ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Saturday December 1st, 2018
Posted by Wendy Wagner on 12/01/18 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.

A  MODERATE  avalanche danger exists above 2500′ in the Alpine. Triggering a slab avalanche 1-3′ thick remains possible due to a layer of buried surface hoar under the Thanksgiving weekend storm snow. Additionally, watch for increasing winds today from a southerly direction that may form wind slabs in unusual places.

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 today – 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

The mountain snowpack was tested yesterday with Mother Nature’s violent shake down. The Nov 30th 7.0 earthquake rocked the region. Luckily visibility was good enough to get eyes on some of the aftermath. Girdwood Valley was the most exciting as large slabs were triggered along with rock fall. Turnagain Pass fared better, as most the snowpack stayed intact. The most notable slabs on Turnagain were on the east facing slopes of Twin Peaks. Otherwise there were some shallow slabs on the southern end of Seattle Ridge and small cornice falls along ridgetops. 

What does this say about the snowpack? Well, the storm snow from last weekend that fell on buried surface hoar (a weak layer) and in some areas old weak snow, is definitely adjusting – the snowpack did not totally shake off the mountainsides. One can deduce that triggering a large avalanche is becoming less possible. Despite this, keeping in the forefront of our minds that a weak layer sits 1-3′ below the snow surface is still essential. Safe travel protocols and watching our partners shouldn’t be forgotten. Additionally, any signs telling us a slope could be unstable are not likely to be seen. 

 

Earthquake triggered avalanches in upper Goat Mt and Goat Couior in the Girdwood Valley

 

Magpie Peak in the upper Crow Creek Valley – avalanche debris and some slabs seen triggered by the earthquake

 

 

Rock fall off peak 4710′, north aspect

 

 

Rockslide off the east face of Rainbow Peak in Chugach State Park – triggered by earthquake

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

Winds are slated to bump into the 40’s mph today from a southerly direction along the ridgetops. This is an unusual flow direction. Watch for terrain forcing to shift winds either westerly or easterly. Be on the lookout for how winds may be transporting snow and avoid any fresh wind drifts or slabs that may form. Above 2,500′ there is loose snow available for transport plus a possible 1-3″ of snowfall today. 

Mountain Weather

Yesterday:   Partly cloudy skies were over the region with a few snowflakes falling last night, adding a trace. Temperatures have been steadily in the 20’sF and teens at the high elevations. Light and variable winds yesterday turned easterly overnight, with a bump in speed to 15mph with gusts to 30mph.

Today:   Light snowfall with 1-3″ accumulation is expected today before diminishing tonight. Temperatures should rise significantly, turning snowfall to rain as high as 2,000′ by the afternoon. Cross your fingers the cool air stays in place! Winds are forecast to increase up to 40mph from the SW with stronger gusts, also diminishing to the 15-20mph range tonight.

Tomorrow:   Another band of precipitation is expected to move through tomorrow. This one is expected to be cooler, bringing a chance for a few inches of snowfall to lower elevations.  

*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′) 26   Trace   0   10  
Summit Lake (1400′) 26   1   0.1   1  
Alyeska Mid (1700′) 27 Trace   0.03   0  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 19   NE   9   31  
Seattle Ridge (2400′) 23    *no data  *no data    *no data