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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, April 6th, 2019 - 7:00AM
Expires
Sun, April 7th, 2019 - 7:00AM
Forecaster
Heather Thamm
Avalanche risk The Bottom Line

The avalanche danger is MODERATE today at all elevations. Triggering a wind slab up to 10″ thick will be possible on steep leeward features due to new snow and strong wind. Below 3000′ glide cracks may avalanche without warning in popular terrain. Plan your route to avoid traveling under existing cracks. Additionally triggering a wet loose avalanche may be possible with rain in the lower elevations if surface crusts become unsupportable. Be prepared for natural wet loose activity on solar aspects in the alpine if the sun makes an appearance today.

PORTAGE VALLEY: Summer trails with avalanche terrain overhead, such as Byron Glacier Trail, are not recommended due to the possibility of an avalanche or cornice fall sending debris over the trail.

Special Announcements

If you’re headed to Alyeska today, bring your avalanche beacon. Backcountry Access (BCA) and Alyeska Ski Resort will have a beacon training park set up all day. Check in at any Ski Patrol station or visit the  Snow Report  for the location of the search park.

Are you wondering what the snowpack is like in Hatcher and Chugach State Park with all this warm spring weather? Check out  Hatcher Pass Avalanche Center  Saturday advisory  and  recent observations from  Chugach State Park  and  consider bringing bear spray.

Sat, April 6th, 2019
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
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
  • Wind Slabs
    Wind Slabs
  • 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.
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

Warm temperatures and Low pressure in the Gulf of Alaska will bring rain to ~2000’ today and snow showers above. In the alpine Easterly winds 20-35mph will form isolated wind slabs on leeward terrain features. The size of these wind slabs will depend on how much snow falls. Areas closer to the coast (Portage, Girdwood and North end of Turnagain Pass) could see 4-6” of snow by early evening. Interior areas will see less precipitation. New snow will be falling on a variety of slick surfaces and bonding may be poor. Surfaces include firm crusts on W-S-E facing aspects in the alpine and surface hoar and near surface facets on shaded Northern aspects. Look for blowing snow, shooting cracking – steep leeward terrain features will be the most suspect for triggering a wind slab. Keep in mind its spring-time and be ready for wet loose activity on solar aspects if the sun comes out.

WET LOOSE: In the lower elevations below 2000’ where a stout crust has formed rain will start to soften this surface crust. If this crust starts to deteriorate and become wet and unsupportable, triggering a wet loose avalanche in steeper terrain will be possible.

CORNICES: Cornices remain very large along some ridgelines in the alpine. Give them a wide berth and avoid travel directly below them.

South of Turnagain in Summit Lake and areas in the interior Kenai Peninsula have a variety of old weak layers within the snowpack. The last observed slab avalanche was 10 days ago on the West face of Butch. A stout crust has formed on all elevations below 3000’ and on solar aspects in the alpine. Triggering a persistent slab 2-3’ deep is becoming an outlier at this time, but worth noting if headed to Summit Lake. Be aware of a variety of avalanche conditions ranging from small isolated wind slabs to wet loose. This area is more likely to see sun in the afternoon. Otherwise conditions are very similar to Turnagain.

Wind slabs will be forming on a variety of surfaces. Surface hoar  has been seen on North aspect (top photo) and stout crusts (bottom photo) on all aspects below 3000′ and W-S-E aspects in the alpine.

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

Glide cracks have been opening and releasing across our region over the last two weeks. This is the entire snowpack gliding downhill and spontaneously releasing. So far all of this activity has occurred on East, South, and West facing slopes below 3000’. This week Turnagain Pass has been the most active. Glide avalanches are not associated with a human trigger and release without warning. These avalanches, even small ones, are extremely destructive. The best way to manage this problem is avoidance. Identify existing cracks and plan your route to avoid being under a glide. Many cracks are opening in popular terrain and may require an unusual route to avoid their runout. This is a case of ‘don’t be caught in the wrong place at the wrong time’.

Recent glide avalanches and lots of glide cracks on popular terrain in Turnagain. From left to right – Magnum, Corn Biscuit, and Lipps – SW aspects

Weather
Sat, April 6th, 2019

Yesterday: Skies were sunny to partly cloudy. Temperatures reached the mid 40Fs to low 50Fs at sea level. Ridgetop temps were in the upper 20Fs during the day and dropped to the low-20Fs overnight. Ridgtop winds were light from the Northeast increasing into the 20s mph overnight. Light rain starting falling this morning with 0.1″ recorded at Turnagain Pass. Rainline is estimated around 1800′.

Today: Snow showers are expected above 2000′ 4-6″ of snow possible. Rain is expected in the lower elevations with up to 0.5″ of rain. Areas further from the coast will see less precip today and a chance for broken skies in the afternoon. Easterly ridge top winds are expected to be 20-35mph. Temperatures at sea level will be in the mid-40Fs and temps along ridgetops may reach the low-30Fs.

Tomorrow: Scattered snow and rain showers are expected on Sunday. Easterly Ridgetop winds will decrease to 5-15mph. Light precipitation is expected with rain below 2500′ and a few inches of snow possible in the upper elevations. Temperatures at sea level will be in the 40Fs to low 50Fs. Ridge top temps are expected to range from mid-20s to low-30Fs.

**SNOTEL data for Center Ridge and Summit was unavailable after 5am (reads 4am due to daylight savings.) 24-hour precip for Turnagain was recorded on the Turnagain DOT RWIS weather station.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 35 0 **0.1 65
Summit Lake (1400′) 35 0 0 20
Alyeska Mid (1700′) 36 0 .01 57

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 25   ENE   8   24  
Seattle Ridge (2400′) 31   ESE   3   11  
Observations
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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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