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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, February 20th, 2023 - 7:00AM
Expires
Tue, February 21st, 2023 - 7:00AM
Forecaster
John Sykes
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE above 2500′ today. Northwest outflow winds will be forming fresh wind slabs 1-2′ deep at upper elevations and in areas exposed to gap winds. Human triggered avalanches are likely and natural avalanches are possible in areas receiving active wind loading. There are also several buried weak layers in the upper snowpack that could produce larger and more connected avalanches in isolated areas.

Between 1000′ to 2500′ the avalanche danger is MODERATE. Wind slab avalanches are less likely at these elevations, but still possible in areas exposed to gap winds. Persistent weak layers exists above 1000′ which could cause larger avalanches. Below 1000′ the avalanche danger is LOW. 

SUMMIT LAKE / LOST LAKE / SNUG HARBOR: NW Outflow winds will impact the mountains across the region today and cause increased avalanche danger at upper elevations.

Mon, February 20th, 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

There was a human triggered avalanche in the Tincan Library yesterday afternoon that caught two skiers and fully buried one of them. The fully buried individual was shallowly buried and able to dig themselves out. The second skier deployed their airbag and took a long ride with the avalanche but ended up on top of the debris. The avalanche was approximately 2′ deep and propagated across the gully feature that the group was skiing. We are still collecting information on this accident and want to extend our gratitude to the group involved who reached out to us to report the incident.

Otherwise we have not heard of any other large avalanches yesterday.

Photo of Tincan Library prior to the avalanche described above. You can see that even a small avalanche in this terrain can have high consequences. Photo 2.19.23 from Megan Guinn

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

Northwest outflow winds are expected to pick up during the day today with averages of 15-30 mph and stronger gusts. These winds will be transporting snow from a different direction than our last few easterly storms, which can make it difficult to read the wind loading patterns on the snow surface. This wind direction typically has the greatest impact along Turnagain Arm, Portage, Seattle Ridge, Crow Pass, and higher elevation ridgelines like Pastoral or Silvertip. Wind slabs 1-2′ deep are likely to be human triggered today, especially in those favored areas and along upper elevation ridgelines and cross loaded gullies. To identify terrain harboring wind slabs keep an eye out for active wind loading and step off the beaten track to check for firm or hollow feeling wind transported snow.

With mostly clear skies expected today the sun could also have an impact on the snow surface, especially in the afternoon when the solar intensity is highest. The winds might be enough to keep the snow surface cool, but in sheltered areas it is possible that sunny slopes could have natural loose snow avalanches and cornice fall is more likely with direct sunlight.

Active wind transport along the ridge of Pastoral yesterday. This will be more of a concern today as moderate to strong NW outflow winds impact the area. Photo 2.19.23 from Kit Barton & Sage Robine

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

We have a variety of potential weak layers in the snowpack right now which could produce larger and more connected avalanches. Within the upper 1-2′ of the snowpack there are several layers of buried surface hoar and facets that have developed between our recent storms. Over the past few days we have seen several avalanches that possibly involved one of these surface weak layers (Notch, Crow Creek, Tincan Library). The specific layers we are concerned about are the 2/5 storm interface, which is a 1.5-2.5′ deep layer of facets at upper elevations and a melt freeze crust with facets on top below 2000′, and the 2/15 surface hoar which was widespread in the forecast area before being buried 6-12″ deep. These weak layers may be more deeply buried in areas closer to Prince William Sound where there has been a lot more recent snowfall.

To determine whether these weak layers are an issue where you are travelling we recommend keeping a close eye out for any collapsing or shooting cracks, using small test slopes to check for unstable surface conditions, and digging hand pits or snow pits to look for the weak layers. Snow pits are the best way to test more deeply buried weak layers and get a sense of how sensitive they currently are. To avoid these avalanche problems you can always stick to lower angle terrain.

In addition to these weak layers in the upper snowpack there is also the 1/10 buried surface hoar that is closer to 2-4′ deep. This layer has been largely dormant for several weeks, but we continue to get unstable test results in isolated areas where the surface hoar layer is well preserved. This layer is unlikely to produce human triggered avalanches at this point in time, but if you find the wrong spot it could produce a very large avalanche. The likelihood is highest in areas with a thin overall snowpack like the southern end of Turnagain Pass and the Summit Lake area.

Example of the snowpack structure in Turnagain Pass with a variety of potential weak layers within the upper 3′. Photo 2.19.23 from Andy Moderow

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 snowpack (i.e. Silvertip and the southern end of the forecast region to Summit Lake) there are various weak layers near the base of the snowpack that remain a concern. For these areas, triggering a larger avalanche is not totally out of the question and a more cautious mindset is recommended.

Weather
Mon, February 20th, 2023

Yesterday: Calm to light winds averaging 0-5 mph and temperatures in the teens to 20s F at upper elevations and low 30s F at lower elevations. No new snowfall. Mid level cloud cover lingered throughout the day at 2000-3000′ leaving some areas socked in and some above the clouds and basking in the blue skies.

Today: Moderate to strong northwest outflow winds are expected to start this morning and persist until the evening. Wind speeds should average 15-30 mph with stronger gusts. Temperatures should remain in the teens at upper elevations and mid-20s F at lower elevations. No new snowfall is expected. Cloud cover is expected to dissipate and give way to mostly sunny skies.

Tomorrow: Outflow wind speeds should decrease on Tuesday with averages of 10-20 mph in the morning then dropping to 5-10 mph in the afternoon. Cloud cover should increase in the evening as a low pressure system moves into the area. No new snowfall is expected during the day, but light snowfall is expected Tuesday night into Wednesday with minimal accumulation.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 22 0 0 67
Summit Lake (1400′) 20 0 0 37
Alyeska Mid (1700′) 23 0 0 71
Bear Valley – Portage (132′) 29 0 0

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 16 W 5 16
Seattle Ridge (2400′) 18 N 1 4
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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.