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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Thu, November 19th, 2020 - 7:00AM
Expires
Fri, November 20th, 2020 - 7:00AM
Forecaster
HPAC Staff
Conditions Summary

Today’s problems are persistent slab, dry loose, and wind slab. The most likely and largest consequence hazard today are loose dry avalanches in steep terrain. Persistent slabs are unlikely, and wind slabs are likely, but very small in size.

Coverage is still thin with 1-2 ½ feet of snow throughout the area. There are plenty of shallowly buried and exposed hazards. Even a small avalanche could result in compounded risk and consequence if swept into rocks. 

For more information on this weeks weather, click the weather tab below.

 

Marmot looking tracked out. 11/18/20

 

 

Lower Presidents Ridge 11/18.

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Thu, November 19th, 2020
Recent Avalanches

Dry Loose, Marmot, south facing slope, 4000′ 11/15

Wind Slab D1, Marmot, 4500′ NW 11/18

 

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

The current snowpack has been slowly stabilizing over time, and the risk for human triggered persistent slab avalanches has decreased since the Election Dump on 11/7-8. It may be possible to human trigger small to large persistent slab avalanches in very isolated locations with previous wind loading, on all aspects, but primarily on west to north aspects, at upper elevations.

It’s been nine days since our last human triggered persistent slab avalanche. The faceted snow at the ground is generally buried underneath 1-2’ of snow. This will be found at all elevations and all aspects. Look at this observation on 11/15/20 to get a better idea of snowpack structure.

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

Cold temperatures have allowed surface snow to facet, and become weak sugary snow. This low-density snow will be capable of producing small dry Loose avalanches. Loose dry avalanches are probable on all aspects, on steep slopes 40° and steeper. Loose dry avalanches will travel fast and entrain enough volume to catch and carry a person. The main danger here is being swept into shallowly buried hazards.Users were observed triggering dry loose avalanches earlier in the week. Expect this problem to remain present throughout the week.

 

Dry Loose, Marmot, south facing slope, 4000′

 

Dry loose, Marmot, 4500′ NW

 

Avalanche Problem 3
  • 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

Small, fresh wind slabs, 2-3” thick, have formed recently in specific locations at the upper elevation near ridgelines on West to North aspects. These avalanches will be easily triggered on terrain 40° or steeper. Shooting cracks, stiffer snow overlying weaker snow, and smooth bulbous pillows, will be bulls-eye clues for this problem. Even a small wind slab could sweep you into dangerous, shallowly buried hazards.

Natural avalanches are unlikely.

 

Wind Slab D1, Marmot, 4500′ NW 11/18

 

 

Wind effect on Marmot ridge top. 11/18

Weather
Thu, November 19th, 2020

No new precipitation this week.

Weather this week has been relatively stable until Wednesday(read more about winds below). This relatively cool clear weather, has allowed for snow surface to begin to facet. In some areas surface hoar 3mm in size has been observed. Temperatures at different elevations have been relatively uniform throughout the week. Starting in the teens at the beginning of the week, and increasing to the mid 20’s by Wednesday.

Winds from the east increased Wednesday with sustained winds up to 18 mphs and gust up to 27mph for 7 hours. These winds were able to transport low density surface snow and create some wind slabs.

NWS Rec Forecast here.

NWS point forecast here.

State Parks Snow Report and Motorized Access information here.

Independence Mine 11/15-11/19
Marmot Wx 11/15-11/19
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