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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Tue, December 22nd, 2020 - 7:00AM
Expires
Wed, December 23rd, 2020 - 7:00AM
Forecaster
HPAC Staff
The Bottom Line

Heads up! the pattern has shifted.

Human triggered avalanches are likely today, natural avalanches are possible! Remote triggering will be possible.

Today’s avalanche problems: Fresh Wind Slabs up to 2 feet thick and Persistent Slabs 2-4 feet deep.

It will be possible for wind slabs to step down into the deeper, persistent slab problem, failing at or near the ground.

 

The avalanche hazard will continue to remain elevated with strong winds and new snow through Wednesday.

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Tue, December 22nd, 2020
Upper Elevation
Above 3,500'
Considerable (3)
Avalanche risk
Mid Elevation
2,500'-3,500'
Considerable (3)
Avalanche risk
Low Elevation
Below 2,500'
Moderate (2)
Avalanche risk
Upper Elevation
Above 3,500'
Avalanche risk
Considerable (3)
Mid Elevation
2,500'-3,500'
Avalanche risk
Considerable (3)
Low Elevation
Below 2,500'
Avalanche risk
Moderate (2)
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

Sensitive wind slabs have formed over the last 24 hours. Shooting cracks, whumphing, active wind loading, and recent avalanches are all red flags for this avalanche problem.

Moderate to strong ESE/SE winds with gusts 30-50mph over the last 12 hours have transported approximately a foot of low density snow (12/19-22) and built wind slabs up to 2 feet thick on leeward aspects, West to North aspects.

These slabs will fail in both the new snow and on persistent grains. Human triggered wind slab avalanches will be likely and natural avalanches will be possible on leeward aspects at mid to upper elevations. At low elevation human triggered wind slabs will be possible and natural avalanches will be unlikely. Remote triggers will be possible where these slabs sit on persistent grains.

Yesterday on a Northwest aspect, at 3000′, on a 35° slope, a backcountry traveler remotely triggered this freshly formed wind slab, likely failing on persistent grains, while digging a test pit. Today these slabs will be twice as deep:

 

It will be possible for wind slabs to step down into the deeper persistent slab problem, increasing the size and danger of triggering any avalanche. These avalanches will fail at or near the ground, be full depth, up to 2-4 feet deep.

Marmot Weather Station Last 24 hours:

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

Heads up! The pattern has shifted. Shooting cracks, whumphing, active wind loading, and recent avalanches are all red flags for this avalanche problem.

A foot of new snow since 12/19, plus strong winds in the last 24 hours will have overloaded the persistent slab problem on leeward aspects, West to North.

As winds and new snow continue through today and into Wednesday, expect persistent slabs to be likely to human trigger and for natural avalanches to be possible at mid to upper elevations, on all aspects, but more problematic on freshly loaded West to North aspects. At low elevation, on all aspects, expect persistent slabs to be possible to human triggered and natural avalanches to be unlikely.

Remotely triggered avalanches will be possible.

Persistent slabs could reach depths of 2-4 feet deep.

Stack the deck in your favor: The persistent slab will be difficult to predict. Use safe travel protocol and choose slopes without terrain traps. 

 

 

 

 

Weather
Tue, December 22nd, 2020

NWS Rec Forecast here.

NWS point forecast here.

State Parks Snow Report and Motorized Access information here.

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