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ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, January 11th, 2023 - 7:00AM
Expires
Thu, January 12th, 2023 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger remains CONSIDERABLE above 2500′. Several inches of new snow with strong easterly winds yesterday likely formed fresh wind slabs. These should be around a foot thick and could be easy to trigger. Additionally, either a wind slab or person has the potential to trigger a very large avalanche on weak snow buried 3-6′ deep. Dangerous avalanche conditions will require cautious route finding, which means avoiding steep avalanche terrain. The danger is MODERATE between 1000′ and 2500′, where the winds were not quite as strong and the deeper weak layers are less likely to fail. The danger is LOW below 1000′.

Special Announcements

Cornbiscuit Avalanches:  We have posted the Near Miss Report for the avalanches that occurred last Saturday on Cornbiscuit. We are grateful everyone is OK after this incident and would like to sincerely thank all involved for sharing their experiences.

Forecaster Chat #2:  Join us at the Girdwood Brewing Co. from 5:30-7:00 p.m. on Thursday, Jan. 19! CNFAIC forecaster Andrew Schauer will open the night with an overview of the state of the snowpack, followed by a discussion on how safe terrain management changes depending on the type of avalanche problem at hand. More details here.

Wed, January 11th, 2023
Alpine
Above 2,500'
3 - Considerable
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.
Recent Avalanches

The last known avalanches were two large slabs that occurred last Saturday on Cornbiscuit Ridge. This was a unique occurrence as a very large slab was triggered by a skier on the SW face of the ridge, which then triggered another large avalanche on the north side. One skier was caught, carried, and partially buried. See the Near Miss Report HERE.

Avalanche Problem 1
  • 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

A fast moving weather system rolled through yesterday with light snowfall and strong ridgetop winds. The peak snowfall was overnight with 3-5″ of new snow at Turnagain pass, a couple inches to the south at Summit Lake, and the winner was Portage and Girdwood Valley where the mid and upper elevations saw 5-8+”. The system is moving out now and quiet weather is forecast today. That means we can expect some lingering wind slabs in the Alpine, which will be today’s avalanche ‘problem 2’. More on that below.

Avalanche ‘problem 1′ is the deadly, but less likely to find and trigger, large slab that breaks in buried weak layers, potentially taking a whole slope out and then some. This type of avalanche is what has been occurring over the past week, and most recently on Cornbiscuit. Slab depths are in the 3-6′ range with thinner areas around 2’ deep. Last night’s snow and wind wasn’t too much of a new load onto the snowpack as a whole, meaning it wasn’t likely enough to cause avalanches in buried weak layers. Yet, with so much uncertainty in the pack right now, our hackles should still be up that a bit of change could push slopes closer to the breaking point; so maybe it’s a hair easier for a person to trigger a large slab today. Things to think about if headed out to Turnagain Pass or surrounding areas:

  • Triggering a large dangerous slab is still possible.
  • No signs of instability are likely to be seen to warn us – this was a common theme voice by groups on Cornbiscuit last Saturday – and is common with this kind of avalanche problem
  • Slopes can release after many people have already been on them. Tracks do not mean the snowpack is stable.
  • Most likely places to trigger a large slab is near rocks and in thin areas of the snowpack; these areas can be hidden and hard to avoid.
  • Terrain with a shallower snowpack in general is more suspect – such as the south end of Turnagain Pass, Lynx Ck, Silvertip.
  • Travel Advice: sticking to the lower angle slopes and out from under large slopes is the best way to manage this issue.

 

Andrew investigating the large slab on the SW face of Cornbiscuit last Sunday (avalanche occurred Saturday 1/7). Note the hidden rock in the crown face. Photo: Matti Silta 01.08.2023.

 

As time goes on and no rapid loading by snow or wind occurs, it will be harder to trigger these large slabs. We are entering into the Deep Persistent Slab category. Meaning harder to trigger, triggering is generally from thin areas in the slab, no red flags to warn us, and stability tests don’t tell us much. In fact, tests can even give false positive results, luring us even more into thinking all is ‘OK’. Unfortunately, we’ll be talking about this layer for a while.

 

This is the same avalanche on the SW face of Cornbiscuit, showing the five tracks from just prior to the avalanche. A portion of the up-track was also covered in debris from this avalanche. Photo taken the day of the avalanche by the group involved, 1.7.23.

 

Avalanche Problem 2
  • 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

Unlike the hidden dragon above, wind slabs should be much easier to see and feel. Look for areas of wind loading and stiffer snow over softer snow. These should be found in the higher terrain just off ridgelines and in cross-loaded gullies. Wind slabs could be up to a foot thick and still touchy as they are likely sitting on surface hoar and faceted snow. Before the wind and snow yesterday, widespread surface facets and surface hoar existed. Something we’ll be watching when the next snowfall rolls in. It is worth noting, a small wind slab does have the potential to trigger a large slab, turning a small problem into a much larger one. One more reason to keep playing it safe today and sticking to mellow terrain.

Weather
Wed, January 11th, 2023

Yesterday:  A quick weather system moved through yesterday bringing cloudy skies and light snow showers during the day, picking up overnight. Snowfall amounts as of 6am are 6-8″ in upper Girdwood Valley and 3-6″ at Turnagain Pass. Ridgetop winds have been easterly, 20-25mph with gusts in the 40’s. Temperatures are warm, mid 30’sF at sea level near 30F at mid elevations and the mid 20’s along the peaks.

Today:  The system is moving out this morning and precipitation should taper off quickly as clouds start to break up. Ridgetop winds are also decreasing and forecast to blow 5-10mph with gusts in the teens from the east. Temperatures look to remain warm, 30’sF at the lower elevations and in the mid 20’sF along ridgelines.

Tomorrow:  A break in weather extends tomorrow through Friday with partly cloudy skies and light to moderate ridgetop winds (easterly 10-20mph). Temperatures remain on the warm side, 30’sF at the lower elevations and mid 20’sF along ridgelines. The next shot for snowfall is looking like possibly this weekend.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 31 3-4 0.3 58-60
Summit Lake (1400′) 31 1-2 0.1 33
Alyeska Mid (1700′) 31 6-8 0.6 55
Bear Valley – Portage (132′) 36 trace 0.8

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 23 NE 22 46
Seattle Ridge (2400′) 26 SE 23* 11*

*Only 12 hour data from 6am to 6pm, anemometer rimed over shortly after 6pm.

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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.