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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Tue, December 4th, 2018 - 7:00AM
Expires
Wed, December 5th, 2018 - 7:00AM
Forecaster
Aleph Johnston-Bloom
The Bottom Line

The avalanche danger is  MODERATE  above 2500′ in the Alpine. As easterly winds increase today triggering a wind slab in steep leeward terrain will be possible. Additionally, human triggered slab avalanches 1-3′ thick remain possible due to a weak layer of snow under the Thanksgiving weekend storm snow.      

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Tue, December 4th, 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
    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

Be on the lookout for wind slabs in steep unsupported terrain. Strong easterly winds on Saturday afternoon loaded leeward slopes in the Alpine. Surface conditions at upper elevations yesterday varied from wind skin to rime crust to sastrugi to wind slab. Winds are forecast to increase again today and there is still snow available for transport. Look for stiff, pillowed snow and cracking and listen for hollow, drum-like sounds. Loading patterns can be very localized and it is crucial to look for clues indicating where the snow is being distributed. 

 

Sunburst weather station wind profile from Friday afternoon to Tuesday morning

 Surface conditions on Sunburst and cross-loading on the north side of Magnum, December 3, 2018. Photo: Heather Thamm

Avalanche Problem 2
  • 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

In the Alpine, sitting anywhere from 1 to 3′ below the snow surface, is a thin layer of weak snow (buried surface hoar/BSH). It got a good shake on Friday during the earthquake and a number of quake triggered avalanches are thought to have run on this BSH (buried November 23rd) layer.  Did that shake it up enough??? That is the hard to answer question. An observer Sunday found this layer to still be reactive in a snowpit on Sunburst right around 2500′. Yesterday at 3100′ on Sunburst there were no results testing this layer but it was very easy to spot laid over in the snowpack. The concern is finding a slope with buried surface hoar that is still intact, reactive and that propagates into an avalanche. At this point obvious signs of instability may not be observed but some lingering suspicion is advised even as the likelihood decreases. As always use safe travel protocol and choose terrain with consequences in mind i.e. where is the avalanche path and where would I end up if the slope slides? 

Weather
Tue, December 4th, 2018

Yesterday:  Skies were partly sunny with temperatures in the low 30Fs to mid 20Fs. Winds were easterly 5-15 mph with gusts in the 30s. Overnight temperatures dropped into the mid 20Fs to high teens.  

Today:  The forecast is for mostly cloudy skies with a chance of rain/snow showers with rain/snow line around 800′. Temperatures will be in the mid 30Fs to mid 20Fs depending on elevation. Winds are forecast to be easterly 20-40 mph with gusts into the 50s.  

Tomorrow: Cloudy skies and rain/snow showers are forecast to continue. Winds should decrease and temperatures will be slightly warmer. The National Weather Service long term discussion had a couple of interesting quotes this morning,  “A high degree of uncertainty will make for an interesting forecast  for the end of the week and into early next week. In short it looks like an active and wet period, but the  details are murky due to model disagreement and inconsistency  being far below normal.”

 *Seattle Ridge wind sensor is rimed over. Alyeska Mid Wx Station and Summit Lake Snotel snow depth sensor are not functioning.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 27   0  0 13  
Summit Lake (1400′) 17   0   0   *no data
Alyeska Mid (1700′) 28    0     0   *no data

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 20    E  8 30  
Seattle Ridge (2400′) 25 *no data   *no data   *no data  
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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.

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