Hatcher Pass Avalanche Center

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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Sat, December 19th, 2020 - 7:00AM
Expires
Sun, December 20th, 2020 - 7:00AM
Forecaster
Allie Barker
The Bottom Line

Persistent slabs will be possible to human trigger today on all aspects at upper elevation.  Large avalanches will be possible in isolated locations on southerly aspects where stiffer snow overlies weaker snow.  Persistent slabs will be unlikely to trigger on all aspects, at mid and low elevation. Low danger does not mean no danger.

Small, dry loose sluffs will be possible to human trigger in steep locations, on slopes 40º and steeper, on all aspects, at all elevation.

NWS is forecasting for 1-3″ of new snow above 1000′ today.

Terrain choice will be key today for avoiding avalanches and finding the best square powder in a environment that consists of highly variable snow conditions. Let’s hope that Winter Solstice brings us some snow!

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Sat, December 19th, 2020
Upper Elevation
Above 3,500'
Moderate (2)
Avalanche risk
Mid Elevation
2,500'-3,500'
Low (1)
Avalanche risk
Low Elevation
Below 2,500'
Low (1)
Avalanche risk
Upper Elevation
Above 3,500'
Avalanche risk
Moderate (2)
Mid Elevation
2,500'-3,500'
Avalanche risk
Low (1)
Low Elevation
Below 2,500'
Avalanche risk
Low (1)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
Recent Avalanches

No recent avalanches have been observed since 12/5-12/8.

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

This problem’s behavior will be difficult to predict. Use safe travel protocol and choose slopes without terrain traps. Making conservative terrain choices will be our decision making tool of the weekend.

Persistent slabs 1-3 feet deep, will be possible to human trigger today in isolated locations at the upper elevation on all aspects. More likely locations for triggering a large persistent slab are on southerly aspects where previously cross-loaded features contain firm snow overly weaker snow. It will be unlikely to trigger persistent slabs on all aspects, at mid and low elevation.

Although the last human triggered avalanche occurred almost 11-14 days ago, the snowpack continues to resemble a sandbox (of facets) with a slab of varying thickness on top. Be cautious of varying slab thickness. It will be easier to trigger an avalanche from a location where the slab is thinner. Hard slabs tend to allow you to get out on slope before breaking above you, increasing the hazard and diminishing the likelihood of getting off the slab.

In isolated areas, remotely triggering a hard slab avalanche is still possible. In this case you will be able to trigger an avalanche in the flats below steeper slopes or in adjacent terrain. You could unknowingly remotely trigger an avalanche which puts other parties at risk, or trigger an avalanche above you.

A few test results are not indicative of the entire snowpack stability picture. Use multiple pole tests, hand shears, and instability tests for a more accurate assessment of the problem.  Whumphing and shooting cracks are red flags.

Below is a video from a south facing slope at 3000′, showing the entire, shallow snowpack consisting of weak, faceted snow. These zones will be the best for riding and skiing.

Below is pit from the same area as the video above. It is showing consistent results with other recent pits: Poor structure, moderate to strong strength, and poor propagation potential.

 

Below is a good overview of the Persistent Slab Problem. Click on the photo to link to a page describing all avalanche problems.

 

 

Looking to the future: When another loading event occurs, such as 12″ of new snow or 6″ of new snow combined with strong wind, it will likely be enough to stress the weak layers and result in another avalanche cycle.

 

Avalanche Problem 2
  • Dry Loose
    Dry Loose
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
Dry Loose
Dry Loose avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.

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

1-3″ of new, low density snow is expected today. In wind sheltered areas, approximately 3-4″ of old, low density snow exists. The combination of new snow and old snow will add up to 3-7″ of low density snow  by the end of today, which could be triggered as loose dry avalanches in steep terrain.

Small dry loose sluffs will be possible to human trigger today on steep slopes, 40º and steeper , on all aspects, at all elevations. Even a small dry loose can can have a large consequence if the terrain funnels into a terrain trap, over a cliff, or drags you through shallow, rocky terrain.

 

 

Weather
Sat, December 19th, 2020

New snow totals 12/13-19:

Independence Mine: 1-2″ overnight

Frostbite bottom: 1″ overnight

Last significant precipitation, 3-4″ on 12/8.

NWS is forecasting a 90% chance for 1-3″ of new snow above 1000′ today. Winds are forecasted to remain light. Overnight gusts at Marmot (4500′) were mostly light, with some moderate gusts.  Temps should be in the lower 20’s today, dipping into single digits overnight at 1-3000′.

Marmot Weather Station:

Temps this week:

Winds overnight:

 

Independence Mine Snotel (3550′)

Temps this week:

 

Frosty Bottom Snotel (2700′)

Temps this week:

NWS Rec Forecast here.

NWS point forecast here.

State Parks Snow Report and Motorized Access information here.

Observations
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