Hatcher Pass Avalanche Center

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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Sat, November 28th, 2020 - 7:00AM
Expires
Sun, November 29th, 2020 - 7:00AM
Forecaster
Jed Workman
The Bottom Line

A Moderate Hazard exists for persistent slabs and wind slabs. Human triggered avalanches will be possible and natural avalanches are unlikely.

Persistent slabs exist at all elevations and on all aspects. These will be hard to predict, and small to large in size releasing at or near the ground, up to 2 feet deep. See yesterday’s human triggered avalanche HERE.

Small wind slabs, up to 4-12″ deep, exist in specific locations, at upper elevation, on West to North aspects.

Shooting cracks and whumphing are red flags for today’s avalanche problems. Any avalanche has the added consequence of a shallow snowpack with rock hazards.

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Sat, November 28th, 2020
Upper Elevation
Above 3,500'
Moderate (2)
Avalanche risk
Mid Elevation
2,500'-3,500'
Moderate (2)
Avalanche risk
Low Elevation
Below 2,500'
Moderate (2)
Avalanche risk
Upper Elevation
Above 3,500'
Avalanche risk
Moderate (2)
Mid Elevation
2,500'-3,500'
Avalanche risk
Moderate (2)
Low Elevation
Below 2,500'
Avalanche risk
Moderate (2)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
Recent Avalanches

Yesterday a human triggered avalanche occurred in the Martin Mine area. This avalanche is indicative of the current persistent slab problem. The skier who triggered the avalanche was approaching a steep slope when the avalanche failed at his feet. He was not caught. This avalanche failed near the ground, which exposed rocks, an additional hazard if you happen to be caught and carried. The group then retreated via their uptack and descended the green line.

11/27 – Martin Mine, SE, 4100′. Human triggered, full depth, large persistent slab avalanche

Yesterday we intentionally triggered a number of cornices which then triggered small wind slabs along the Marmot ridgelines, on both the SW face and the North face along the Rae Wallace chutes. Wind slabs were failing within the new storm snow and at the new/old snow interface of near surface faceted snow.

11/27 – Marmot Mt, NW, 4500′. SS-ACS-D1-I, Wind slab failed approximately  12+” deep x 10′ wide

Natural avalanches released during and just after the recent snow storm on 11/25.  Avalanches released within the new snow, at the new/old snow interface, and some released at the ground. A disproportionate amount of persistent slab avalanches occurred on the Punk spines compared to the core area, which saw much fewer natural avalanches in the 11/25 cycle. We hypothesize this is because of stronger winds in the Arkose Ridge area.

11/25 – Punk Spines, cropped photo of entire mountainside showing 5 of 7 natural avalanches. Winds during the storm cross-loaded terrain features, overloaded weaker, old snow, resulting in large slab avalanches failing near the ground. Notice the old avalanches in next photo from 11/9. There are some repeat offenders. This is a great puzzle to study closely.

11/26 – NW, 2700′. Just above “what used to be” the Motherload, ridge approach to Delia Creek. Small human triggered avalanche on facets near the ground. Tracks on left were from 11/26, which were partially covered with wind blown snow . Tracks on right are from 11/27. See observation HERE.

Avalanche Problem 1
  • 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

Persistent slab avalanches, 1-2 feet deep, will be possible to human trigger and natural avalanches will be unlikely on all aspects and at all elevations. It will be possible to remotely trigger persistent slabs in isolated locations, see recent observation HERE.

Winds and new snow amounts have been greater at the mid to upper elevations, so expect the persistent slab problem to be more prevalent there than at the low elevations. However, be cautious at the lower elevations where persistent slab problems may linger in isolated locations, on all aspects, on steep (35°+) terrain features.

Expect these slabs to fail near the ground, and therefore have enough volume to burry, injure or kill a person. Red flags include whumphing collapses and shooting cracks.

Persistent slabs will be difficult to predict. Hand pits, pole tests, test slopes, and instability tests will be useful in identifying the problem of stiffer snow overlying weaker snow. Collapses, whumphing and shooting cracks are red flags for this avalanche problem.

Yesterday’s human triggered avalanche in Martin Mine is the perfect example of what will continue to be a problem this weekend.

Below is a study pit from yesterday showing this season’s storm layers and weak layer problems. Stability has increased since the last loading cycle, with instability tests showing strong force needed to propagate an avalanche. While it is becoming more difficult and less likely to trigger a persistent slab, it is still possible and carries a significant consequence.

Avalanche Problem 2
  • 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

Moderate to strong winds over the last 48 hours have continued to build wind slabs at the upper elevations at and near ridgelines, on West to North aspects.

Wind slabs will be 5-12″ thick in pockets, possible to trigger, but small in overall dimensions and size.  Any wind slab triggered will have the possibility of entraining loose, weaker snow near the ground, which may increase the volume of the avalanche. With shallow coverage in the upper elevations, if caught and carried, expect to encounter rock hazards which will compound the hazard. Overall, the upper elevations steeps are just not quite ripe. As a good friend likes to say, “why sour your mouth with green bananas?”

Expect to find a combination of weak layers, including failures in the new, 11/25 snow, and failures on older, mixed forms, which are technically considered persistent grains. These soft, fresh wind slabs have been formed by recent winds over the last 48 hours, are located on leeward aspects, and are small in overall size.

Along with recent winds, note that small, extremely weak, cornice features are failing easily. This cornice failure triggered a small wind slab, difficult to see in the video:

Wind loading yesterday on Marmot Mountain:

11/27 – Wind loading on marmot ridge features, West, 3700′

 

 

 

 

Weather
Sat, November 28th, 2020

NWS Rec Forecast here.

NWS point forecast here.

State Parks Snow Report and Motorized Access information here.


This week we received 4″ of snow on Sunday and 8″ on Wednesday evening, and a trace to 1″ new snow overnight at 3000′. Moderate to strong S, E and SE winds have been consistently transporting snow at mid to upper elevations most of this week. Moderate to Strong winds overnight have continued to transport snow. The last 4 days of winds have reduced the amount of available snow for transport for the last 24 hours.


Marmot Weather Station

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Frostbite Bottom Weather Station

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Observations
Recent Observations for Hatcher Pass