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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Tue, January 29th, 2019 - 7:00AM
Expires
Wed, January 30th, 2019 - 7:00AM
Forecaster
Wendy Wagner
The Bottom Line

The avalanche danger remains CONSIDERABLE  on slopes above 1,500′. Fresh wind slabs and cornice falls, due to strong wind and 6-12″ of new snow overnight, may release naturally along ridgelines and  human triggered avalanches will be likely. The new snow and wind loading  may also overload and trigger much larger avalanches  that break in a weak layer of buried surface hoar sitting 2-5′ below the surface. Continued cautious route finding and conservative decision-making will be essential for a safe day in the mountains.  

SUMMIT LAKE / JOHNSON PASS:    Natural avalanches are possible and human triggered avalanches are likely due to  strong wind, 4-6″ of new snow and a poor snowpack structure.  

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Tue, January 29th, 2019
Alpine
Above 2,500'
Considerable (3)
Avalanche risk
Treeline
1,000'-2,500'
Considerable (3)
Avalanche risk
Below Treeline
Below 1,000'
Low (1)
Avalanche risk
Alpine
Above 2,500'
Avalanche risk
Considerable (3)
Treeline
1,000'-2,500'
Avalanche risk
Considerable (3)
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 overnight with an additional 3-6″ expected today along with strong ridgetop winds are overloading the tenuous snowpack. If you’ve been following along, you’ll know that a weak layer of buried surface hoar sat 1-3′ below the surface before this new snow. The layer was highly reactive during last weekend’s clear skies and resulted in nine human triggered avalanches. Today, we will see how reactive it still is with another shot of stress by new snow and wind loading. 

Total snowfall as of 6am this morning for the past 24-hours:

  • Turnagain Pass – SNOTEL (1800’):  4″ at station, 6-8″ estimated above 2,500′
  • Girdwood – Alyeska Midway (1700’):  5″ at station, 10-12″ estimated above 2,500′
  • Portage – Bear Valley Tunnel (100’):  1.7″ of rain, 16″ – 2′ estimated above 2,500′
  • Summit Lake – SNOTEL (1400′):  4″ at station, 5-6″ estimated above 2,500′

Wind loaded slopes above 1,500′ are the most suspect for both naturally occurring avalanches and human triggered avalanches. Below 1,500′ the snowpack is composed of mostly crusts and avalanches are unlikely. Although fresh wind slabs and cornice falls may be the most common avalanche today, the more dangerous avalanche is the one that breaks deeper in the snowpack. If you are headed into the mountains and skies clear enough for travel above treeline keep in mind:

1-  Large avalanches have the potential to occur naturally and send debris to valley floors
2-  Fresh wind slabs and cornice falls may step-down and trigger a large avalanche
3-  Sticking to lower angel slopes, less than 30 degrees, with nothing steeper above you is a way to avoid these dangerous avalanche conditions. 

 

Storm snow avalanches from last week’s storm cycle in Lynx Creek Drainage (1/25). The debris in these slides covered the route into the upper portion of the valley. Additional slides like this are expected off steep ridgelines today. 

 

The thin gray line easily seen about a foot deep in the pit wall is the MLK buried surface hoar responsible for the rash of human triggered avalanches last Saturday and our worrisome weak layer being overloaded today. (Lynx Creek Drainage, 1/27/19, 2,800′ north aspect)

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 have been blowing strong out of the east (20-35mph) for the past 24-hours. We can expect fresh wind slabs and cornices to be forming as winds transport not only the new snow but older soft snow. Wind slabs are likely to be in the 1-2′ foot deep range and easily triggered. This is also a prime condition for step-down avalanches. Meaning a fresh wind slab that triggers a deeper slab underneath. One that breaks 2-5′ deeper in the pack creating a much larger and dangerous avalanche. 

Weather
Tue, January 29th, 2019

Yesterday:   Overcast skies with light rain below ~500′ and light snow flurries above dominated yesterday. Snowfall increased overnight with a total 24-hour accumulation of between 3-6″ at mid-elevations as of 6am this morning. Ridgetop winds have been strong, averaging 20-35mph from the east with gusts to the 60’s, for the past 24-hours. Temperatures have been in the upper 30’s to 40F at sea level, 32F near 1,000′ and in the mid 20’sF along ridgelines.  

Today:   Cloudy skies with snow falling above ~500′, rain below, is expected to continue through today. The mountains should see between 3 and 6″ of snow (.3 and .5″ of water equivalent) today with an additional 2-3″ tonight (~.25 water). Ridgetop winds are expected to remain in the 20-35mph range from the east. Temperatures look to sit in the upper 30’sF at sea level, in the low-30’sF at 1,000′ and mid-20’sF along ridgetops.

Tomorrow:    The low pressure systems and associated fronts look to move north tomorrow decreasing ridgetop winds and snowfall. We could see some flurries tomorrow before skies begin to break. A return to clearer skies and cooler temperatures heads into the region for Thursday/Friday.    

*Seattle Ridge weather station was heavily rimed and the anemometer (wind sensor) was destroyed.   We are currently working to replace it.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 32   3    0.4 60  
Summit Lake (1400′) 32   4     0.3   24  
Alyeska Mid (1700′) 32   5   0.8   48  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 23   NE   24   62  
Seattle Ridge (2400′) 28   *N/A   *N/A     *N/A    
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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

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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.