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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Mon, February 4th, 2019 - 7:00AM
Expires
Tue, February 5th, 2019 - 7:00AM
Forecaster
Wendy Wagner
The Bottom Line

The avalanche danger remains a ‘scary’ MODERATE  on slopes above 2,000′. A worriesome weak layer of buried surface hoar is lurking 2-3′ below the surface and it remains possible to trigger a large and dangerous hard slab avalanche; similar to those triggered last Friday.  On the surface, there are more obvious avalanche problems associated with yesterday’s snowfall. These are fresh wind slabs, up to a foot thick, and loose snow sluffs. Additionally, cornices are growing and remember to give them a very wide berth.  

SUMMIT LAKE / JOHNSON PASS:    Roughly 4-6″ of new snow has fallen with wind in this zone. A variety of weak layers exist in the snowpack and human triggered slab avalanches 1-3′ thick remain possible on slopes with recent wind loading.  

LOST LAKE:   New snow and wind yesterday in the Lost Lake and Seward area is expected to have increase the avalanche danger.  

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Mon, February 4th, 2019
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
  • Persistent Slabs
    Persistent Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

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

New snow and wind loading yesterday has added weight onto a snowpack with a troublesome buried weak layer that sits 2-3′ deep. This is our primary concern today despite the surface instabilities of fresh winds slabs and sluffs in the 4-8″ of new snow. It was only three days ago that three very large and dangerous avalanches were triggered by people along Seattle Ridge. This problem layer, which is buried surface hoar, was responsible for the avalanches. That layer has not gone away and although it has become more stubborn to trigger with time, every new load can cause it to become more reactive. This is the ‘scary’ part of the situation as there are a lot of unknowns. 

What to keep in mind if skies clear enough for travel above treeline:
   1-  Around 2-3 feet under your feet or snowmachine sits a weak layer
   2-  The weak layer may or may not be reactive, this is the tricky part
   3-  If an avalanche is triggered in this deeper weak layer, it can be very large, propagate across the entire slope and fill the valley floor with debris
   4-  No obvious signs of instability are likely to be seen before a slope releases
   5-  Remotely triggering a slab from the ridge, sides or below is possible
   5-  Safe travel protocol and assessing the consequences if the slope slides, the larger the terrain the larger the potential avalanche

To complicate matters, other areas such as the Girdwood Valley have seen other weak layers  of facets associated with crusts. These are now roughly 2-3′ below the surface. All this said, it’s important to keep in mind that the snowpack harbors buried weak layers. 

 

Photo of the crown of the Widowmaker avalanche from Feb. 1st. It’s a bit tough to see, but the bed surface is covered in 1cm feathers of buried surface hoar that was the responsible weak layer.

Avalanche Problem 2
  • Wind Slabs
    Wind Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    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

Ridgetop winds associated with yesterday’s snowfall were moderate from the east. This is the perfect recipe for building wind slabs along ridgelines. Furthermore, a new batch of surface hoar grew last weekend and these slabs could be more touchy than expected if sitting on the surface hoar. Watch for cracking in the surface snow and stiffer snow over softer snow. Although wind slabs are likely to be shallow, they could be more dangerous if they were to step-down and trigger a large slab that breaks in the buried surface hoar discussed above. 

Loose snow sluffs:  Sluffs on steep slopes within the new snow should be expected.

Cornices:  We had a report of a large cornice fall in upper Seattle Creek drainage over the weekend. Heads up to give these an extra wide berth as they continue to grow. 

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

Glide cracks are opening again. We know of one glide avalanche that has released recently in the Summit zone just north of Manitoba. Heads up to look for glide cracks and limit exposure under them! 

Note the glide cracks in Seattle Creek’s Main Bowl (1st Bowl) on the slopes near the top of the photo. This photo was taken on Feb. 2nd, near the trigger point of the very large Widowmaker avalanche, which occurred Friday, Feb. 1st. 

Weather
Mon, February 4th, 2019

Yesterday:    While heavy snow fell near and north of Anchorage yesterday, only light snowfall was seen in our forecast regions of Girdwood, Turnagain and Summit Lake. Roughly 4-5″ of snow fell at the mid elevations and up to 8″ at the higher elevations. The Girdwood Valley saw closer to 8″ at the mid elevations. Ridgetop winds averaged in the 20’smph with stronger gusts from the east over the course of the day and have decreased to the 5-10mph overnight.

Today:   A break in weather is expected today with a chance for clearing skies and valley fog. Cloud cover and light snow showers are expected to pick back up tonight as another front pushes moisture in from the southwest.   Ridgetop winds are expected to remain light, 5-15mph, from the east today before increasing to the 20-30mph range tonight. Temperatures are slowing climbing into the 20’sF at the upper elevations, where they will remain, and should hover near 30F at sea level.  

Tomorrow:   Light snow showers are expected to continue through tomorrow with accumulations in the 2-6″ range at the mid elevations.  

*The Seattle Ridge anemometer (wind sensor) was destroyed. We have a replacement on the way and it should be operational by mid February.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 26   4   0.2   60  
Summit Lake (1400′) 26   5   0.4   27  
Alyeska Mid (1700′) 26   5   0.37   52  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 18   NE   12   36  
Seattle Ridge (2400′) 23   *N/A   *N/A     *N/A    
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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

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Closed as of May 1. Thanks for a fun, safe season!
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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.