ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Monday December 3rd, 2018
Posted by Aleph Johnston-Bloom on 12/03/18 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 2500′ in the Alpine. Human triggered slab avalanches up to 1-3′ thick remain possible due to a weak layer of snow under the Thanksgiving weekend storm snow. Additionally, watch for lingering wind slabs in steep wind-loaded terrain.

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.
0. No Rating
Below Treeline
/ Below 1,000'
Avalanche risk

Special Announcements

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

Avalanche Problem 1
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

In the Alpine, sitting anywhere from 1 to 3′ below the snow surface, is a thin layer of weak snow (buried surface hoar/BSH). An observer yesterday found this layer to still be reactive in a snowpit on Sunburst right around 2500′. This buried weak layer remains a concern. At this point obvious signs of instability may not be observed but the possibility of triggering an avalanche cannot be ruled out. As always use safe travel protocol and choose terrain with consequences in mind i.e. where is the avalanche path and where would I end up if the slope slides? The snowpack profile below illustrates the slab sitting over the weak layer.

 

         

 Snowpit on Sunburst, December 2, 2018. Observer: Chris Robertson 

  Snow cover looking south on Turnagain Pass, December 2, 2018. Photo: Allen Dahl

Avalanche Problem 2
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

Be on the lookout for wind slabs in steep unsupported terrain. Strong easterly winds on Saturday afternoon may have loaded leeward slopes in the Alpine. It may still be possible to trigger a lingering pocket that could be dangerous. Look for stiff, pillowed snow and cracking and listen for hollow, drum-like sounds. 

Mountain Weather

Yesterday:  Skies were overcast and there were very light rain/snow showers. Temperatures were in the mid 30Fs to mid 20Fs. Winds were easterly 10-20 mph with gusts into the 30s.  

Today:  Cloudy skies in the morning becoming partly sunny. Temperatures will be cooling from the 30Fs into the lower 20Fs as skies clear and colder air moves over the region. Winds will be northeasterly 5-15 mph with gusts into the 20s. Expect overnight temperatures in the low 20Fs and high teens. Clouds will move back in overnight.  

Tomorrow:  Chance of snow showers and increased easterly winds in the afternoon as the next front moves over the area. The pattern remains active through the week. As usual the temperatures and timing are uncertain. Stay tuned!

*Seattle Ridge wind sensor is rimed over.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 33   1 0.1 13  
Summit Lake (1400′)  29     3 0.3 2
Alyeska Mid (1700′)   33     0 0.19   N/A

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 24  NE  11 32  
Seattle Ridge (2400′)  29    *no data *no data   *no data