Hatcher Pass Avalanche Center

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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Thu, December 10th, 2020 - 7:00AM
Expires
Fri, December 11th, 2020 - 7:00AM
Forecaster
Jake Kayes
Conditions Summary

Today’s avalanche problem will be persistent slab and dry loose. Human triggered avalanches will be possible and natural avalanches are unlikely. While persistent slab avalanches will be possible on all aspects, we have observed a recent pattern of human triggered avalanches on SE to SW aspects at mid and upper elevations. To identify this avalanche problem look for stiff hard snow over loose weak sugary snow.

At low elevations, persistent slabs are unlikely and small dry loose avalanches are possible. Natural avalanches are unlikely at low elevations.

While some new snow has improved coverage, the snowpack is still thin and shallow in many areas. Triggering any avalanche will have higher consequences, due to the potential of carrying a person through thinly buried rocks and other hazards. 

Recent snowfall has improved riding conditions in isolated areas that are protected from the wind.

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Thu, December 10th, 2020
Recent Avalanches

One human triggered avalanche has been observed this week.

location of avalanche pictured above

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

Human triggered persistent slab avalanches are possible, natural avalanches are unlikely. This avalanche problem will be found at mid and upper elevations and on all aspects, in terrain 30° or steeper. These avalanches will be large enough to bury, injure or kill a person. At lower elevations persistent slab avalanches will be unlikely.

Hard slabs will allow you to travel out onto a slope before failing above you.

While persistent slab avalanches can be found at mid and upper elevations and on all aspects, human triggered avalanches have been occurring on south facing slopes. South facing slopes that are cross loaded with thinner slabs will be more susceptible to avalanching. A human triggred avalanche on Marmot last week had a measured slope angle of 28°.

Two weak sugary layers of faceted snow exist in the snow pack(see photo below). One of these layers is thin and buried approximately 12-18” from the surface. The other layer of basal facets is buried under 1-3’ of snow and will be found at and near the ground. 

If you find dense hard snow it’s most likely sitting on top of weak sugary snow and should be considered suspect. Using hand pits, pole tests, and instability tests will help you identify this avalanche problem. Shooting cracks, collapsing, and whumphing are all red flags for this avalanche problem.

Make sure you have adequate spacing when traveling uphill, ride one at a time down slopes, spot your partners and use safe zones to stay out of harm’s way. Choose slopes to ride that are not above terrain traps. This includes creeks, gullies, or slopes with abrupt transitions from steep to flat. If you trigger an avalanche above a terrain trap it will increase your burial depth, reducing your chances of survival

Pit from 12/09

Lower Eldorado 3700′ Thin conditions with vegetation poking out of the snow pack

Cross loaded gullies on marmot

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

Human triggered dry loose avalanches are possible, and natural avalanches are unlikely. This avalanche problem will be found at all elevations and on all aspects and in terrain 40° or steeper. 4-6” of newer, low density snow will be easy to trigger.

Loose dry avalanches will be small in size, but may be able to catch and carry a person into more consequential hazards, such as rocks or over cliffs. These dry loose avalanches will also be able to entrain old sugary facets, increasing their size and consequences. 

 

Weather
Thu, December 10th, 2020

New snow totals 12/6-10:

Independence Mine: 3-4″

Frostbite bottom: 1-2″


A large storm system to the south, increased temperatures at Hatcher Pass earlier in the week. Temperatures rose above freezing starting Sunday morning and remained above until tuesday morning. On Monday temperatures peaked with Independence Mine reaching 34°F and frostbite hitting 38°F. Marmot weather station reported light winds from the east to southeast from Sunday morning to Tuesday. On Tuesday Marmot received moderate winds from the SE for 8 hrs before dissipating.


NWS Rec Forecast here.

NWS point forecast here.

State Parks Snow Report and Motorized Access information here.


Independence Mine 3550′:

 


Frostbite Ridge 2700′:

 


Marmot 4500′:

Observations
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