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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Thu, December 6th, 2018 - 7:00AM
Expires
Fri, December 7th, 2018 - 7:00AM
Forecaster
Heather Thamm
Avalanche risk The Bottom Line

The avalanche danger remains  MODERATE  above 2500′ in the Alpine where triggering a wind slab in steep terrain will be possible.  Additionally, triggering a  slab avalanche 1-3′ thick, releasing on buried surface hoar, is possible though trending toward a lower likelihood.

SUMMIT LAKE: A very shallow snowpack exists, but triggering a slab large enough to bury a person is possible in the upper elevations.

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Thu, December 6th, 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
  • Wind Slabs
    Wind Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    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

Overnight moderate Easterly ridgetop winds were accompanied by 2-3” of new snow in the upper elevations. Be aware of fresh shallow wind slabs on steep leeward features. Earlier in the week moderate winds re-distributed the snow in the alpine and older wind slabs may be hard and hidden. Pillowed snow, cracking and hollow, drum-like sounds are clues wind slabs may be reactive. Yesterday on Tenderfoot observers noted recent wind slab formation on top of weak faceted snow. Wind slabs in the Summit Lake area may be more reactive due to a weaker foundation. As always evaluate the terrain for consequences and avoid terrain traps like gullies, rocks, trees below. Also keep in mind very little snow exists below 2000’.  

Wind drifted snow on Sunburst from Monday (12-3-18)

Avalanche Problem 2
  • Persistent Slabs
    Persistent Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    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

In the upper elevations we are keeping tabs on a thin layer of buried surface hoar sitting 1-3′ below the surface. This layer has been documented in a number of snowpits above 2500’ over the last week. In many places this layer is laying down and unreactive, where in other places it may be upright and still able to propagate. Overnight 3” of snow fell above 2500’ in Turnagain Pass and Girdwood. This isn’t enough weight to be concerned about natural activity, but it could be adding some stress to the snowpack. As always use safe travel protocol and obvious clues like ‘whumpfing’ may not be present.

*Also be aware of a shallow snowpack with weak faceted snow near the ground in the Summit Lake area. Persistent slabs may be more reactive in this zone and could be large enough to bury a person on a large connected slope.

Surface hoar found 2′ below the surface on Sunburst on 12-3-18

 

In this area there wasn’t much of a slab, but be aware of facets under wind drifted snow in Summit Lake. This structure was found in most hand pits above 2500′ on Tenderfoot. 

Weather
Thu, December 6th, 2018

Yesterday: Temperatures increased from mid-20F’s to mid-30F’s in the mid-elevations. Light rain started falling below 1000′ yesterday and 2-3 € of new snow fell in the upper elevation overnight. In Portage where precip totals are typically higher 0.5 € of rain fell near Bear Valley. Ridgetop winds were moderate from the East 15-35mph overnight.    

Today: Temperatures will remain in the mid to upper-30F’s near sea level and rain snowline is expected to be around 1300′. Precipitation below this elevation will fall as light rain (up to .15 € of water) and another 1-2 € of snow is possible in upper elevations. Easterly ridgetop winds have already started to decrease into the 5-15mph range, but will increase again this evening.

Tomorrow: Get ready as a fast moving system moves into Prince William Sound and with it another round of strong winds, warm temps and a period of heavy rain for our region. Expect above freezing temps in the lower to mid-elevations with rain/snow line around 1500′. Easterly winds will build overnight into sustained 30-40’s mph ridgetop winds with gusts up to 50’s mph.

* Warm temps and winds have finally freed up the Seattle Ridge Anemometer. Wind data is from 12pm – 6am.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 35   3   0.3   16  
Summit Lake (1400′) 26   trace   0.1   3  
Alyeska Mid (1700′) 35   2   0.25   *N/A  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 27   ENE   12   38  
Seattle Ridge (2400′) 31   ESE   12   34  
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Riding Areas
Updated Mon, December 05th, 2022

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
The Forest has issued a closure order due to inadequate snow cover for resource protection. Conditions will be monitored daily. Scheduled opening would have been Dec 1st.
Placer River
Closed
The Forest has issued a closure order due to inadequate snow cover for resource protection. Conditions will be monitored daily. Scheduled opening would have been Dec 1st.
Skookum Drainage
Closed
The Forest has issued a closure order due to inadequate snow cover for resource protection. Conditions will be monitored daily. Scheduled opening would have been Dec 1st.
Turnagain Pass
Closed
The Forest has issued a closure order for Turnagain Pass due to inadequate snow cover for resource protection. Conditions will be monitored daily. Between 14-16” of snow exists at the parking lot. The scheduled opening would have been the Wednesday before Thanksgiving per Forest Plan.
Twentymile
Closed
The Forest has issued a closure order due to inadequate snow cover for resource protection. Conditions will be monitored daily. Scheduled opening would have been Dec 1st.
Seward District
Carter Lake
Closed
The Forest has issued a closure order due to inadequate snow cover for resource protection. Conditions will be monitored daily. Scheduled opening would have been Dec 1st.
Lost Lake Trail
Closed
The Forest has issued a closure order due to inadequate snow cover for resource protection. Conditions will be monitored daily. Scheduled opening would have been Dec 1st.
Primrose Trail
Closed
The Forest has issued a closure order due to inadequate snow cover for resource protection. Conditions will be monitored daily. Scheduled opening would have been Dec 1st.
Resurrection Pass Trail
Closed
Closed to motorized use for the 2022/23 winter season per Forest Plan. Open next season.
Snug Harbor
Closed
The Forest has issued a closure order due to inadequate snow cover for resource protection. Conditions will be monitored daily. Scheduled opening would have been Dec 1st.
South Fork Snow River Corridor
Closed
The Forest has issued a closure order due to inadequate snow cover for resource protection. Conditions will be monitored daily. Scheduled opening would have been Dec 1st.
Summit Lake
Closed
The Forest has issued a closure order due to inadequate snow cover for resource protection. Conditions will be monitored daily. Scheduled opening would have been Dec 1st.

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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.