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Fri, December 25th, 2020 - 7:00AM
Sat, December 26th, 2020 - 7:00AM
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE in the Alpine (above 2,500′) for wind slab avalanches. An increase in winds overnight should have been enough to create fresh wind slabs 1-2′ thick. These are expected to be easy to trigger on slopes over 35 degrees with new wind deposited snow. The danger is MODERATE for wind slabs between 1,000′ and 2,500′, where fresh slabs should be shallower and less likely to find. A LOW danger remains below 1,000′.

SUMMIT LAKE: The snowpack in the Summit Lake area is thinner and weaker. In addition to the wind slab issue there is still a chance of triggering a slab that breaks in a buried weak layer mid-pack or near the ground.

Fri, December 25th, 2020
Above 2,500'
3 - Considerable
Avalanche risk
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
Avalanche risk
0 - No Rating
1 - Low
2 - Moderate
3 - Considerable
4 - High
5 - Extreme
Avalanche risk Avalanche risk Avalanche risk Avalanche risk Avalanche risk
Travel Advice Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features. Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern. Dangerous avalanche conditions. Dangerous avalanche conditions. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended. Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain.
Likelihood of Avalanches Natural and human-triggered avalanches unlikely. Natural avalanches unlikely; human-triggered avalanches possible. Natural avalanches possible; human-triggered avalanches likely. Natural avalanches likely; human-triggered avalanches very likely. Natural and human-triggered avalanches certain.
Avalanche Size and Distribution Small avalanches in isolated areas or extreme terrain. Small avalanches in specific areas; or large avalanches in isolated areas. Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas. Large avalanches in many areas; or very large avalanches in specific areas. Very large avalanches in many areas.
Avalanche Problem 1
  • Wind Slabs
    Wind Slabs
  • Certain
    Very Likely
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
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.

Likelihood of Avalanches
Terms such as "unlikely", "likely", and "certain" are used to define the scale, with the chance of triggering or observing avalanches increasing as we move up the scale. For our purposes, "Unlikely" means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. "Certain" means that humans will be able to trigger avalanches on many slopes, and natural avalanches are expected.

Size of Avalanches
Avalanche size is defined by the largest potential avalanche, or expected range of sizes related to the problem in question. Assigned size is a qualitative estimate based on the destructive classification system and requires specialists to estimate the harm avalanches may cause to hypothetical objects located in the avalanche track (AAA 2016, CAA 2014). Under this schema, "Small" avalanches are not large enough to bury humans and are relatively harmless unless they carry people over cliffs or through trees or rocks. Moving up the scale, avalanches become "Large" enough to bury, injure, or kill people. "Very Large" avalanches may bury or destroy vehicles or houses, and "Historic" avalanches are massive events capable of altering the landscape.

Signal Word Size (D scale) Simple Descriptor
Small 1 Unlikely to bury a person
Large 2 Can bury a person
Very Large 3 Can destroy a house
Historic 4 & 5 Can destroy part or all of a village
More info at Avalanche.org

‘Twas the night before Christmas, when all through the hills,
Neither skier or rider was looking for thrills.
With a blast of wind and light snow falling,
There were hopes of blue skies in the early chill morning.

Happy Holidays everyone! And, that’s right, many folks are hoping for some clearing skies today after a quick pulse of moisture and wind moved in last night. This brought 4″ of new snow to the mid elevations and ~6″ to the higher elevations. Ridgetop winds picked up from the east averaging 25-35 mph for a period of 8 hours last night with gusts to 56mph on Sunburst. Since then, winds have been steady around 20mph with gusts near 40mph; where they are expected to remain today. With around 6-10+” of existing loose snow on the surface and other 6+” from last night in the high elevations, we can expect to find some wind loaded slopes and touchy wind slabs today. The good news is, new wind slabs should be easy to identify and relegated to the higher terrain. Seeking out areas without wind effect will be your best bet for fun riding conditions and avoiding a wind slab avalanche.

Wind slab avalanches:  Triggering a fresh wind slab is the main concern today. If skies clear enough for travel into the bigger terrain watch for:

  • Slopes or gullies with fresh wind deposited snow.
  • Cracking in the snow around you and stiff snow over softer snow.
  • Recent avalanches? There could have been some small natural wind slabs overnight. Keep on the lookout for these signs.
  • Wind slabs could be anywhere from 1-2 feet thick pending the wind loading.
  • Older, stubborn, but larger wind slabs could be lurking from the very strong winds on Tuesday.

Cornices:  As always, watch for cornices and give them a wide berth.

Loose snow sluffs:  If you are lucky enough to find a slope without wind effect and lots of loose snow, watch your sluff.

Warm-up Bowl (-1 Bowl) avalanche from Wednesday 12/23 – UPDATE:
We were able to get a look at the large avalanche triggered on the backside of Seattle Ridge yesterday. This slide was 2-4’+ feet deep, 1,000′ wide and ran around 800 vertical feet. It was triggered by a snowboarder who was the 5th person down the slope. The boarder deployed their airbag and when the debris settled, was buried up to their neck. They are thankfully OK and a few more details can be found HERE (from the boarder involved) and HERE.

Much of the crown face can be seen in the photo below with its wide propagation. This did turn out to be a large wind slab that had not bonded yet after Tuesday’s 2-3′ of snow and extremely strong winds. The weak layer was precipitation particles (stellar crystals) and not a persistent weak layer – however, these stellars can act like a persistent grain type for a few days before bonding, hence the wide propagation. Today is day three, and there just might be a lurking larger wind slab not yet bonded under the new smaller slabs formed overnight.

Warm-up Bowl avalanche from Wednesday 12.23.20. Photo: Anonymous

Crown face of one section of the slide. The crown was 2′ thick here but grew to over 4′ in sections. Photo taken 12.24.20.


Additional Concern
  • Deep Persistent Slabs
    Deep Persistent Slabs
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.
More info at Avalanche.org

What to do about those old October facets near the ground in the shallow snowpack zones? In Summit Lake and the central Kenai Mtns the snowpack is much thinner than Turnagain Pass. This area is outside our forecast zone, but we want to be sure folks know the difference in snowpack. As Aleph mentioned yesterday, this layer continues to show less and less reactivity over time and triggering an avalanche releasing near the ground is unlikely. Because of all the crusts in the mid-pack from 2,500 and below, this issue would be above, in the Alpine, where no crusts exist.

Fri, December 25th, 2020

Yesterday:  Overcast to obscured skies filled the region. Light snow started falling in the evening with rain below 500′. Accumulations are in the 4″ range at mid-elevation and 6″ in the Alpine. Easterly ridgetop winds bumped up with the precipitation and have averaged 20-35mph with gusts into the 50’s for a 8 hour period ending at midnight. Temperatures were warm, in the mid 20’s F along ridgelines and mid 30’s below 1,000′.

Today:  Cloudy skies and light precipitation will slowly move out this morning and and skies should begin to clear by midday. Ridgetop winds are expected to average in the 10-20mph range and decrease as the day goes on. Temperatures will start to cool off with the clear skies and possibly it the teens along the ridgetops and mid 20’s F in the parking lots.

Tomorrow:  Another round of wind and snow is on tap for tomorrow. This is a cooler and quick moving system which should bring a few inches of snow, even to sea level. Models are showing another larger scale precipitation event for early next week. This one also looks to be on the cooler side with snow close to sea level. Stay tuned!

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 30 4 0.4 80
Summit Lake (1400′) 25 0 0 31
Alyeska Mid (1700′) 30 4 0.4 78

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 22 E 20 56
Seattle Ridge (2400′) 25 *N/A *N/A *N/A

*Seattle Ridge anemometer is rimed over again and not reporting wind data.

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Date Region Location
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This is a general backcountry avalanche advisory issued for Turnagain Arm with Turnagain Pass as the core advisory area. This advisory does not apply to highways, railroads or operating ski areas.