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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, February 25th, 2023 - 7:00AM
Expires
Sun, February 26th, 2023 - 7:00AM
Forecaster
John Sykes
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE above 2500′. Northwest outflow winds up to 30 mph are expected today which will make human triggered avalanches up to 1′ deep likely and natural avalanches possible. It is also possible to trigger an avalanche on a buried weak layer 1-3′ deep. Careful snowpack evaluation is recommended, especially in areas with active wind loading.

From 1000′ to 2500′ the avalanche danger is MODERATE. Wind slab avalanches are possible at these mid elevations in areas that are typically exposed to NW winds like Turnagain Arm. Buried weak layers also exist in this elevation band and human triggered avalanches 1-3′ deep are possible. Below 1000′ the avalanche danger is LOW. 

SUMMIT LAKE / LOST LAKE / SNUG HARBOR: Moderate NW outflow winds will impact these areas over the weekend and create increased avalanche danger. In areas with a thinner and weaker overall snowpack, like Summit Lake, this wind loading could be enough to reactivate buried weak layers and make larger avalanches more likely.

Sat, February 25th, 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. 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 no recent avalanches observed or reported yesterday. We had good visibility for the first part of the day and saw evidence of recent wind transport on ridgelines but no fresh avalanches. The last known avalanche activity was a human triggered persistent slab in the Tincan Library which caught and carried two skiers last Sunday. See report here for more details.

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

This weekend looks like it will be cold with mostly clear skies and moderate NW outflow winds. We are expecting winds to reach 20-30 mph in areas that are favored by this NW wind direction like Turnagain Arm, Crow Pass, Portage, Seattle Ridge, and upper elevation ridgelines. Human triggered avalanches up to 1′ deep are likely in areas experiencing active wind loading. To identify areas harboring wind slabs you can look for signs of wind transport on the snow surface, step off the beaten path to feel for firm or hollow wind drifted snow, and use small test slopes to check for shooting cracks. To avoid this problem you can stick to treeline and below treeline areas which should be largely sheltered and are holding good snow conditions thanks to the last few cold storms.

Even though temperatures were cool with light winds yesterday we still saw sun crusts forming just underneath the surface on steeper south facing terrain features. This is not concerning from an avalanche perspective right now but is worth noting that snow conditions will become more aspect dependent moving forward. In areas with soft snow still on the surface and steeper terrain loose snow avalanches are possible today. Cornice fall is also more likely with sun shining so it is worthwhile to be aware of any large cornices overhead.

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 past week or so the persistent weak layers in the upper snowpack have been sending mixed signals. Other than the avalanche in the Tincan Library we do not know of any recent avalanche activity in the area and it seems like folks have been travelling all over the place. In our snow pit tests we have seen a few concerning results where the weak layer has shown the potential to propagate across the slope and create a larger avalanche, but we have also had a bunch of tests where these layers have not shown concerning results. This leaves us in a tricky place as far as deciding whether or not to be concerned about these weak layers.

On the one side, the presence of these weak layers in the upper snowpack and the few signs we have seen that they have the potential to produce larger avalanches could be enough to justify sticking to lower angle terrain. On the other side, the lack of recent avalanche activity is a strong sign that these weak layers might not be an issue until the next time we get a loading event. That new loading event could be today in areas getting wind loading, but in sheltered areas we will have to wait until the next storm to test whether these persistent weak layers will become more active. For now we continue to consider it possible for human triggered avalanches 1-3′ deep on these persistent weak layers, but with no new snow expected over the next few days the likelihood of triggering should be trending towards unlikely.

One of the persistent weak layers (2/5 facets) in the upper snowpack which failed in one extended column test and propagated across the column. Photo 2.24.23

Snowpack structure in treeline elevation band on S aspect of Tincan, concerning test result is highlighted in red but other test results are not very concerning. Photo 2.24.23

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

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
Sat, February 25th, 2023

Yesterday: Broken cloud cover with some areas covered in thick mid elevation valley clouds and other areas with mostly clear skies. No new snow. Light winds at upper elevations averaging 0-10 mph. Temperatures around freezing at sea level and in the mid teens at upper elevations.

Today: Northwest outflow winds are the main weather feature today. We are expecting 20-30 mph winds in locations favored by gap winds, like along Turnagain Arm, Crow Pass, Portage, and Seattle Ridge. Temperatures are expected to decrease today starting in the mid teens at upper elevations and dropping to single digits by this evening. No new snow expected. Cloud cover should decrease throughout the day and trend toward mostly sunny.

Tomorrow: Sunday looks very similar to Saturday, with moderate NW outflow winds up to 20-30 mph in favored locations. Temperatures continue to drop on Sunday with highs in the teens and lows around 0 F. Mostly sunny skies should remain through Sunday. The next storm system expected to bring new snowfall looks like it will arrive on Tuesday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 26 0 0 66
Summit Lake (1400′) 24 0 0 39
Alyeska Mid (1700′) 25 0 0 71.5
Bear Valley – Portage (132′) 30 0 0.01

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 16 NW 4 19
Seattle Ridge (2400′) 20 NNW 4 21
Observations
Recent Observations for Turnagain Pass
Date Region Location
11/16/23 Turnagain Observation: Tincan Common
11/14/23 Turnagain Observation: Sunburst/Magnum
11/14/23 Turnagain Observation: Seattle Ridge
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11/12/23 Turnagain Observation: Sunburst
11/12/23 Turnagain Avalanche: Goldpan – avalanche
11/11/23 Turnagain Observation: Tincan Common
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11/10/23 Turnagain Observation: Tincan Trees
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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.