ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Sunday December 16th, 2018
Posted by Aleph Johnston-Bloom on 12/16/18 at 7:00 am.
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. Human triggered avalanches are possible. Be on the lookout for wind slabs in steep, wind-loaded terrain. Pay attention to snowfall intensity today. The danger could rise to CONSIDERABLE as storm slabs develop. Additional concerns: watch your sluff in steep terrain  and avoid traveling underneath  glide cracks.

GIRDWOOD VALLEY:  The storm that moved through Friday night/Saturday morning favored the Girdwood Valley. Look for wind slabs to be thicker and sluffs to be larger.  

SUMMIT LAKE:    Additional new snow today may overload weak layers near the ground, increasing the chance a person could trigger a larger slab avalanche.  

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

New snow and wind has increased avalanche conditions in zones outside of the Chugach National Forest. #knowbeforeyougo

Avalanche Stories from Sunburst at Powder Hound Ski Shop, Girdwood €“ FREE.  Join Chugach National Forest Avalanche Forecaster, Heather Thamm, for an evening discussion on avalanche safety and awareness. This talk will cover some basic things you need to know before going into the backcountry. Expect to hear lessons learned and stories from Sunburst in Turnagain Pass. This talk is geared towards any level of backcountry experience, novice to the seasoned Powder Hound.

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

Observers yesterday reported triggering small wind slabs in steep, wind-loaded terrainLook and feel for stiff, pillowed snow and cracking and listen for hollow, drum-like sounds. Loading patterns can be very localized and it may be hard to tell where the wind effect is as new snow today covers up the evidence.  There is a lot of soft snow available for transport and more on the way. Winds today could form new wind slabs and/or increase the depth of existing ones. Keep in mind the consequences of the terrain should even a small slab pull out from underneath you.  

Loose Snow Avalanches: In steep terrain where winds aren’t an issue sluffs may be large enough to knock you off your feet and carry you. 

Storm Slabs: Pay attention to changing conditions and to how the new snow falling today bonds to the snow below. If the snow is cohesive enough shallow storm slabs may form. Temperatures are forecast to be warmer than the previous storm so slabs may form more quickly with the density difference. Hand pits and small tests slopes will be useful ways to assess bonding.

Wind affected surface snow on Seattle Ridge. 12-15-18

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

We are tracking buried layers of facets and crusts that sit 1-3′ under the snow surface. These layers are more prevalent in the mid-elevations (2000’ – 2700’). Snow pit data and a lack of avalanche activity has been pointing to an unlikely chance for an avalanche releasing deeper in the pack. Additional snow load especially in the Summit area could start to tip the balance. Evaluate the snowpack and terrain as you travel. The thinner the snowpack the more suspect it is. Observers keep finding spots that whumpf on facet/crust combinations reminding us not to forget about this potential avalanche hazard.

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

Glide cracks are beginning to open up in many places, including Sunburst’s SW face under the weather stationSW face of Tincan Proper, Gold Pan area (behind Cornbiscuit/Magnum) and the one shown below in the Johnson Pass area. These cracks can release at any moment, as this one did below in Johnson Pass. They are not associated with human triggers and the best way to manage the hazard is to avoid being on or beneath slopes with cracks. 

Johnson Pass area glide avalanche, 12-13-18. Photo:Matt McKee.

Mountain Weather

Yesterday:  1-2″ of snow fell in the morning before the system moved out around 9 am.  Skies were overcast, winds were easterly 10-20 mph with gusts into the 40s. Temperatures were in the 20Fs. Overnight winds were light and temperatures remained in the 20Fs.  

Today:  Skies will be overcast becoming obscured as the next storm moves in. 2-7″ of snow is forecast to fall with a rain/snow mix possible at sea level. Temperatures will be in the mid 20Fs to low 30Fs. Winds will be easterly 15-25 mph with gusts into the 30s. Overnight an additional 5-10″ of snow is forecast with temperatures staying in the 20Fs to low 30Fs.  

Tomorrow:  Snow showers continue with calm winds and temperatures dropping into the low 20Fs as cooler air moves into the region by the afternoon/evening. Cooler temperatures and a chance of snow are the theme for the week. We will pay close attention to the next low developing over the Aleutians. Stay tuned!  

*Seattle Ridge anemometer (wind sensor) is rimed over and not reporting.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′)  24     1   0.1    31
Summit Lake (1400′) 21   0        0        11  
Alyeska Mid (1700′) 24   2   0.2  21

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 20   NE   12   41  
Seattle Ridge (2400′) 24 *no data   *no data   *no data