ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Saturday December 15th, 2018
Posted by Wendy Wagner on 12/15/18 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  in the Alpine and Treeline. A bump in easterly winds today along with a few inches of snow this morning will likely form fresh wind slabs, up to a foot thick, that could be easy to trigger. If snowfall and wind intensify more than forecast, the danger will rise to CONSIDERABLE. Additional concerns: watch your sluff, as these have been getting larger, and keep an eye out for glide cracks and avoid traveling underneath them.

SUMMIT LAKE:   New snow overnight may overload weak layers near the ground, increasing the chance a person could trigger a larger slab avalanche.  

2. Moderate
Alpine
/ Above 2,500'
Avalanche risk
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
2. Moderate
Treeline
/ 1,000'-2,500'
Avalanche risk
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
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

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

Watch for changing conditions. Ridgetop winds have picked up and should be blowing the existing loose surface snow into sensitive wind slabs this morning. These fresh slabs are likely to be shallow, around a foot thick, and easy to recognize if you are looking for them. Watch for areas with wind deposited snow and loading patterns in general. Shooting cracks will be an obvious clue wind slabs are tender. Feel for punchy or upside down snow and keep in mind the consequences of the terrain should even a small slab pull out from underneath you. In areas where winds aren’t an issue loose surface snow could move faster and farther than expected. 

Trevor Grams sent in this photo from the Library area further back along Tincan Ridge yesterday. Note the loose surface snow available for transport with today’s winds. 

Avalanche Problem 2
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

As is often the case, the snowpack in the periphery zones of our advisory area such as, Crow Pass and Summit Lake have a shallower snowpack. In these areas, and even those closer to Turnagain Pass itself, we are tracking buried layers of facets and crusts that sit 1-2′ under the snow surface. These layers are most prevalent in the mid-elevations (2000’ – 2700’) and though snow pit data and a lack of avalanche activity has been pointing to an unlikely chance for an avalanche releasing deeper in the pack, additional snow load in the Summit area could start to tip the balance. Evaluate the snowpack and terrain as you travel and be aware that obvious clues like whumpfing may not be present.

Additional Concern
Glide Avalanches
Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. The are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.
  • TYPE
    Glide Avalanches

Glide cracks are beginning to open up in many places, including Sunburst’s SW face under the weather stationSW face of Tincan Proper, Gold Pan area (behind Cornbiscuit/Magnum) and the one shown below in the Johnson Pass area. These cracks can release at any moment, as this one did below in Johnson Pass. They are not associated with human triggers and the best way to manage the hazard is to avoid being on or beneath slopes with cracks. 

 

Photos of the Johnson Pass area glide avalanche couresy of Matt McKee.

 

An unnamed longtime CNFAIC pro observer puts his take on this avalanche problem…

Mountain Weather

Yesterday:    Overcast skies were over region yesterday. Evening snow showers favored the Summit Lake area on the Kenai and further South where 4-6″ fell along the Seward Highway. Turnagain Pass and Girdwood only picked up 1-2″. This storm also favored the Anchorage region and North with 4-6″. Ridgetop winds have remained easterly in the 10-20mph range with gusts in the 30’s. Temperatures warmed dramatically overnight and are sitting in the teens along the ridgetops and 20’s at 1,000′.  

Today:  Snow showers have picked up this morning in Portage Valley and Girdwood. This quick moving front should give the mountains 2-4″ of snow before moving out by midday. Ridgetop easterly winds have picked up as well and should stay in the 15-25 mile range with gusts in the 40’s. Temperatures continue to rise and are expected to hit the upper 20’s at 3,000′ and mid 30’s at sea level.

Tomorrow:    What looks like a stronger front associated with the low-pressure in the Gulf moves in on Sunday. This should give the Turnagain area another chance to build up the snowpack. Warmer temperatures could bring a rain/snow mix to sea level, but so far it looks like it will be all snow at 1,000′. Stay tuned!

*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′) 20   1   0.1   30  
Summit Lake (1400′) 18   4   0.5   13  
Alyeska Mid (1700′)  21 3   0.3   17  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 14   ENE   14   41  
Seattle Ridge (2400′) 20   *no data   *no data     *no data