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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sun, February 18th, 2024 - 7:00AM
Expires
Mon, February 19th, 2024 - 7:00AM
Forecaster
Andrew Schauer
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE above 2500′. The easterly winds are expected to back off slightly, but will still be moving snow into sensitive slabs in the upper elevations, making it easy to trigger an avalanche. There is also a weak layer of snow buried deeper in the snowpack that has the potential to make bigger avalanches, especially in the mid elevations. The danger is MODERATE below 2500′.

Special Announcements

Summit Lake Avalanche Accident:  An avalanche was triggered on Tuesday February 13th by a group of three backcountry skiers on John Mtn. Two of the three people involved sustained injuries and the third did not survive. Our deepest condolences go out to the friends and family of the deceased. A preliminary accident report is available here and we will publish a full report by the end of next week.

Forecaster chat Friday, February 23 from 5-6 pm at the Seward Community Library and Museum community room. For our friends down on in Seward and Moose Pass, come chat with us about the product we are producing called the “Weekend Avalanche Outlook”. We’ll also be talking about the state of the snowpack in Summit and Seward, and any other questions you have. More info Here.

Sun, February 18th, 2024
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
Avalanche risk
Mon, February 19th, 2024
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
Mon, February 19th, 2024
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

The last human triggered avalanche that we know of was a large avalanche in the Squirrel Flats area above the Placer Valley back on Wednesday Feb. 14. There have been many natural wet loose avalanches since the temperatures started to climb in the middle of last week.

Large natural wet avalanche debris in the upper Girdwood Valley. This occurred late last week. Photo: Billy Finley, 02.17.2024

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

The easterly winds that have been howling through our forecast zone since early last week are easing slightly, but will likely still be strong enough to be moving snow around at the upper elevations today. After nearly a week of strong winds, the amount of soft snow on the ground available to be blown into fresh wind slabs is becoming limited, and that will most likely keep any fresh wind slabs on the smaller side. However, with sustained speeds of 15 to 30 mph avalanches may still be large enough to bury a person.

Look out for dangerous conditions on steep slopes below ridgelines, convex rolls, and in gullies. Pay attention to warning signs like cracks shooting out from your snowmachine or ski tips as clear indicators of unstable snow, but keep in mind you won’t always see these signs when conditions are dangerous. You may be able to assess this problem with travel tests like slope cuts or hand pits, but you can also manage the problem by avoiding the common problematic terrain features mentioned above. Remember, we are still dealing with a dangerous persistent weak layer that is buried deeper in the snowpack and this week-long wind event is still adding stress to that weak layer. More on this in Problem 2 below.

Wet Loose Avalanches will remain likely, especially in areas that are seeing rain on snow. These have been small for the most part, but they can be dangerous if they carry you into terrain traps like trees, cliffs, or gullies where debris can pile up deeper.

The winds were still able to find snow to blow around yesterday down in Summit, and it will be looking similar throughout the advisory area today. Photo: Trevor Clayton, 02.17.2024.

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

The deeper weak layer of faceted snow that formed back in January is trickier than the wind slabs mentioned above. We’ve found this layer to be most problematic at the mid elevations between around 1000′ to 2500′. This is concerning during a long wind event like we’ve been seeing since the most dangerous slopes are probably in the same elevation band where we are going to find the best surface conditions. From what we have seen lately, it seems like the weak layer is less concerning in Turnagain Pass than it is in Girdwood, Placer, or Summit. However, it still exists in Turnagain, and we think a person can still trigger a big avalanche on that weak layer.

The problem layer is around 1-2 feet deep on average, but it may be deeper where the winds have loaded the surface. As opposed to the wind slabs mentioned above, quick travel tests do not give good feedback on these layers. This one is going to be too deep to reliably assess with a hand pit, and there have been numerous accident reports in the past where people comment that they didn’t see any red flags before triggering a large avalanche. If you are thinking about stepping out into steep terrain, the only way to reliably assess this weak layer is to dig a snowpit near the piece of terrain you are considering. Stability tests can give you some information, but you can also just look for the thick layer of weak, sugary snow as a warning sign. The problem can be tricky to identify, and you can play it safe by sticking to low angle terrain a little longer while this layer continues to heal.

The AAS motorized Rec 2 group was out on the front side of Seattle Ridge digging in the snow yesterday. They were able to identify the January facet layer but did not get concerning test results. Photo: Graham Predeger, 02.17.2024

Weather
Sun, February 18th, 2024

Yesterday: Skies were mostly cloudy with warm temperatures in the upper 20s F at the highest elevations and upper 30s F to low 40s F below around 3000’. Winds were strong out of the east at 15 to 40 mph with gusts over 60 mph. Portage valley picked up around 0.2” precipitation, and no other stations are showing precipitation in the past 24 hours. Rain line was somewhere around 2000 to 2300 feet.

Today: Today is looking to be similar to yesterday, with warm temperatures, cloudy skies, and some light precipitation possible. Easterly winds are expected to average 15 to 25 mph with gusts of 20 to 30 mph. High temperatures should be in the low to mid 30s F, only cooling a couple degrees overnight tonight. Most areas will only see a trace of precipitation today, but some coastal zones could see up to 0.5” precipitation falling as rain up to 2000’ to 2500’.

Tomorrow: A stronger system is expected to start impacting our area tomorrow afternoon, bringing heavy precipitation to the Seward zone first. Those southern mountains are expected to receive 2-4” snow during the day, and up to 2 feet by Tuesday morning. Most of the rest of the advisory area should only see an inch or two during the day tomorrow, and 4 to 6” by Tuesday morning. The rain level should drop at least a little bit, down to around 1500’ to 2000’. Winds are expected to increase again with sustained speeds of 15 to 35 mph and gusts of 30 to 50 mph. Temperatures are expected to stay warm with highs in the mid to upper 30s F and lows in the low to mid 30s F.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 37 0 0 83
Summit Lake (1400′) 34 0 0 42
Alyeska Mid (1700′) 36 0 0 92
Bear Valley – Portage (132′) 41 0 0.21
Grouse Ck (700′) 36 0 0 61

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 26 ENE 20 64
Seattle Ridge (2400′) 30 SE 11 28
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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.