ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Tuesday December 11th, 2018
Posted by Aleph Johnston-Bloom on 12/11/18 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. Watch for wind slabs in steep, wind-loaded start zones. Triggering a slab avalanche is still possible. As new snow accumulates assess how well it bonds to the snow surface below.   Pay attention to changing conditions.  

SUMMIT LAKE:    A 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

If you are heading to Hatcher Pass  make sure  to check out  the Hatcher Pass Avalanche Center’s forecast  update  and the  latest observations  about the human triggered avalanches that occurred Sunday.

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

Leeward, steep, unsupported slopes above 2500′ may still harbor wind slabs today from the winds Sunday night/Monday morning. Look 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 with the few inches that fell yesterday and overnight. Another 2-5″ is forecast to fall today. 

Additionally, pay attention to how new snow falling today bonds to the snow below. As the inches slowly stack up over the hard wind-blown snow surface or over the melt-freeze crust at lower elevations loose snow avalanches may be a possibility in steep terrain. If the snow is cohesive enough shallow storm slabs may form. Hand pits and small tests slopes will be useful ways to assess bonding.  

Wind effect in Tincan Common, 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

As is often the case we are concerned with the snowpack in the periphery zones of our advisory area, Crow Pass and Summit Lake.  Where the Alpine 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 near the ground in Summit Lake. We have limited information from north of Girdwood but suspect a weak set-up.  If traveling to these areas the possibility 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.

Tenderfoot, 12-5-18 

Mountain Weather

Yesterday:  Skies were cloudy with snow showers throughout the day. Temperatures were in the 20Fs to low 30Fs. Winds were easterly 5-15 mph with gusts into the 30s. Snow showers continued overnight, winds were light and temperatures dropped a few degrees.  

Today:  Snow showers continue with 2-5″ forecast to fall today with an additional 2-6″ tonight. Temperatures will start in the mid to high 20Fs and drop slowly throughout the day into the low 20Fs and teens tonight. Winds will be variable and light.  

Tomorrow:  Snow showers and cooler temperatures are the trend for the week. Thursday may see a brief window of clearing. The pattern remains active into the weekend. Stay tuned as we watch storm tracks and temperatures!  

*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′)  29     4    0.4     29  
Summit Lake (1400′)  27      1     0.1   5  
Alyeska Mid (1700′) 28   3   0.3   12

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 21    NE 9    36
Seattle Ridge (2400′) 25   *no data   *no data     *no data