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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, March 28th, 2019 - 7:00AM
Expires
Fri, March 29th, 2019 - 7:00AM
Forecaster
Heather Thamm
Avalanche risk The Bottom Line

The avalanche danger is MODERATE across the Advisory area this morning but will rise to CONSIDERABLE  above 2000′ on South (SE-SW) aspects due to the possibility of wet loose avalanches and glide avalanches in the afternoon. With warm temperatures and direct sunshine on steep, solar aspects triggering a large, dangerous deep slab avalanche in the Alpine is also still a concern. Give cornices and glide cracks a wide berth. Pay attention to changing surface conditions and avoid solar aspects later in the day.

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 cornices to fall. Due to steep avalanche terrain directly above Byron Glacier Trail – this summer trail is 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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Thu, March 28th, 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

Wet loose avalanches are a concern today on solar aspects due to above freezing temperatures and cloudy skies trapping in the warmth. Many weather stations have remained above freezing for the last two days including Seattle Ridge. Yesterday light to moderate winds helped to keep surfaces cooler, but today’s winds are expected to decrease to calm this afternoon. Lack of wind combined with radiation from the sun could play a bigger role in heating up South (SE – SW) facing aspects in the afternoon and evening. Also thin cloud cover today has the potential to trap more heat and “greenhouse” the snow. All of these subtle factors are impossible to fully forecast and evaluating the snowpack as you travel is essential. Today it will be important to pay close attention to surface conditions. If you are sinking in on your skis or trenching on your machine or punching through with your boots it’s a good idea to get off the slope. In the Alpine drier snow may feel like mashed potatoes as it heats up. Be on the lookout for roller balls or small avalanches initiating from rocks. On slopes steeper than 35 degrees you could initiate a small avalanche that entrains a larger volume of snow as it picks up momentum. Plan your route to avoid travel on or under steep, solar slopes later in the day and recognize that the greenhouse effect could warm shaded aspects in the Alpine as well. Choose terrain carefully!

Southwest facing slopes of Magnum presents a variety of avalanche issues including a large cornice, glide cracks widening, and the potential for triggering wet loose avalanche with warming later in the day. 

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

Two days ago a very large natural slab avalanche was reported north of Girdwood high in the alpine around 4500’. A wide crown was described as very deep. Evidence of deeper instabilities is visible all over the mountains following a widespread wet slab avalanche cycle that ended on Monday. Now the deeper instability is more of concern in the alpine above 3000’, above where it rained last week. This colder snow in the Alpine is now getting warmed by direct sunshine and rising temperatures during the day, but the potential for triggering a deep slab exists on all aspects. This is a very difficult avalanche problem to evaluate due how deeply buried (5-10+’) the old snow (faceted) interface lies. As we move away from a two week storm cycle that created this problem, a lot uncertainly remains for triggering this kind of avalanche. 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. Use good travel protocol and think about consequences if a slope does slide. In addition, if traveling in the alpine look for areas that are more wind-loaded, watch for cracking and listen/feel for hollow snow indicating wind slabs in upper elevation terrain.

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.

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.

This crown was visible on Monday in the Twentymile zone and is a good example of how deep a slab might release.

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 week including several yesterday on the Southeast slopes of Seattle Ridge. One of these glide avalanches pulled out a slab along with it. Above freezing temps and solar warming today may allow for more glides to release naturally. 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.

One of several glide avalanches that released yesterday on the SE face of Seattle Ridge. This one also initiated a slab with it. Keep in mind this elevation band did not freeze overnight and may see more activity with daily warming and sun today.

Weather
Thu, March 28th, 2019

Yesterday: Skies ranged from cloudy to hazy with the sun poking through on occasion. Temperatures reached the mid 50Fs near sea level, mid 40Fs in the mid elevations and mid 30Fs in the upper elevations. Overnight temps at sea level and in the mid elevations remained above freezing. Seattle Ridge weather station has also remained above freezing overnight, but Sunburst and Summit Lake dipped into the upper 20Fs. Ridgetop winds were 8-10mph with gusts 15-20mph from the East all day and overnight.

Today: Skies could range from partly cloudy to mostly cloudy with a chance of scattered showers near coastal areas. No accumulation is expected. Daytime temperatures are expected to reach the mid 40Fs in the mid and lower elevations. In the alpine temps could reach the upper 30Fs. Ridge top winds from the NE will be light (5-15mph) but are expected to decrease into the afternoon.

Tomorrow: High pressure will continue tomorrow and into the weekend. Some cloud cover is expected through tomorrow. Temperatures are expected to remain in the 30Fs to mid 40Fs with light and variable winds. No precipitation is expected.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 42    0 0   69  
Summit Lake (1400′) 37    0 0   22  
Alyeska Mid (1700′) 39   0   0   64  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 31   ENE   9   23  
Seattle Ridge (2400′) 37   SE   9   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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