ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Friday November 30th, 2018
Posted by Heather Thamm on 11/30/18 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 above 2500′ where triggering a slab 1-3′ thick is possible on slopes steeper than 35 degrees. Be aware of changing weather and wind slabs forming late afternoon.

At treeline (below 2500′) the avalanche danger is LOW where a strong crust has formed and capped the snowpack.

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

The Friends of the CNFAIC have two scholarships dedicated to avalanche education. The funds generated to make these possible are in celebration of Rob Hammel and Amy Downing, their love and passion for the mountains, and to help others stay safe. We encourage you to read each one and apply if you fit the need, or pass along to someone who could benefit.  Applications due on Dec 1st.

Rob Hammel Scholarship Fund   €“ For recreational users and professional avalanche workers.

Amy Downing Scholarship Fund   €“ For recreational users.

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 a thicker snowpack exists and with it more uncertainty.  Observers yesterday found depths to be in the 3-5’ range near 3000’. Two pit locations above 3000’ (on Tincan & Sunburst) found a layer of surface hoar buried 2-3’ below the surface. One party on Sunburst felt a collapse ‘whumpf’ around 2800’ along the West ridge and several large piles of debris were noted on steep Northern terrain from avalanches earlier in the week.

What does this all mean?

Triggering a large persistent slab is possible in steep terrain and will be more likely in thinner areas of the snowpack. It remains unknown how widespread this layer is across our region or how much avalanche activity occurred on it early in the storm that may be hidden. It may be more likely to trigger a persistent slab in places that received less overall precipitation like the Southern end of Turnagain Pass and Summit Lake. Persistent weak layers are tricky and hard to manage. Evaluate your consequences and choose conservative terrain. Be on the lookout for any obvious signs on instability like whumpfing, shooting cracks or any new avalanche activity.

 

Several snowpits dug in the upper elevations yesterday found buried surface hoar

 

Rollerballs and a small mid storm avalanche were visible yesterday. This visual is a little deceiving. Loose snow is covering a thin crust, but gradually disappears around 2700′. 

 

A strong crust has formed below 2500’ but the snowpack remains very thin to nonexistent below 1500′.

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

Easterly winds are expected to increase to Moderate (15-25mph) late afternoon and into the evening. In the alpine there is snow available for transport and triggering a shallow wind slab on leeward features may be possible. Pay close attention to changing weather and be on the lookout for shooting cracks and drifting snow.

Mountain Weather

Yesterday: Temperatures remained below freezing with ridgetops near 15F most of the day and mid 20F’s near 1000′. Winds were light and variable. No measureable amount of precipitation fell.   Skies were broken in the morning becoming overcast in the afternoon.

Today: Winds are expected to be light and variable in the morning shifting to an Easterly pattern and increasing to Moderate (15-30mph) by tonight. Temperatures are expected to remain below freezing in the mid and upper elevations for today. Scattered snow showers are possible in the upper elevations and skies will be overcast.    

Tomorrow:  Saturday temperatures are expected to rise as a second front moves up Cook Inlet. Rain/snowline may push as high as 1500′ by late morning. A few inches of snow is possible above this elevation and rain showers at lower elevations. Easterly ridgetop winds will remain Moderate (15-25mph) through Saturday evening.  

*Seattle Ridge weather station stopped collecting wind data at 10 pm on Nov 27, 2018.  *Center Ridge weather station depth sensor showed 2″ increase, but no actual accumulation occurred.    

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 24   0 0   *10
Summit Lake (1400′) 19   0   0   0  
Alyeska Mid (1700′) 25   0   0    *N/A

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 15   Variable   1   11  
Seattle Ridge (2400′) 21   *N/A   *N/A   *N/A