ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Monday December 10th, 2018
Posted by Aleph Johnston-Bloom on 12/10/18 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. As easterly winds continue today triggering a wind slab in steep leeward terrain will be possible. Watch for changing conditions. If snow amounts are higher than forecast expect danger to rise to CONSIDERABLE  and human triggered avalanches to be likely.  

SUMMIT LAKE:    A  very weak and  shallow snowpack exists  under the recent storm snow. Slab avalanches 1-2′ deep may to be easy to trigger on steeper slopes.

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.
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

Looking for avalanche courses or evening presentations? Check out our  calendar page! There are two CNFAIC evening discussions coming up – these are FREE and a great way to get your head back in the avalanche game. This week –  ‘Tales from the Pit’ at Blue & Gold Boardshop in Anchorage!

 

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

Easterly winds blowing 15-30 mph with gusts into the 50s overnight will have moved any soft storm snow left available to transport into leeward areas. Additional snow falling today may also get blown around with 1-4″ in the forecast. Wind effect from the very strong winds during the wet storm on Friday and Saturday was evident in the Alpine yesterday. However, there was minimal signs of wind slab. Today that may have changed, wind-loaded, steep unsupported slopes above 2500′ may be tender.  Look for stiff, pillowed snow and cracking and listen for hollow, drum-like sounds. Loading patterns can be very localized and it is crucial to look for clues indicating where the snow is being distributed. 

Wind effect on the Tincan uptrack, 12-9-18

 

Additional Concern
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

Observations yesterday in Turnagain Pass found signs that the storm snow is bonding well to the old snow surfaces. We do not have evidence that there currently is a persistent weak layer issue in the this core area of the advisory zone. Above 2,500′ there is a thin layer of buried surface hoar anywhere from 2-5′ deep in the snowpack but it is not showing signs of reactivity and at this point is being regarded as a dormant layer to track.  As we move south and the snowpack is shallower, the potential to trigger a persistent slab avalanche on weak snow near the ground increases. Observations prior to the storm showed weak facets in Summit Lake. If traveling in this area the likelihood of triggering a dangerous avalanche remains. Choose terrain carefully.

Red flags to watch for:
–  Recent avalanches 
–  Whumpfing (collapsing) of the snowpack, a sign to avoid avalanche terrain.
–  Shooting cracks, likely to be seen near ridgelines where the wind has formed wind slabs.

 

Summit snowpack observation: Tenderfoot, 12-3-18

Mountain Weather

Yesterday:  Skies were mostly clear and sunny. Temperatures were in the 20Fs to mid teens. Winds during the day were light and variable. Overnight easterly winds picked up blowing 15-30 mph with gusts into the 50s. Temperatures initially cooled into the evening and then rose again in the early morning into the 20Fs and low 30Fs.  

Today:  Cloudy skies and snow showers throughout the day. 1-4″ of snow above 300′.   Easterly winds 10-20 mph with gusts into the 30s. Temperatures in the low 30Fs to mid 20Fs. Continued snow showers overnight with snow to sea level, 2-5″.  

Tomorrow: Continued snow showers with more cold air moving into the region Tuesday night. From the National Weather Service:  The colder air which moved across the Gulf and Southcentral  yesterday was the beginning of what appears to be a prolonged  shift to a more “typical” Alaska weather regime for this time  of year.  There are a number of features in the forecast that we will keep tabs on for the week! Stay tuned. Timing and precipitation amounts are still uncertain.  

*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  26    
Summit Lake (1400′) 19    0     0   4  
Alyeska Mid (1700′) 24   0   0  

8  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 14   NE   12   53  
Seattle Ridge (2400′) 20   *no data   *no data     *no data