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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Wed, December 25th, 2019 - 7:00AM
Expires
Thu, December 26th, 2019 - 7:00AM
Forecaster
Ryan Van Luit
The Bottom Line

MODERATE avalanche danger exists above 1000′ and LOW danger below treeline.  As winds transport snow today, it’s possible for a human to trigger wind slabs on steep leeward slopes and cross-loaded gullies. Give cornices and glide cracks a wide berth.

*The National Weather Service has issued a Winter Weather Advisory beginning at noon today for snow and blowing snow along the Seward Highway.

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Wed, December 25th, 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
  • 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

Santa only brought us a trace of snow, but we’re expecting 4″-8″ starting this afternoon!  Winds as low as 10-15mph are capable of transporting loose snow to the lee of ridges and gullies.  Models indicate that winds will pick up today and average from 20-30mph at ridgetops.  Any new or windblown snow will likely sit atop one or two recently formed weak layers (Surface hoar from Christmas Eve and Solstice) within the top 1′ of snow.  If the blowing snow is bonding together, windslabs could be triggered.

Signs that windslabs could be forming:

  • Winds transporting snow
  • Any signs of red flags: shooting cracks, recent avalanches, whumphing sound from collapsing snow, sudden loading from new precipitation or windloading
  • Touchy hand shear or and shallow hand/snow pit tests indicating a slab over a weak layer

Cornices:  As always – give cornices a wide berth.  As winds transport snow, cornices will continue to grow and may be easily triggered.

Cornice over Hippy Bowl on Tincan Ridge. Credit Troy Tempel.

Storm slabs:  In steeper areas of treeline, and areas out of the wind, shallow soft slabs sitting atop a weak layer may be triggered.

Avalanche Problem 2
  • Glide Avalanches
    Glide Avalanches
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
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.

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

There are two glide cracks in the Tincan trees that we know of and many in the Corn Biscuit and Magnum area. Glide cracks are likely obscured and difficult to see with poor visibility and light snow covering cracks.  As always, avoid or limit time under glide cracks as they are unpredictable.

Additional Concern
  • Persistent Slabs
    Persistent Slabs
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.
More info at Avalanche.org

In the high Alpine, above 3,000′, we have been tracking a weak layer of snow sitting near the base of the snowpack.  This weak layer is buried anywhere from 1-6+ feet deep due to the variable wind loading during the December storm cycles. There has been a lingering concern a person could trigger a large avalanche if they hit the wrong spot. Data is pointing to this layer being dormant as this point. However, we will keep it on the radar especially in areas where the overall snowpack is shallower – towards the southern end of Turnagain Pass, in Summit Lake and in the Crow Pass terrain north of Girdwood.

Weather
Wed, December 25th, 2019

Yesterday:  Partly cloudy skies were over the region with a few snow flurries here and there. Only a trace to 2″ of snow is being reported at the snow stations. Ridgetop winds have been light from the east with gusts to 20mph. Temperatures remained in the upper teens along ridgetops and in the twenties at 1,000′.

Today:  Cloudy skies are forecast with snowfall expected to begin around noon today.  Between 2-4″ of new low density snow is expected today (.2-.4″ of water equivalent) at sea level and 4-8″ the high elevations and areas closer to Portage Valley and Turnagain Arm. Ridgetop winds will increase to the 20-30mph range from the East. Temperatures are on a decline and should sit near 20F at 1,000′ and near 10F along ridgetops.

Tomorrow:  Cloudy skies with intermittent snow and easterly ridgetop winds 15-30mph are expected. Periods of snow showers will be over Southcentral for the remainder of the week. A chance for a larger snowfall event is taking shape for this weekend.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 22 trace trace 30
Summit Lake (1400′) 18 trace trace 8
Alyeska Mid (1700′) 22 2 0.1 20

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 17 E 7 22
Seattle Ridge (2400′) N/A* N/A* N/A* N/A*

*Rime has covered the wind sensor on Seattle Ridge.

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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
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Closed
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Closed
Skookum Drainage
Closed
Turnagain Pass
Closed
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.