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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, December 20th, 2021 - 7:00AM
Expires
Tue, December 21st, 2021 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger remains MODERATE on all aspects above 1,000′. There still exists the chance for a person to trigger a large slab that breaks in old weak layers anywhere from 2-5+’ deep. This type of large slab avalanche could be triggered from below, the side, or on top of a slope. Additionally, a few inches of new snow with moderate south winds today may just be enough to form small new wind slabs in the higher elevations.

Below 1,000, the danger is LOW where triggering an avalanche in unlikely.

*Roof Avalanches: There is a lot of frozen snow and ice hanging on roofs in Girdwood. Warming temperatures later today may be enough to weaken some of the snow/ice and cause it to fall. As always, watch the roofs around you and pay attention to children and pets.

Special Announcements
Mon, December 20th, 2021
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
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. Careful snowpack evaluation, cautious route-finding, and conservative decision-making essential. 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
  • Deep Persistent Slabs
    Deep Persistent Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
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.

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

Although there is weather headed in for Anchorage and Hatcher Pass, our forecast region isn’t in the bull’s eye this time. It’s looking like Girdwood and Turnagain Pass could see snowfall totals of 4-6″ by early tomorrow morning. Around 2-4″ of that falling during today’s 5 hours and 33 minutes of daytime hours (one minute longer than tomorrow’s shortest day of the year).

Accompanying the snowfall will be southwest winds in the 10-20mph range. With not a whole lot of new snow to work with and varying degrees of older existing snow available for transport, new wind slabs will have a tough time forming. However, if snowfall and/or wind increases more than forecast, that could change things, something to keep an eye out for. How much snow has fallen? Is it being transported by the winds?

What we really can’t forget about is what is hiding down in the snowpack, under multiple layers of settled storm snow, which are those old faceted layers we keep harping on. We are finding the facets everywhere above ~1,000′. Although the depths they are buried vary greatly; from 6′ to 2′ deep (plus or minus…). They are on the deeper side in Girdwood/Placer Valley/north end of Turnagain and shallower the more south one goes into the interior Kenai (Summit Lake). They last produced avalanches during the storm on Dec 9-10 and most of these avalanches were in the 1,500′ to 3,000′ elevation band. This is the same elevation band where the Halloween crust sits under the facets. It is this band that continues to be the most concerning for someone to trigger one of these slabs.

A few inches of new snow and warming temperatures today will be adding some stress to the snowpack. But, it is not likely enough to change the fact these large slabs will continue to be stubborn to trigger. That said, outside of the forecast zone in Summit Lake yesterday where the weak layer was only buried 18″ deep, a group got a very large collapse in the facets. This is a big red flag and reinforces our travel advise of still playing it safe. Triggering one of these slabs could be very large, propagate across terrain features and simply have too high of high consequences.

 

 

A cross-section of the snowpack at lower elevations (1,300′) where many folks are riding in the Placer/Skookum area. This pit shows the poor structure of the snowpack and the faceted layer that failed with a lot of force.


Glide Avalanches: 
The last known glide avalanche was sometime in the past week in the Lynx Creek drainage. There have also been some glide cracks creeping open in Lynx Creek as well as in the Summit Lake area. Glide avalanches are destructive and unpredictable, so as always, limit your time spent traveling below glide cracks.

Weather
Mon, December 20th, 2021

Yesterday:  Partly cloudy skies with some periods of sunshine were over the region. Ridgetop winds were light from north and west. Temperatures were generally in the 15-25F range from valley bottoms to ridgelines.

Today:  A frontal system is moving through today that will bring warming temperatures and snowfall to much of the region. Between 2-4″ is expected with another 2-4″ overnight tonight. Ridgetop winds should be southwesterly in the 15-20mph range. Temperatures are climbing now and should sit near 32F at sea level and in the mid 20’s at the higher elevations.

Tomorrow:  Skis should clear up tomorrow as a cold northwest wind sets up over the region. These NW winds are looking like they could ramp up to the 30-40mph range along the ridgelines and something we’ll be watching for. Stay tuned.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 21 0 0 68
Summit Lake (1400′) 12 0 0 21
Alyeska Mid (1700′) 18 0 o 41

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 19 W 4 14
Seattle Ridge (2400′) 20 N 2 6
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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.