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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, December 28th, 2019 - 7:00AM
Expires
Sun, December 29th, 2019 - 7:00AM
Forecaster
Ryan Van Luit
Avalanche risk The Bottom Line

Today above 1,000′ the avalanche danger is MODERATE where steep windloaded slopes are the most likely places for humans to find and trigger an avalanche. These wind slabs are estimated to be 1-2′ thick. Watch for signs of instability and give cornices and glide cracks a wide margin.

Sat, December 28th, 2019
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
Avalanche risk
0 - No Rating
1 - Low
2 - Moderate
3 - Considerable
4 - High
5 - Extreme
Avalanche risk Avalanche risk Avalanche risk Avalanche risk Avalanche risk
Travel Advice Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features. Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern. Dangerous avalanche conditions. Dangerous avalanche conditions. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended. Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain.
Likelihood of Avalanches Natural and human-triggered avalanches unlikely. Natural avalanches unlikely; human-triggered avalanches possible. Natural avalanches possible; human-triggered avalanches likely. Natural avalanches likely; human-triggered avalanches very likely. Natural and human-triggered avalanches certain.
Avalanche Size and Distribution Small avalanches in isolated areas or extreme terrain. Small avalanches in specific areas; or large avalanches in isolated areas. Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas. Large avalanches in many areas; or very large avalanches in specific areas. Very large avalanches in many areas.
Recent Avalanches

There was one human triggered avalanche reported to us yesterday. This was on the smaller side, 10 inches thick and 30 feet wide. It was a soft wind slab on the West face of Magnum, pictured below. With another 2-4″ of snow falling last night along with easterly winds, we can expect similar avalanche conditions today.

Observers reported intentional human triggered soft wind slab avalanche on west face of Magnum at 2500′

Photo credit: Emily Sullivan

Avalanche Problem 1
  • Wind Slabs
    Wind Slabs
  • 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

All wind slabs are suspect for being unstable. The new snow and wind events over the last three days in combination with buried weak layers has created the recipe for avalanches.  Wind effected slopes and cross loaded gullies in the treeline and alpine are the most concerning.

Keep a close lookout for windloading and areas where the top foot of snow is stiffer than the snow underneath. Any shooting cracks in the snow or whumpfing (collapsing) are clear signs that layer of snow could avalanche.

Once the newer snow becomes more consolidated, it could become cohesive and form a slab. Once it does, we could start seeing an increase in avalanche activity.

Sluffs:   In steeper areas, sluffs could entrain enough snow to knock a person off their feet.  Give extra caution while traveling above cliffs and rocky terrain.

Cornices:  With winds capable of transporting snow and loose snow available for transport, cornices continue to form and build throughout the region.  Give cornices a wide margin.

Additional Concern
  • Glide Avalanches
    Glide Avalanches
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.
More info at Avalanche.org

Glide cracks and avalanches are still possible.  New snow and wind is likely filling in or bridging glide cracks throughout the region and they could be more difficult to see. Remain prudent when visibility is low, and avoid being underneath glide cracks when possible.

Weather
Sat, December 28th, 2019

Yesterday:  Cloudy and obscured skies with 2″-4″ of snow overnight at sea level. Ridgetop winds from the east at 10-35mph. Temperatures were below zero and into the teens.

Today:  Mostly cloudy skies with intermittent snowfall today may provide an additional 1-3″ before this system pushes out. Ridgetop winds from the east will likely blow from 5-15mph gusting to 35mph. Temperatures will range from 0°F to the low 20’s today.

Tomorrow:  Cloudy skies with continued intermittent light snowfall.  Temperatures should bump back up to the teens and low 20°F as the next front approaches on Sunday afternoon bringing another round of precipitation into Monday.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 5 3 0.3 38
Summit Lake (1400′) 3 3 0.2 13
Alyeska Mid (1700′) 4 4 0.2 30

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 9 ENE-Variable 13 35
Seattle Ridge (2400′) N/A* N/A* N/A* N/A*

*Rime has covered the wind sensor on Seattle Ridge.

Observations
Recent Observations for Turnagain Pass
Date Region Location
11/16/23 Turnagain Observation: Tincan Common
11/14/23 Turnagain Observation: Sunburst/Magnum
11/14/23 Turnagain Observation: Seattle Ridge
11/13/23 Turnagain Avalanche: Tincan Trees
11/12/23 Turnagain Observation: Sunburst
11/12/23 Turnagain Avalanche: Goldpan – avalanche
11/11/23 Turnagain Observation: Tincan Common
11/11/23 Turnagain Observation: Taylor Pass – Sunburst
11/10/23 Turnagain Observation: Tincan
11/10/23 Turnagain Observation: Tincan Trees
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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.