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ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, December 21st, 2022 - 7:00AM
Expires
Thu, December 22nd, 2022 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

A MODERATE avalanche danger exists above 1,000′. Fresh wind slabs up to 1′ deep could be forming today along the higher elevations due to an increase in easterly ridgetop winds. Watch for active wind loading along ridgelines. Additionally, triggering a larger slab avalanche that breaks in weak snow 2-4′ deep is possible. These larger slabs are most concerning in Girdwood Valley and the south end of Turnagain Pass. The danger is LOW below 1,000′.

SUMMIT LAKE:  The snowpack in the Summit Lake area is generally thinner and weaker than Turnagain Pass. Winds picking up today may create wind slabs that could overload buried weak layers, creating a lager avalanche. Extra caution is warranted in this area.

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Wed, December 21st, 2022
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. 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.
Recent Avalanches

The last known avalanches in our forecast zone were immediately following the Dec 14-15 storm. Multiple large avalanches appeared to have failed in weak faceted snow near the Thanksgiving crust, with the largest avalanches in the Girdwood Valley and on the south end of Turnagain Pass.

Avalanche Problem 1
  • Wind Slabs
    Wind Slabs
  • Aspect/Elevation
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
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.

Aspect/Elevation of the Avalanche Problem
Specialists develop a graphic representation of the potential distribution of a particular avalanche problem across the topography. This aspect/elevation rose is used to indicate where the particular avalanche problem is thought to exist on all elevation aspects. Areas where the avalanche problem is thought to exist are colored grey, and it is less likely to be encountered in areas colored white.

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

HAPPY SOLSTICE everyone!! Starting tomorrow, we’ll have 6 more seconds of daylight to play in than we will today, our shortest day of the year. Today’s solstice will bring with it a few high clouds and an increase in ridgetop winds. Winds are expected to blow 10-20mph with stronger gusts from the east, which could be enough to form new wind slabs in the high elevations. Slabs should be building here and there through the day, so watching for active wind loading will be our best bet to knowing what slopes are being loaded.

Newly formed wind slabs could get up to a foot deep or so and should be easy to see. They may be touchy as they will likely be sitting on weak faceted snow. In shallower snowpack areas such as Lynx creek and outside our forecast zone in Summit Lake, a wind slab may overload buried weak layers. In this case a smaller wind slab could step down creating a larger avalanche.

Dry loose avalanches:  The surface snow is getting looser by the day with the cold temperatures. Sluffs on steep slopes should be expected.

 

Avalanche Problem 2
  • Persistent Slabs
    Persistent Slabs
  • Aspect/Elevation
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
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.

Aspect/Elevation of the Avalanche Problem
Specialists develop a graphic representation of the potential distribution of a particular avalanche problem across the topography. This aspect/elevation rose is used to indicate where the particular avalanche problem is thought to exist on all elevation aspects. Areas where the avalanche problem is thought to exist are colored grey, and it is less likely to be encountered in areas colored white.

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

As time goes on, we are still concerned about those layers of faceted snow sitting above and below the Thanksgiving crust. This is our ‘persistent slab‘ problem. These layers create the potential for a person to trigger a slab avalanche, in this case 2-4′ deep. That potential is decreasing with time and in the absence of new loading by snow/wind. The winds today are not expected to transport enough snow to overload these layers in Turnagain, but the Summit Lake area could be different as mentioned above.

The mountains around Girdwood and towards the south end of Turnagain Pass are the most suspect for triggering a persistent slab; along with many areas outside our forecast zone like Chugach State Park. In short, this is a tricky situation, especially as the snowpack slowly stabilizes and triggering one of these slabs gets tougher. Tracks on a slope doesn’t mean that slope is safe, yet at the same time, unless it avalanches we won’t know for sure one way or the other. What we can do is implement good travel practices by exposing only person at a time, watching our partners, or just sticking to the lower angle slopes to avoid the problem.

 

Many tracks in steep terrain in Superbowl without any skier triggered avalanches. It is easy to forget there is some funny business buried in the snowpack. Debris off the north side of Cornbiscuit in the foreground is from natural activity during the Dec 14-15 storm. Photo taken 12.18.22.

Weather
Wed, December 21st, 2022

Yesterday:  Clear and cold conditions were over the region once again. Temperatures in valley bottoms were -15 to 0F while mid and upper elevations were near 10F. Ridgetop winds were light from the east (5-10mph).

Today:  High clouds look to filter in today with a chance for 2-4″ of snow to fall overnight tonight. Ridgetop winds are picking up from the east and are expected to blow 10-20mph with gusts up to 30mph by this afternoon. Temperatures are warming in the mid and upper elevations, into the teens, while valley bottoms remain in the minus single digits.

Tomorrow:  Partly cloudy skies are expected tomorrow with a few snow flurries in the morning. Ridgetop winds look to remain breezy from the east, in the 15-25mph range with stronger gusts. The breaking down of the blocking high-pressure and the next series of storms looks to be setting up for Sunday into the early part of next week. Stay tuned!

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 9 0 0 37
Summit Lake (1400′) -10 0 0 30
Alyeska Mid (1700′) 8 0 0 39
Bear Valley – Portage (132′) 4 0 0

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 10 E 6 17
Seattle Ridge (2400′) 12 SE 4 12
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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.