalanche in terrain above 1,000′. Identify ridgelines with cornice features and give them a wide berth. Pay attention to changing weather, shooting cracks or blowing snow €“ these will be obvious clues wind slabs are touchy. Glide cracks are still opening and releasing, minimize exposure under them.  Keep in mind that limited visibility could make navigating these hazards challenging today.

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 have limited information from Johnson Pass and Lynx Creek and would love any info from these areas. Consider submitting an observation  .

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 your 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

For the Hatcher Pass Avalanche Center mid-week summary click 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

Several inches of new snow (2-6”) is possible today and ridgetop winds are expected to be light. Should you see more snow than forecasted or an increase in winds be ready to adapt to any changing weather. Storm slabs are unlikely unless we see over 6″ of new snow.

CORNICES: Today new snow may obscure visibility. Having some familiarity of known cornice hazards is helpful, but even so, it could be challenging to see them today. Earlier in the week cornices were quite active. A few close calls occurred on Sunday where several people triggered large cornices including one person who went for a ride without any reported injury. Two days ago we saw a period of strong winds and this may have added more weight to some already large cornices. Give cornices a lot of space and be aware of groups traveling in the same zone as you who could inadvertently triggered one from above.

WIND SLABS: Triggering a lingering wind slab is possible on steep leeward terrain features. A period of strong winds on Tuesday formed shallow wind slab that may be on weak snow or buried surface hoar. Feel for denser snow on top of loose snow that feels upside down and be weary of supportable hollow sounding snow.  Although winds are expected to be light, don’t be caught off guard if you see any sign of blowing snow or shooting cracks. These will be obvious clues wind slabs are tender.

This photo is from Monday (12-24-18) and is a good example of how difficult it is to see cornices until you’re too close. This cornice fall triggered a wind slab pocket below. Photo by Wendy Wagner.

 

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

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 evaluate terrain for consequences before selecting a route to travel. 

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

Evidence of two glide avalanches (actual releases, not just cracks) were observed in the Lynx Creek drainage on Monday. Numerous glide cracks have been opening up in more commonly traveled areas of our forecast zone including the SW aspect of Sunburst and Corn Biscuit, and in Warm-up (-1) Bowl on Seattle Ridge. It is important to remember glide cracks can release into full-blown avalanches at any time and are not associated with human triggers. The best way to manage this problem is to keep your eyes peeled for cracks and limit travel underneath them.

Photo taken yesterday (12-26-18) of glide cracks on the SW face of Corn Biscuit. Photo by Andy Moderow. 

Mountain Weather

Yesterday: Skies were overcast. Temperatures were in the mid 30F’s at sea level. Freezing line was near 1000′ and temperatures at ridgetops were in the low to mid 20Fs. Ridgetop winds were light to moderate from the Northeast. No precipitation was recorded.

Today: Cloudy skies and a 2-6 € of snow (0.37 Snow water equivalent) is for forecasted today. A slight cooling trend will help bring rain/snow line from 1000′ to just above sea level (200′) by this evening. Temperatures at 1000′ will be in the low 30Fs to upper 20Fs. Expect ridgetop winds to by light 5-15mph from the East.

Tomorrow: A period of clearing skies is expected overnight through tomorrow morning with another chance for scattered snow showers Friday evening into Saturday. Temps will be in the mid to upper 20Fs at 1000′ and rain/snow line may be right around sea level. Easterly ridgetop winds will be light to moderate.  

*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′) 31   0   0   56  
Summit Lake (1400′) 30   0   0   12
Alyeska Mid (1700′) 31   0   0   34  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 22   ENE   9   28  
Seattle Ridge (2400′) 27   *N/A   *N/A     *N/A