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ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, January 21st, 2020 - 7:00AM
Expires
Wed, January 22nd, 2020 - 7:00AM
Forecaster
Ryan Van Luit
Avalanche risk The Bottom Line

Today the avalanche danger is MODERATE at all elevations.  With 10″ inches of low density snow in the past 36 hours it’s possible for a human to trigger fast moving loose snow sluffs, which could easily entrain the older loose snow within the snowpack.  Soft slab avalanches composed of the new snow may also be a concern where the new snow is more consolidated by warmer temperatures or wind loading. In isolated areas in the Alpine, older wind slabs under the new snow may be triggered.

SUMMIT LAKE: This region is just out of our advisory area to the south. The snowpack is shallow, composed of several weak layers, and experienced more wind effect and wind slab development. Extra caution is advised for triggering a slab avalanche.

Tue, January 21st, 2020
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
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.
Avalanche Problem 1
  • Storm Slabs
    Storm Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    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

Over the last 36 hours we’ve received 10″ of storm snow which will consolidate slowly as temperatures remain below freezing with calm to light winds.  Although it’s not expected today, if temperatures warm to near freezing, the latest storm snow will gain slab character more rapidly.  In areas that saw enough wind to transport snow, soft wind slabs may be found and could still be possible to trigger. This is mostly likely in the higher Alpine zone. In the meantime, in many areas we have unconsolidated new snow over a thick layer of unconsolidated facets.

This is a common structure in upper snowpack where wind has had little effect – 10″ of recent storm snow over 10″ of facets.  1.20.20  Photo: H. Thamm

 

Loose snow sluffs: In many areas, the upper snowpack is comprised of powder over unconsolidated facets. Our storm snow over the last 36 hours is likely to exacerbate the loose snow sluffing problem by adding volume to a thick layer of loose faceted snow near the surface.  With this in mind, if you’re venturing into steep or channeled terrain remain aware of the consequences. It could be easy for enough loose snow to become entrained and take a rider off their feet through the fall-line.

 

Avalanche Problem 2
  • Persistent Slabs
    Persistent Slabs
  • 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

We still carry concern about a crust and facet interface from New Years Eve, in addition to isolated old wind slabs within the upper 4′ of the snowpack.  In steeper and channeled terrain it could be easy to trigger sluff, and in turn gain enough volume and mass to trigger the underlying facets and a slab layer deeper in the snowpack. This would be most suspect to occur where older wind slabs remain from the dry spell and just below the recent storm snow.  Frequent hand shear and probing will assist in identifying this potential.

Weather
Tue, January 21st, 2020

Yesterday:  10″ of snow accumulated under cloudy skis and calm to light winds from the southeast.  Temperatures reached into the low 20’s with lows in the single digits °F.

Today:  Mostly cloudy with a high near 20°F and low around 11°F. Winds will likely be from the southeast around 5 mph becoming calm into the afternoon and evening. Intermittent light precipitation can be expected throughout the day.

Tomorrow: Skies begin sunny trending toward partly cloudy by late afternoon.  Temperature high in the upper teens, with a low near 4°F.  Winds are expected to be calm to light from the northwest.  These relatively cold and clear conditions will likely be the norm for at least two to three days.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 18 0 0 47
Summit Lake (1400′) 11 0 0 16
Alyeska Mid (1700′) 16 3 0.23 47

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 18 ENE 11 31
Seattle Ridge (2400′) 20 VAR 6 11
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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.