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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Thu, December 17th, 2020 - 7:00AM
Expires
Fri, December 18th, 2020 - 7:00AM
Forecaster
Jake Kayes
Conditions Summary

Today’s avalanche problems will be persistent slab and dry loose. Human triggered persistent slab and dry loose avalanches will be possible on all aspects at mid to upper elevations. Natural avalanches are unlikely. Stiff hard snow on top of weak sugary facets is a sign that the persistent slab problem is present.

Remote triggering a persistent slab avalanche is possible. Choose safe zones carefully and beware of steep terrain that is above you.

At lower elevations, persistent slab problems are unlikely and small dry loose avalanches are possible. 

A few new inches of new snow has improved coverage but the snowpack is still thin and shallow in many areas. Getting caught in any avalanche will have high consequences, due to the potential of carrying a person through thinly buried rocks and other hazards.

Riding conditions have improved slightly in isolated areas.

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Thu, December 17th, 2020
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 at mid to upper elevations on all aspects, in terrain 30° or steeper. Natural avalanches are unlikely. Expect these avalanches to be large enough to bury, injure or kill a person. Persistent slab avalanches will be unlikely at lower elevations. Remote triggered avalanches will be possible.

Instability tests continue to be reactive in areas where a stiff hard slab can be found. Reports of whumping and shooting cracks earlier in the week are a great reminder that this problem has not gone away. 

This type of avalanche problem is difficult to predict. It may allow you to travel multiple times on a slope before triggering an avalanche. Tracks on a slope are not an indicator of stability. A single snow pit with stable results will not necessarily be representative of the slope you are testing.

Hard slabs will allow you to travel out onto a slope before failing above you making escape difficult to impossible. Avoid cross loaded gullies and slopes with old stiff snow.

Two weak sugary layers of facets still exist in the snowpack. One layer of basal facets is found at or near the ground, and is buried under 1-3’ of snow. The other layer of rounding facets is closer to the surface and is buried 12”-18” from the surface.

Any hard dense snow sitting on top of weak sugary facets, should be given extra caution. To identify these conditions use hand pits, pole tests and instability tests. Shooting cracks, collapsing and whumping will all be red flags for this problem.

To increase your margin of safety for this avalanche problem, use disciplined safe travel protocol and avoid slopes with terrain traps. Use adequate spacing when traveling uphill, ride one at a time down slopes, spot your partners, and use safe zones that are out of harm’s way.

Avoid any slope that is above a terrain trap. Getting caught in an avalanche above a creek, gullie or slope with an abrupt transition will increase your burial depth. Terrain traps increase your burial depth and make a successful rescue more challenging.

2-3mm basal facets

Snow pit from marmot 12/16

Coverage is slowly improving but hazards sit just below the surface

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. 1-2” of newer, low density snow will be easy to trigger, and may entrain deeper, weak snow.

Loose dry avalanches will be small in size, but may be able to catch, carry, and sweep a person into more consequential hazards, such as rocks or over cliffs. 

Columns and stelar dendrites. Yesterdays storm snow, todays dry loose problem

Weather
Thu, December 17th, 2020

New snow totals 12/13-17

Independence mine: 1-2″

Frostbite ridge: 2-3″


This week the weather has been relatively stable. On Sunday frostbite weather station reported temps above freezing for most of the day. Independence Mine was a little bit cooler but still made it above freezing for a few hours. On monday temperatures began to drop into the lower 20s at all the weather stations and have remained cool throughout the week. Calm east to south winds have spared our snowpack and made for pleasant riding conditions. On Wednesday 1-2″ of snow fell just barely hiding old tracks, rocks, and other hazards. 


NWS Rec Forecast here.

NWS point forecast here.

State Parks Snow Report and Motorized Access information here.


Frostbite

 


Independence Mine

 


Marmot

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