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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Mon, March 19th, 2018 - 7:00AM
Expires
Tue, March 20th, 2018 - 7:00AM
Forecaster
Wendy Wagner
The Bottom Line

Today is a day of transition. Triggering a large, dangerous slab avalanche 2-4+’ thick is still possible on all aspects and all elevations due to weak snow under a dense slab. However, there are additional avalanche problems closer to the surface.

This morning, the avalanche danger is  CONSIDERABLE  on all aspects below 1000′ where rain is falling. Wet snow avalanches will be likely at these lower elevations before colder temperatures move in later this afternoon.  This afternoon, strong winds will impact the region and may increase the avalanche danger to CONSIDERABLE at the upper elevations by tonight where cornice falls and wind slabs may occur naturally. The mid elevation band is expected to remain generally MODERATE. Pay attention to changing snow conditions and remember remote triggering a large slab remains possible. Evaluate terrain consequences and practice safe travel protocol.  

Dangerous avalanche conditions exist in Summit Lake, check out the most recent  Summit snowpack and avalanche summary  if you are headed South of Turnagain Pass.

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Mon, March 19th, 2018
Alpine
Above 2,500'
Considerable (3)
Avalanche risk
Treeline
1,000'-2,500'
Moderate (2)
Avalanche risk
Below Treeline
Below 1,000'
Considerable (3)
Avalanche risk
Alpine
Above 2,500'
Avalanche risk
Considerable (3)
Treeline
1,000'-2,500'
Avalanche risk
Moderate (2)
Below Treeline
Below 1,000'
Avalanche risk
Considerable (3)
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

This morning will be the final push of warm and rainy weather before skies clear and a cold Northwest wind sets in to freeze the wet and saturated snow. Up to 2-3″ of new snow could fall above 1,000′. This dramatic change in the weather pattern is expected around noon today and by late this afternoon, strong winds along with sunny skies are forecast. The cooling trend will stabilize the lower elevations quickly. At the higher elevations, the winds will search out any remaining soft snow, along with any new snow, and attempt to build wind slabs and grow cornices. Wind slabs could be touchy and may release naturally, yet they should be on the smaller side – up to a foot thick. Wind slabs may also overload weak layers deeper in the pack, contributing to a much larger slide.

This larger avalanche issue is our deep persistent slab problem, which boils down to: triggering a large destructive avalanche 2-4+ feet thick is still possible. For the first time in over a week, there were no new slab avalanches reported yesterday. The last avalanche was a snowmachiner triggered slab in Main Bowl of Seattle Ridge on Saturday. This slope had tracks on it from the day before and was triggered from a flatter area below (remotely triggered from below). To recap, over the past week many large human triggered avalanches and/or natural avalanches have released from Girdwood to Lost Lake (please see Heather’s video if you have not already). Some of these avalanches have been remotely triggered and some with skier/snowmachiners on the slope. Most of these avalanches have occurred below 3000’, releasing on weak faceted snow on a slick crust, 2-4 feet below the surface. A lot of uncertainty remains in the Alpine where slab depths are much deeper and triggering an avalanche could be more stubborn. Widespread buried surface hoar and facets have been well documented at all elevations. 

*With a deep slab problem it is important to remember no signs of instability may be present before a slope releases. It may be the 10th person onto the slope that finds the trigger point and slopes may be triggered remotely. It is crucial to visualize the consequences if the slope does slide. Are there terrain traps below?  Bigger slope = Bigger avalanche. Thin spots near rocks and along ridgelines are likely areas to find the trigger point. 

Main Bowl avalanche from Saturday, March 17th, East aspect. This was a remote snowmachine trigger from below. No one was caught. Photos: Brian Bird

Avalanche Problem 2
  • Wet Loose
    Wet Loose
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    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

Until the skies clear and temperatures cool down later today, wet loose and wet slab avalanches remain likely. The exception will be Southerly slopes that see sunshine and are out of the winds. These slopes may continue to see wet avalanche activity through the day. A wet point release could be small to large and has the potential to trigger a much larger, more dangerous slab. Pay attention to the surface conditions and avoid slopes with saturated wet snow. 

Additional Concern
  • Cornice
    Cornice
Cornice
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
More info at Avalanche.org

Cornices are large and looming. Give these an extra wide berth and limit exposure underneath them. Winds today may find enough snow to add to their size and this along with sun later today, may be enough to cause some of break off naturally. A cornice fall could trigger and propagate an avalanche on a slope below.

Weather
Mon, March 19th, 2018

Mainly obscured and overcast skies filled the region yesterday. Temperatures were warm, in the upper 30’sF at 1,000′ and in the upper 20’sF along the higher ridgelines. Winds have been Easterly in the 5-15mph range with stronger gusts during the past 24 hours. Between .1 – .2″ of rain has fallen up to 1,000′ overnight in the Girdwood Valley, but Turnagain Pass and Summit Lake weather stations are reporting no precipitation.

For today, Monday, we can expect .2-.3″ of rain up to 1,000′ this morning with 2-3″ of snow falling above 1,000′. By the afternoon, skies look to clear as this system moves out and strong Northwest outflow winds move in; ridgetop winds look to be in the 25-40mph range with strong gusts. Temperatures will be warm (upper mid 30’s at 1,000′ and mid 20’sF along ridgetops) before cooling down with the NW winds this afternoon to the upper 20’sF at 1,000′ and teens along the ridgetops.

Tuesday and into the work week, the cold Northwest outflow winds look to remain entrenched for much of Southcentral Alaska. Sunny skies will accompany the wind along with a cooling trend at all elevations.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 32   0   0   82  
Summit Lake (1400′) 34   0   0   30  
Alyeska Mid (1700′) 33   2   0.2   77  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 24   E   7   18  
Seattle Ridge (2400′) 29   E   11   23  
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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.