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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Mon, December 7th, 2020 - 7:00AM
Expires
Tue, December 8th, 2020 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE at all elevations today as snow, rain and steady ridgetop winds continue. Natural avalanches and cornice falls are possible and human triggered avalanches are likely. Additionally, above 2500′, there is a chance a natural avalanche or a person could trigger a deep slab avalanche in weak snow near the ground. Cautious route-finding and conservative decision-making are essential.

SUMMIT LAKE: The snowpack in the Summit Lake area is generally thinner and weaker, making it easier to trigger a deep slab avalanche near the ground. Extra caution is advised. Avoid steep, rocky terrain where these avalanches will be most likely.

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Mon, December 7th, 2020
Alpine
Above 2,500'
Considerable (3)
Avalanche risk
Treeline
1,000'-2,500'
Considerable (3)
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
Considerable (3)
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
  • Storm Slabs
    Storm Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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

With the active weather pattern continuing and the possibility of natural avalanches, today is a day to stick to low angle terrain and avoid avalanche runout areas. Yesterday morning brought a pulse of  3-6″ of light fluffy snow that was followed by rising temperatures, heavier snow and rain overnight (with rain/snowline around 1500′).

24 hr storm totals:

  • Turnagain Pass at 1,880′:  0.7″ of water equivalent, roughly 8-12″ of snow above treeline.
  • Girdwood Valley at 1,700′:  1.31″ of water equivalent, roughly 1-2′ above treeline.
  • Portage Valley at sea level:  1.23″ of water equivalent, roughly 1-2′ of snow above treeline.
  • Summit Lake at 1,400′: 0.3″  of water equivalent, roughly 5-8″ of snow above treeline.

With heavier snow falling on lighter snow and the old surface snow being weak, expect storm slabs to be sensitive today. In addition, below 2500′ this will all be resting on top of the rain crust. This could easily act as a bed surface with the weak snow sandwiched between it and the storm snow. Observers yesterday noted snow not bonding well to the crust. Steady ridgetop winds will have transported snow to leeward terrain and created touchy wind slabs. Cornices will also be larger and more sensitive to triggering today. Look for roller balls and loose snow avalanches at low elevations as rain falls onto dry snow. With all these storm related avalanche issues today use good travel protocol, avoid small terrain trap features and look for signs of instability.

Red flags to watch for:
–  Recent avalanches, from yesterday or today?
–  Whumpfing (collapsing) of the snowpack.
–  Shooting cracks, likely to be seen near ridgelines where the wind has formed wind slabs.

Corniced ridgeline on Notch Mountain, 12.6.20. Expect cornices to be sensitive today.

Sparkling surface hoar and small near-surface facets are now buried at the new snow/old snow interface and could be quite reactive today. Photo: Manitoba. 12.5.20

The stout rain crust to around 2500′ has the potential to act as bed surface. Crust @ 2400′, Notch Mountain. 12.6.20

 

 

Avalanche Problem 2
  • 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

Another reason to keep it mellow today is that once you travel into terrain where the stout rain crust ends (around 2500′), we cannot rule out the potential for large avalanches failing at the ground. Weak, faceted snow still exists at the bottom of the snowpack. Saturday’s snowboarder-triggered avalanche on Silvertip is reminder of this lingering deep slab avalanche problem. Snowfall and wind-loading will add stress to the snowpack and an avalanche failing in the in the upper snowpack or a cornice fall could make these more sensitive today.

Cornice triggered deep slab in upper Bertha Creek (Gold Pan). Photo from 12.4.20 by Alan Abel. Cornice falls today could trigger deep slab avalanches.

Weather
Mon, December 7th, 2020

Yesterday: Skies were overcast with snow falling throughout the day, turning to rain at lower elevations in the late afternoon. Winds were easterly 15-25 mph gusting into the 40-50s. Temperatures started in the teens and rose to the high 20°Fs to mid 30°Fs. Overnight snow and rain continued. Temperatures were in the high 30°Fs at sea level and mid to high 20°Fs in the alpine. Ridgetop winds remained easterly and steady, averaging 10-20 mph and gusting into the 40s.

Today: Rain and snow continue today with another 5-10″ of snow (0.5 inches of H2O) forecast to fall. Temperatures will be in the high 20°Fs to mid 30°Fs depending on elevation. Rain/snowline could be as high as 1500′ and should lower this evening. Winds remain easterly 10-20 mph gusting into the 40s. Rain and snow continue tonight becoming all snow with temperatures decreasing to the 20°Fs and winds easing off.

Tomorrow: Tuesday is forecast to be mostly cloudy with snow showers, light east winds and temperatures in the 20°Fs. The active pattern looks to persist this week but there is still some uncertainty in the details. As always think cold thoughts! #snowtosealevelplease

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 31 8 0.7 63
Summit Lake (1400′) 30 3 0.3 24
Alyeska Mid (1700′) 31 10 1.31 60

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 24 NE 24 56
Seattle Ridge (2400′) 26 E 9 23
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Riding Areas
Updated Thu, April 01st, 2021

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
Open
No parking in turnaround at end of the road near the outhouse.
Placer River
Open
Please do not ride along Railroad tracks. Cross tracks at 90 degree angle and clear the right of way.
Skookum Drainage
Closed
The Skookum Valley is closed to snowmachines. This closure occurs annually on April 1 as per the CNF Forest Plan.
Turnagain Pass
Open
Twentymile
Open
Please do not ride along Railroad tracks. Cross tracks at 90 degree angle and clear the right of way.
Seward District
Carter Lake
Open
Lost Lake Trail
Open
Primrose Trail
Open
Resurrection Pass Trail
Closed
Closed for the 2020/21 winter season.
Snug Harbor
Open
South Fork Snow River Corridor
Open
Summit Lake
Open

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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.