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Issued
Tue, February 7th, 2023 - 7:00AM
Expires
Wed, February 8th, 2023 - 7:00AM
Forecaster
John Sykes
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE above 1000′ today. Human triggered avalanches 1-2’+ deep in areas with recent wind loading or where there was a lot of recent storm snow (12″+) are still likely. It is also possible to trigger a larger avalanche on a layer of buried surface hoar 2-4′ deep. The snowpack needs some time to adjust to the new storm snow and we are recommending a cautious approach to decision-making and terrain selection today.

Below 1000′ the avalanche danger is MODERATE. The new snow that fell at these elevations was moist and we expect it will bond quickly with the old snow surface.

SUMMIT LAKE: Snowfall has been trickling into the Summit Lake area this week and the existing snowpack is very weak. It is important to be aware of the potential for large avalanches on buried weak layers with the addition of new snow or active wind loading.

SEWARD/LOST LAKE/SNUG HARBOR: These areas are expected to be favored by the current weather pattern and could see 2-6″ of additional snowfall tonight which will increase avalanche danger.

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.

Tue, February 7th, 2023
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. 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

Poor visibility for most of the day yesterday made spotting avalanches difficult, but we did see very reactive wind slabs at tree line on Pete’s North. We triggered one on a test roll at 2100′ on a wind loaded aspect. As we travelled across the slope to find a pit location we triggered widespread shooting cracks on several wind loaded features that were just not quite steep enough to slide. Freshly built cornices were also quite touchy near tree line.

Small but very touchy wind slab on Pete’s N at about 2100′ on a W aspect. Photo 2.6.23

Avalanche Problem 1
  • Storm Slabs
    Storm Slabs
  • Aspect/Elevation
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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

The latest storm on Sunday through Monday produced quite a bit of snowfall in Turnagain Pass, with up to 16″ of new snow reported from the north end of the pass and closer to 6″ on the south end of the pass near Johnson Pass Trailhead. All this new snow combined with strong winds made for quite a touchy snowpack yesterday. We saw very touchy wind slabs at tree line and also had reactive test results in our snow pits with propagation on several different potential weak layers.

Wind slabs and storm slabs should become less reactive today as the snowpack has some time to adjust to the new load. However, human triggered avalanches 1-2’+ deep are still likely at upper elevations on steeper slopes in areas that saw wind loading yesterday. Using hand pits and small test slopes are great ways to test how well the new surface snow is bonding to the old snow surface. Given the recent loading and the existing weak layers in our current snowpack we are recommending a cautious mindset today. Remember that a layer of buried surface hoar still exists in the upper snowpack and could produce large avalanches (see problem 2).

At lower elevations in areas sheltered from the wind we saw evidence that it is possible to trigger a storm slab on top of the 1/25 melt freeze crust that formed from sea level up to about 1800-2000′. The new snow at these elevations should start to bond with the existing snowpack pretty quickly but it is worth having this on your radar so you are not surprised by the potential for a low elevation storm slab avalanche.

With this amount of low density new snow dry loose avalanches (aka sluffs) and cornice fall are also likely on steeper terrain.

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

For the first time in over a week in Turnagain Pass we saw reactive snow pit test results yesterday on the 1/10 buried surface hoar layer. The area we tested had very large and well preserved surface hoar grains that were quite evident in the snowpack. It took more force to initiate the failure than we saw a few weeks ago when this weak layer was active, but when the layer failed it still had a lot of energy and the block slid right into our pit. This serves as a reminder that large avalanches 2-4′ deep on this layer of buried surface hoar are still possible. With the added load of the last storm this weak layer could be more reactive now than it has been during the recent spell of calm weather. To avoid this problem we recommend sticking to low angle terrain and remaining aware of any steep slopes overhead, especially if other groups are travelling above you.

Snowpit structure from 2100′ on Pete’s N with very intact layer of buried surface hoar from 1/10 that propagation in multiple extended column tests. Photo 2.6.23

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

A weak facet/melt freeze crust layer from Thanksgiving exists at the base of the snowpack 4-8′ deep. Triggering an avalanche on this layer is unlikely, but it would produce a very large avalanche so we are keeping it on our radar as a worst case scenario. The most likely place to trigger an avalanche on this layer is from a thin spot in the snowpack, like where wind has scoured the snowpack down or where rocks are penetrating through the snowpack. We hope to put this layer in our rear view mirror soon, but with the recent loading event it is possible that this layer could be reactivated temporarily.

Weather
Tue, February 7th, 2023

Yesterday: Obscured to overcast skies for most of the day with a clearing trend in the late afternoon. Light to moderate winds at lower elevations increasing near treeline. Temperatures were warm at road elevations with moist snow falling above ~200′ elevation. 1-2″ of snow accumulation during the day.

Today: Partly cloudy skies during the first half of the day before snowfall is expected to start again, with Girdwood and Portage possibly adding another 4-6″ while Turnagain Pass is only expecting 1-3″. Most of that snowfall is expected in the evening and overnight. Winds should be lighter than yesterday with averages of 5-10 mph and gusts into the 20s at upper elevations. Temperatures should remain in the teens to mid 20s F with snow line at sea level.

Tomorrow: Continued light snowfall is expected in the morning before possibly clearing skies in the afternoon and evening. Only about an inch of additional snow accumulation is expected. Winds should remain light at 0-10 mph and temperatures are expected to decrease slightly into the teens as the cloud cover breaks up tomorrow afternoon. Snow line should remain at sea level.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 28 2 0.2 70
Summit Lake (1400′) 25 3 0.2 38
Alyeska Mid (1700′) 27 2 0.2 72
Bear Valley – Portage (132′) 35 0 0.25

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 18 E 14 53
Seattle Ridge (2400′) 22 SE 8 24
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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.