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ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, December 7th, 2022 - 7:00AM
Expires
Thu, December 8th, 2022 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE above 1,000′ due to 6-12″ of new snow coupled with strong northwest outflow winds. Natural avalanches will be possible and human triggered avalanches likely. Wind slabs 1-2′ deep are expected on windloaded slopes and sluffs in the light new snow should be prevalent on the steeper slopes. In areas seeing over 10″ of new snow, soft slab avalanches could be triggered on slopes sheltered from the winds. Below 1,000′ the danger is MODERATE where the winds should have less of an impact.

Special Announcements
  • Chugach State Park:  Avalanche danger is increasing. More snow, up to a foot and possibly more, has fallen across the Anchorage Front Range. With strong northerly outflow winds forecast today, expect dangerous avalanche conditions.
Wed, December 7th, 2022
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
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

There were likely small avalanches last night associated with the new snow. Otherwise, the last known avalanches were small natural wind slabs from 11 days ago.

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

After a welcome overnight snowfall, to sea level, the northwest outflow winds should waste no time in moving the new snow around, especially in the higher elevations. For snowfall totals, as of 6am this morning, up to a foot has fallen in the upper Girdwood Valley and 6-8″ at Turnagain Pass and Summit Lake. Avalanche concerns will be directly related to the amount of new snow and windloading today.

In general, the new snow is not likely to bond very quickly with the old surfaces; from faceted loose snow to harder old wind blown snow. The new snow is light and in areas with only 6″ the avalanche conditions will be less than areas seeing up to a foot. The key for today will be to 1) watch for how much new snow has fallen and 2) look for active windloading that is creating fresh wind slabs.

Wind Slabs:  Any slope with wind loading today will be worth avoiding. Wind slabs could be anywhere from 1-2′ thick and should be easy to trigger. The outflow winds funnel in all directions at Turnagain Pass, so our best bet is to simply watch where the wind is transporting snow, feel for stiffer snow over softer snow, and note any cracks that shoot out from us.

Storm Slabs:  In sheltered areas with up to 10″ of new snow or more, soft slab avalanches breaking at the new/old snow interface are possible. The new snow may be too light to create a slab without wind effect, but this is something to look for in areas with up to a foot of new snow.

Loose Snow Avalanches – Sluffs:  Triggering a sluff should be fairly easy on the steeper slopes. There will likely be evidence of natural sluffs from overnight seen today.

Quick hand pits to check the amount of new snow, any stiffening from the winds, and looking for easy failures at the new/old interface will good ways to assess today’s issues. The good news is the problems are at the top of the snowpack, making them much easier to identify.

 

 

Snowpack structure at 2,600′ on the backside of Seattle Ridge yesterday, 12.6.22. Roughly 6-8″ of new light snow sits on top of this pit as of this morning. 

Weather
Wed, December 7th, 2022

Yesterday:  Cloudy skies and light snowfall (to sea level) was over the region. Overnight precipitation increased and this morning Girdwood Valley has received 10-12″, while Turnagain Pass and Summit Lake saw 6-8″. Ridgetop winds were easterly, 10-15mph, until last night when they shifted to be light from the northwest.

Today:  A few light snow flurries could add another 1-2″ of snow this morning before tapering off midday. Skies should also begin to clear midday. The northwest ridgetop winds are forecast to blow in the 20-30mph range with gusts in the 40’s. Temperatures look to remain in the 20’sF at the mid elevations and the teens in the Alpine.

Tomorrow:  Mostly sunny skies are expected tomorrow. The strong NW winds look to remain through tomorrow. For Friday and Saturday, mostly clear skies are forecast with light NW winds.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 30 5 0.5 28
Summit Lake (1400′) 29 4 0.5 20
Alyeska Mid (1700′) 30 10 0.9 30
Bear Valley (132′) 33 8 0.75 N/A

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 21 NE 9 18
Seattle Ridge (2400′) 25 SE 8 18
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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.