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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Sat, March 24th, 2018 - 7:00AM
Expires
Sun, March 25th, 2018 - 7:00AM
Forecaster
Heather Thamm
The Bottom Line

The avalanche danger is  MODERATE  above 1000′ where triggering a large, destructive slab avalanche 2-4+ feet thick is possible on all aspects and may be remotely triggered. Likely trigger spots will be thinner areas of the snowpack near rocks or scoured areas near ridgelines. Watch for wind slabs and avoid cornices. Pay attention to afternoon warming. Evaluate snow and terrain carefully.  

Below 1000′ avalanche danger is LOW where a stout surface crust has formed.  

Check out the most recent  Summit snowpack and avalanche summary  if you are headed South of Turnagain Pass.

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Sat, March 24th, 2018
Alpine
Above 2,500'
Moderate (2)
Avalanche risk
Treeline
1,000'-2,500'
Moderate (2)
Avalanche risk
Below Treeline
Below 1,000'
Low (1)
Avalanche risk
Alpine
Above 2,500'
Avalanche risk
Moderate (2)
Treeline
1,000'-2,500'
Avalanche risk
Moderate (2)
Below Treeline
Below 1,000'
Avalanche risk
Low (1)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
Avalanche Problem 1
  • Deep Persistent Slabs
    Deep Persistent Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    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

The avalanche danger is best described as SCARY MODERATE because our current hard-pack snow conditions look and feel deceivingly stable. Triggering a dangerous slab 2-4+ feet deep may be stubborn or could catch you by surprise in an unusual spot. Yesterday a skier remotely triggered a large avalanche while descending a low angle slope on Raggedtop Mountain in Girdwood Valley on a Southeast aspect. This avalanche released on a steep rocky terrain feature 200 yards away from an adjacent slope. No obvious clues were present until the slope released. This is the scary part about our current snowpack.  It may be the 10th skier or snowmachiner onto a slope that finds a thin part of the snowpack (a trigger point) or it could be triggered remotely from an adjacent slope or below. 

All aspects are suspect due to widespread and well preserved facets under a connected slab 2-4+ feet thick. These facets may be sitting on a slick melt/freeze crust or mixed with buried surface hoar from January. Strong Northwest winds that ended Thursday have caused unusual loading patterns opposite our usual Easterly storm track direction. This means the typical windward slopes with thinner weaker snow may be more loaded than usual and ‘trigger spots’ may be lurking just below the surface in unexpected places. Observers over the last few weeks have found poor structure along scoured ridges and under sastrugi we all assume is bomber and safe. Knowing where in the terrain a large and destructive avalanche could be triggered is a difficult question without x-ray vision. Evaluate terrain for consequences and don’t assume the ‘typical safe zones’ are truly safe. The bigger and steeper the terrain the more potential for a very large and destructive avalanche. 

**Over the last few days only very steep South and Southeast slopes have been softening in the sun. Gentler slope angles have remained dry on these aspects. Surface warming may not be present until committed to steeper features. Don’t underestimate the power of the sun. Subtle warming may make it easier to trigger a slab later in the afternoon.  

 Skier triggered slab on SE aspect of Raggedtop at ~3000′ at 3pm yesterday from 200 yards away on low angle terrain. This photo was taken later in the evening. The skiers route can be seen along the lookers left ridge. 

 This avalanche occurred on a thinner rocky area on a steep terrain feature. Don’t forget larger more connected slopes have the potential for an even larger avalanche. 

 

Loose facets sitting on a hard melt/freeze crust found under 5 feet of hard snow in Lynx Creek on Tuesday. This structure has been found across our forecast zone. 

Additional Concern
  • Wind Slabs
    Wind Slabs
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.
More info at Avalanche.org

Wind Slabs: Old wind slabs may be lurking on a variety of aspects due to unusual loading patterns this week. Smooth supportable surfaces where the snow is hollow sounding are suspect, especially if the slope is unsupported. Triggering an old wind slab is becoming less likely, but warming from the sun can make triggering easier in the afternoon. It will be impossible to tell the different between a small isolated wind slab versus the deeper more dangerous avalanche problem described above.   

Cornices: Cornices are large and looming and the sun and above freezing temperatures can make them more unstable. Give cornices lots of space and limit exposure underneath them.

Northwest winds can funnel through Turnagain Pass from the Southwest and load Northern as on Magnum North facing terrain. These slopes are often thin and scoured.    

Weather
Sat, March 24th, 2018

Yesterday skis were clear and winds were light and variable. Daytime high temperatures near sea level reached the low 40F’s and overnight dipped into the teens F.   Ridgetop temperatures reached the mid 30F’s and cooled to teens (F) overnight.   No precipitation was recorded.  

Today expect similar either with clear and sunny skies. Winds will be light and variable. Daytime temperatures may reach the mid 30F’s in the upper elevations and low 40F’s at sea level. Overnight temperatures will drop back into the teens F at all elevations.  

Tomorrow skies are expected to become overcast as low-pressure moves into our region. Scattered snow showers are possible Sunday night and Monday, and a few inches of new snow is forecasted. Temperatures are expected to stay cool enough to keep this precip as snow. Ridgetop winds will build tomorrow to Moderate (15-30mph) from the East through Monday.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 27   0   0   80  
Summit Lake (1400′) 19   0   0   32  
Alyeska Mid (1700′) 27   0   0   75  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 26   variable   4   15  
Seattle Ridge (2400′) 24   variable   3   15  
Observations
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Riding Areas
Updated Fri, May 01st, 2020

Status of riding areas across the Chugach NF is managed by the Glacier and Seward Ranger Districts, not avalanche center staff. Riding area information is posted as a public service to our users and updated based on snow depth and snow density to prevent resource damage at trailhead locations. Riding area questions contact: mailroom_r10_chugach@fs.fed.us

Area Status Weather & Riding Conditions
Glacier District
Johnson Pass
Closed
Placer River
Closed
Skookum Drainage
Closed
Turnagain Pass
Closed
Closed as of May 1. Thanks for a fun, safe season!
Twentymile
Closed
Seward District
Carter Lake
Closed
Lost Lake Trail
Closed
Primrose Trail
Closed
Resurrection Pass Trail
Closed
Snug Harbor
Closed
South Fork Snow River Corridor
Closed
Summit Lake
Closed

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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.