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ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, March 29th, 2019 - 7:00AM
Expires
Sat, March 30th, 2019 - 7:00AM
Forecaster
Heather Thamm
Avalanche risk The Bottom Line

The avalanche danger is generally LOW in the morning below 3000′, but is expected to rise to CONSIDERABLE by late afternoon on steep Southern aspects where initiating a wet loose avalanche is likely. Above 3000′ the avalanche danger is MODERATE where triggering a deeper more dangerous slab avalanche is possible. Later in the day with warming from the sun, the avalanche danger in the Alpine will also rise to CONSIDERABLE on South aspects where triggering a cornice or wet loose avalanche has the potential to initiate a destructive slab avalanche. Pay attention to changing surface conditions and avoid solar aspects in the afternoon. Natural glide avalanches have been occurring daily. Give cornices and glide cracks a wide berth.

PORTAGE VALLEY: There is a still a chance that direct sunshine and afternoon warming could initiate wet loose avalanches that run to lower elevations or cause cornice fall. Summer trails with avalanche terrain overhead, like Byron Glacier Trail, are not recommended.

SUMMIT LAKE (& INTERIOR EASTERN KENAI MTS): The snowpack in this zone is shallow and has many weak layers. Human triggered slab avalanches remain possible in upper elevation terrain with direct sunshine and warming temperatures. In addition, look for wet loose avalanches and changing surface conditions in the afternoon.

SEWARD/LOST LAKE: Similar to Turnagain – Human triggered avalanches will be possible today with warm temperatures and direct sunshine. Extra caution especially on steep, solar aspects in the Alpine is recommended.

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Fri, March 29th, 2019
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.
Avalanche Problem 1
  • Wet Loose
    Wet Loose
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
Wet Loose
Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
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 snowpack has shifted into a daily pattern of freezing surfaces overnight and melting surface snow in the afternoon and evening. On steep solar aspects radiation from the sun has also caused some natural wet avalanche activity later in the day. The majority of this activity has been on slopes that have rocks or dirt exposed and act as heat conductors, like Seattle Ridge which also saw a big avalanche cycle during last week’s rain event. Today we have a very warm and sunny day and calm winds today could allow surfaces to heat up more than they have in the past few days.

The best way to manage this problem is to shift over to more shaded slopes later in the day. Pay close attention to surface conditions and if you are sinking in on your skis or trenching on your snowmachine or punching through with your boots it’s a good idea to get off the slope. Push-a-lanches where the surface snow picks up momentum and entrains more surface snow is a good indicator of dangerous conditions. Avoid travel directly on or under South facing slopes later in the day. Visual clues such as recent avalanches will be tough to identify due to a widespread avalanche cycle last week. It’s really hard to tell which avalanches are new and which ones are old.

CORNICES: Cornices are very large and warm air temperatures and direct sunshine could destabilize them today. A cornice fall has the potential to trigger a very large avalanche on the slope below and could break farther back than expected. Give them a wide berth.

Seattle Ridge has been falling apart between the big wet avalanche cycle last week during a heavy rain event, natural glide avalanches, and now the sun is heating up loose snow around rocks and dirt daily. 

Avalanche Problem 2
  • Deep Persistent Slabs
    Deep Persistent Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
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.
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

Above 3000’ deeper snowpack instability remains a concern. A big storm event that ended on Monday and lasted two weeks dumped 5-10+ feet of snow across our region. In many places this overloaded an older snow (facets/buried surface hoar) interface and caused some very large natural avalanches. At this point there remains uncertainty if a human could trigger such a large and dangerous avalanche. Triggering this kind of avalanche would either require a very large trigger like a cornice or large wet-loose avalanche later in the day or a skier or snowmachiner finding just the right trigger spot in a shallow part of the slab. Signs of instability won’t necessarily be present and it could be the 1st or 10th skier or rider on a slope that triggers a very large destructive avalanche. All aspects and slopes above 3000’ that haven’t already avalanched are suspect. Snow pits and stability tests won’t necessarily provide representative information. This avalanche problem has become a high consequence low likelihood scenario and managing this problem is tricky. Keep in mind the following:

  • Identify and avoid cornices and dangerous terrain traps 
  • Avoid steep solar aspects that are heating up later in the day
  • Use safe travel protocols like traveling one at a time from established safe zones

South of Turnagain in Summit Lake and areas in the interior Kenai Peninsula have a very poor snow structure with variety of old weak layers in the mid pack (facets and buried surface hoar.) Triggering a persistent slab 2-3’ deep as slopes warm in the afternoon sun is possible in the alpine. 

A recent natural slab avalanche on a West aspect of Butch Mt in Summit Lake. The timing of this avalanche in unknown, but due to how fresh the crown appears we suspect it happened within the last two or three days. Note the older crown from last week in the lookers left corner is more filled in. 

Additional Concern
  • Glide Avalanches
    Glide Avalanches
Glide Avalanches
Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.
More info at Avalanche.org

Many glide cracks have avalanched over the last few days throughout the region. The Southeast face of Seattle Ridge has seen a bunch of natural glide avalanches in the last three days. Glides are very unpredictable and fail at the ground pulling out the entire season’s snowpack. Many cracks are opening up in popular terrain and avoidance is strongly recommended. Plan your routes to avoid spending any time below existing glide cracks. 

 

This glide crack released in the early evening yesterday also pulled out small slab with it.

Weather
Fri, March 29th, 2019

Yesterday: Skies were clear and sunny. Temperatures reached the low 50Fs near sea level, upper 40Fs in the mid elevations and mid 30Fs in the upper elevations. Overnight temps at sea level and mid elevations hovered around freezing. A little cloud cover moved in overnight, but no precip was recorded. Ridgetop winds were 5-15mph from the East all day becoming light and variable overnight.

Today: Skies will be mostly sunny with some clouds in the morning. Daytime temperatures are expected to reach the mid 40Fs in the mid-elevations. In the alpine temps could reach the upper 30Fs. Ridge top winds should remain light and variable today. Overnight temperatures should drop below freezing to the upper 20Fs.

Tomorrow: High pressure and sunny skies will continue tomorrow and into next week. Similar daily temperatures are expected, mid 40Fs during the day and low 20Fs overnight. Light winds and no precipitation is expected.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 38   0   0   69  
Summit Lake (1400′) 36   0   0   22  
Alyeska Mid (1700′) 39   0   0   63  

0  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 29   ENE   6   23  
Seattle Ridge (2400′) 34   ESE   6   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.
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