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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sun, February 12th, 2023 - 7:00AM
Expires
Mon, February 13th, 2023 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger is MODERATE at all elevations. Several different types of avalanches could be triggered today. In the lower and mid elevations, it is still possible to trigger a slab avalanche 1-2′ deep on steep slopes where the storm snow from last week has still not bonded well. In the higher elevations, watch for older wind slabs 1-2′ deep along ridgelines and in cross-loaded gullies. Last, sluffs could be easily triggered on steep slopes.

SUMMIT LAKE:  Several inches of snow has fallen in the past couple days on a much weaker snowpack than is present at Turnagain. Human triggered avalanches are more likely in this area that sits to the south of the forecast zone.

SEWARD/LOST LAKE: The Seward area has been favored by snowfall as well, which has led to an increase in likelihood for human triggered avalanches. Keep an eye out for red flags and signs of recent wind loading to identify avalanche prone slopes.

Special Announcements

Join us on Valentine’s Day (Feb 14th) for Snowball! Dance to lively music by the Jangle Bees, bid on the silent auction, and enjoy 49th State Brewing libations and decadent desserts. Bring your sweetie or your best backcountry partners—or find new ones on the dance floor. All proceeds from this event benefit the Friends of the Chugach Avalanche Center and the Alaska Avalanche School, so you can let loose knowing it’s for a great cause! Tickets are limited, so get yours soon. Click here for tickets and more information.

Sun, February 12th, 2023
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
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

We heard of one skier triggered slab avalanche yesterday. This was on Eddie’s Ridge. A skier reported to have triggered the slab on a lower SW facing steep roll, around 1,600′, and was able to ski off the slab. No photos or additional details. It’s most likely the avalanche failed around 16-20″ deep at the new/old snow interface from Feb 5th (12-18″ of snow fell on Feb 5 and 6). At these lower elevations a crust exists under this interface, which could be keeping that old storm snow from bonding in places.

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

As noted by the skier triggered avalanche yesterday on Eddies, there is still some funny business under the storm snow from last Sunday/Monday. With 4-6″ of new snow and light winds over the past 30 hours, the main concern will lie with this older storm interface. In general the Feb 5/6 storm snow has shown mostly signs of stabilizing, but not everywhere. At elevations below 2,000′ the storm snow fell on a layer of weak snow sitting on a crust that was formed in late January. This lower elevation setup is something to have on our radar as even a small slope could pop out and cause harm. These slabs are in the 1-2′ deep range and something we can look for with quick hand pits.

At the higher elevations there also could still be some slopes harboring surprises. These will most likely be associated with wind slabs (below in Problem 2), but other factors could be present. The sun created a crust on certain steep southerly slopes last week. New snow sitting on these crusts could be suspect for sliding. Once the skies clear enough for travel into the Alpine, we’ll need to be watching for all the red flags.

Another layer that we are still thinking about is the old 1/10 buried surface hoar that is 2-4′ deep in the snowpack. This layer has not caused an avalanche for 2.5 weeks and it’s not likely to cause one today. Yet, it’s good to know that layer is still under our feet, it can be quite easy to see in snow pits, and just more encouragement to always keep with our safe travel protocol.

 

New layers of surface hoar have been forming during short clear spells over the past week. One was on Saturday Feb 4th, another on Tuesday, Feb 7th, and this one photographed by Allen Dahl is from Thursday, Feb 9th. These layers are intermixed with new snow in the top foot of the snowpack and may not end up being a concern, but we’ll be looking for them just in case. 

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

Two days ago, on Friday, the region saw strong easterly winds. Since then the visibility has not been good enough to travel into the Alpine or even see into these upper elevations as to what the winds did and any avalanche activity associated with them. However, once the sky does clear, know that lingering wind slabs could be hiding under several inches of new snow that has been tricking in since the winds stopped.

Wind slabs could be sitting on various layers, one of which could be buried surface hoar and slabs may not be all that well bonded. They are likely to be in the 1-2′ deep range and along ridgelines and in cross-loaded gullies. As always, paying attention to areas winds have loaded slopes, feeling for stiffer snow over softer snow, and looking for cracking or collapsing in the snow under your feet/machine are key red flags.

Loose Snow Avalanches:  Sluffs are likely to be triggered on steep slopes due to several inches of new light snow on the surface.

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

In areas with a thin overall snowpack, such as Silvertip and south of Turnagain Pass, there are deeper weak layers near the base of the snowpack that are concerning. The most widespread weak layer is the Thanksgiving facet/crust combo but in some areas (Summit Lake for example) there is also a weak layer of basal facets. Triggering a large avalanche breaking in these layers is not necessarily likely, but also not out of the question. Additionally anytime we see active weather, it is possible they could become more reactive and catch us off guard.

Weather
Sun, February 12th, 2023

Yesterday:  Cloudy skies and light snowfall was over the region yesterday. Snow fell to sea level and 4-6″ was seen in the Girdwood/Portage zone while Turnagain Pass and Summit Lake picked up 2-4″. Ridgetop winds were light from the east (5-10mph) with gusts in the teens. Temperatures were mild, near 32F at the lower elevations and in the low 20’sF along ridgelines.

Today:  A break is weather is expected today with continued cloudy skies and a few snow flurries. Only a trace to 2″ of new snow is forecast, to sea level. Ridgetop winds should remain light (5-10mph) from the NW with some areas close to Turnagain Arm seeing stronger winds. Temperatures should cool through today into the teens at most locations.

Tomorrow:  Clearing skies are forecast for Monday along with the development of moderate to strong NW outflow winds along ridgelines (25-40mph). Temperatures should fall to the single digits with the cold arctic air. Another system is pushing in Tuesday that should bring strong easterly winds, some snowfall, and slightly warmer temperatures.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 29 2 0.1 72
Summit Lake (1400′) 27 3 0.2 37
Alyeska Mid (1700′) 28 5 0.4 71
Bear Valley – Portage (132′) 32 4 0.4

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 21 NE 6 19
Seattle Ridge (2400′) 24 SE 7 15
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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.