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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Sat, December 1st, 2018 - 7:00AM
Expires
Sun, December 2nd, 2018 - 7:00AM
Forecaster
Wendy Wagner
The Bottom Line

A  MODERATE  avalanche danger exists above 2500′ in the Alpine. Triggering a slab avalanche 1-3′ thick remains possible due to a layer of buried surface hoar under the Thanksgiving weekend storm snow. Additionally, watch for increasing winds today from a southerly direction that may form wind slabs in unusual places.

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Sat, December 1st, 2018
Alpine
Above 2,500'
Moderate (2)
Avalanche risk
Treeline
1,000'-2,500'
Low (1)
Avalanche risk
Below Treeline
Below 1,000'
No Rating (0)
Avalanche risk
Alpine
Above 2,500'
Avalanche risk
Moderate (2)
Treeline
1,000'-2,500'
Avalanche risk
Low (1)
Below Treeline
Below 1,000'
Avalanche risk
No Rating (0)
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

The mountain snowpack was tested yesterday with Mother Nature’s violent shake down. The Nov 30th 7.0 earthquake rocked the region. Luckily visibility was good enough to get eyes on some of the aftermath. Girdwood Valley was the most exciting as large slabs were triggered along with rock fall. Turnagain Pass fared better, as most the snowpack stayed intact. The most notable slabs on Turnagain were on the east facing slopes of Twin Peaks. Otherwise there were some shallow slabs on the southern end of Seattle Ridge and small cornice falls along ridgetops. 

What does this say about the snowpack? Well, the storm snow from last weekend that fell on buried surface hoar (a weak layer) and in some areas old weak snow, is definitely adjusting – the snowpack did not totally shake off the mountainsides. One can deduce that triggering a large avalanche is becoming less possible. Despite this, keeping in the forefront of our minds that a weak layer sits 1-3′ below the snow surface is still essential. Safe travel protocols and watching our partners shouldn’t be forgotten. Additionally, any signs telling us a slope could be unstable are not likely to be seen. 

 

Earthquake triggered avalanches in upper Goat Mt and Goat Couior in the Girdwood Valley

 

Magpie Peak in the upper Crow Creek Valley – avalanche debris and some slabs seen triggered by the earthquake

 

 

Rock fall off peak 4710′, north aspect

 

 

Rockslide off the east face of Rainbow Peak in Chugach State Park – triggered by earthquake

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

Winds are slated to bump into the 40’s mph today from a southerly direction along the ridgetops. This is an unusual flow direction. Watch for terrain forcing to shift winds either westerly or easterly. Be on the lookout for how winds may be transporting snow and avoid any fresh wind drifts or slabs that may form. Above 2,500′ there is loose snow available for transport plus a possible 1-3″ of snowfall today. 

Weather
Sat, December 1st, 2018

Yesterday:   Partly cloudy skies were over the region with a few snowflakes falling last night, adding a trace. Temperatures have been steadily in the 20’sF and teens at the high elevations. Light and variable winds yesterday turned easterly overnight, with a bump in speed to 15mph with gusts to 30mph.

Today:   Light snowfall with 1-3″ accumulation is expected today before diminishing tonight. Temperatures should rise significantly, turning snowfall to rain as high as 2,000′ by the afternoon. Cross your fingers the cool air stays in place! Winds are forecast to increase up to 40mph from the SW with stronger gusts, also diminishing to the 15-20mph range tonight.

Tomorrow:   Another band of precipitation is expected to move through tomorrow. This one is expected to be cooler, bringing a chance for a few inches of snowfall to lower elevations.  

*Seattle Ridge wind sensor is rimed over.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 26   Trace   0   10  
Summit Lake (1400′) 26   1   0.1   1  
Alyeska Mid (1700′) 27 Trace   0.03   0  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 19   NE   9   31  
Seattle Ridge (2400′) 23    *no data  *no data    *no data  
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.