Hatcher Pass Avalanche Center

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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Sat, November 21st, 2020 - 7:00AM
Expires
Sun, November 22nd, 2020 - 7:00AM
Forecaster
Allie Barker
The Bottom Line

It will be possible to human trigger wind slabs, dry loose, and persistent slab avalanches today.

Steady easterly winds over the last 24 hours have transported old snow and built fresh wind slabs on westerly aspects at the upper elevations. Expect these to be small in size, but easy to human trigger.  This is the primary hazard today.

Conditions have been rather stale since the Election Dump. On a positive note, cold temps have recrystallized the snow . Conditions include square powder, sun crust, and wind crusts.  The snowpack varies in depth from 6″ to 2.5′.

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Sat, November 21st, 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'
Low (1)
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
Low (1)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
Recent Avalanches

No new slab avalanches have been observed since Nov. 10. Steady winds overnight and through today will make human triggered small, shallow wind slabs possible. Keep your eyes peeled for this bull-eye clue for this hazard.

This week we observed small human triggered loose dry avalanches on north facing, steep terrain at the upper elevations. This problem will be present this weekend on all aspects, on steep slopes, 40° and steeper.

11/19 Dry Loose, Marmot, 4500′, NW

Avalanche Problem 1
  • 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

Easterly winds increased on Friday and will remain steady through the day.  Wind slabs have formed overnight and will continue to form throughout the day.

Expect 2-6″ thick, small wind slabs, on W to N aspects, at the upper elevations, on slopes 35° and steeper, to be possible to human trigger. Wind slabs will be thicker near ridgelines and therefore more consequential.

Shooting cracks are red flags of this problem. Hand pits can easily identify the stiffer snow failing on the old snow surface. Look for smooth, freshly drifted snow to identify the presence of this problem.

Winds are predicted to taper tonight and into Sunday. Expect wind slab instabilities to decrease within 24-48 hours, once the winds calm down.

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 this week have contributed to faceting in the snowpack. As a result, small dry loose avalanches (sluffs) will be possible to human trigger in steep terrain above 40° and in specific locations. In some protected locations, the entire snowpack consists of facets which in steep terrain funneling into a terrain trap can compound the risk of a small avalanche. Although we expect these sluffs to be small in size, early season rock hazards can increase the consequence of any avalanche.

Avalanche Problem 3
  • 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 slabs up to 1.5′ thick will be possible to human trigger today in isolated locations on all aspect at upper elevation.

Stability has been steadily improving despite the atrocious snowpack structure. A benign weather pattern with cold temps, light to moderate winds, and no recent precipitation has not aggravated our snowpack. You can expect stability to steadily improve until another significant loading event. At that point, expect our weak layers and faceted snowpack to react angrily.

The Persistent Slab problem continues to linger, but the likelihood of triggering has decreased substantially. Anticipate finding this avalanche problem in isolated locations where stiff snow overlies weaker snow near the ground. Probing the snowpack is a useful tool for identifying this hidden structure as are pole tests and hand shears.


The pit below was conducted in a targeted location which is not representative of the snowpack as a whole, but is representative of isolated locations where triggering a slab may be possible. It is important to note that while the test here appears drastic in it’s result, we could not produce shooting cracks or significant collapsing of the snow pack while traveling over this snowpack on skis.  Expect the poor structure in this location to become more problematic after the next storm.

Weather
Sat, November 21st, 2020

NWS Rec Forecast here.

NWS point forecast here.

State Parks Snow Report and Motorized Access information here.


No new precipitation this week.

Winds increased on Friday and remained steady overnight.

Marmot Weather Station last 24 hours


Independence Mine Snotel last 24 hours

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