Hatcher Pass Avalanche Center

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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Sat, December 12th, 2020 - 7:00AM
Expires
Sun, December 13th, 2020 - 7:00AM
Forecaster
Jed Workman
The Bottom Line

The current snowpack varies from a few inches to 3 feet deep. It’s still early for the rocky Smasher’s Pass, which needs more snow for better coverage.

Persistent slabs continue to be the primary avalanche problem and will be possible to human trigger on all aspects, at mid to upper elevations, on slopes 30° and steeper. Remote avalanches will be possible to human trigger. Natural avalanches are unlikely.

Dry loose avalanches are possible to human trigger on all aspects and at all elevations on slopes 40° and steeper.

The risk of triggering any avalanche will be compounded due to shallow coverage in many locations. Even a small sluff could be traumatic, if swept into rocks, over cliffs, or into other shallowly buried hazards. This is reason enough to make conservative terrain choices. It’s a long winter, let’s stay in one piece!

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Sat, December 12th, 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

The last avalanche may have occurred on 12/8. This avalanche date and cause has not been verified. It is strikingly similar to an avalanche that occurred previously this season, however, this is newer. It may have occurred naturally due to wind loading. The left side of the photo clearly shows wind scouring which matched the predominant wind direction. Or, this may have occurred by human trigger, unreported, and the tracks were covered by the most recent new snow and wind transported snow.

Hatch Common, NE, 4200′

 

The last human triggered avalanche was observed on 12/5. This avalanche occurred on a wind scoured aspect where the snowpack is generally thinner. The shallow gully features were cross-loaded, increasing the depth of the snowpack at the site of the avalanche.  Notice there is a lot of rock showing on the untrack and very little snowpack. Thinner, wind scoured snowpacks are often capable of forming weak faceted, sugar snow. This alone is generally not hazardous. However, add wind loaded snow on top, and you now have the recipe for an avalanche.

The red x is the approximated point where the rider triggered the avalanche.  This was a close call. The crown was approximately 2 feet deep, failed near the ground and was D2 in size, large enough to bury, injure or kill a person. This avalanche is now obscured by new snow.

 

 

 

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

Persistent slabs will continue to be the primary avalanche concern today, on all aspects, at the mid to upper elevations, on slopes 30° and steeper, where it will be possible to human trigger large avalanches. This avalanche problem may allow you to get out onto a slope before you trigger it, making escape difficult to impossible. Natural avalanches will be unlikely. At low elevation persistent slabs are unlikely to human trigger and natural avalanches are also unlikely.

There are two persistent weak layers within the snowpack (pit link here). The first one consists of old, near surface facets buried in the mid pack, 12-18″ below the surface. The second persistent weak layer of basal facets is buried 1-3 feet deep, at, and near the ground, surrounding an old crust. Both of these avalanches will vary from soft slab to hard slab depending on location.

The deeper layer near the ground is the culprit in most of the human triggered avalanches over the last two weeks. This deeply buried layer will continue to be difficult to predict. Remotely triggering this layer will remain possible. A pattern of human triggered persistent slabs on SE to S to SW aspects at upper elevations has been observed over the last two weeks. These aspects contain a thinner, weaker snowpack, where any wind loaded features will be easier to trigger, the classic recipe for an avalanche. Be conservative and use extra caution in these locations.

Note that any one stability test does not provide conclusive information for this avalanche type. We will be playing it smart by employing safe travel protocol, avoiding slopes with terrain traps, and testing the snow in many locations to increase our safety margins. Shooting cracks and whumphing are red flags for this avalanche problem.

There are numerous locations out there where the snowpack is right side up, lacks a slab component, and is fairly shallow, less than two feet deep. These areas tend to be located in the low to mid elevations, in wind protected areas, and on lower angle slopes. In these locations the snowpack is made up of almost entirely weak faceted snow, which is also fun to ride. The biggest hazard here is shallowly buried hazards such as rocks.

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

Loose dry avalanches will be possible on all aspects, at all elevations, on slopes 40° and steeper but more prevalent on slopes above 45°. These will be small in size.

Some areas, especially at the upper elevations, will have a stiff layer of snow buried in the snowpack with about 4″ of low density snow on top, “dust on crust”. This low density snow will slide easily on the firmer, buried layer. In other areas, where the snowpack is generally more faceted and less firm, it may be possible for dry lose avalanches to dredge and entrain older snow, deeper in the snowpack, which will increase the volume of the avalanche and likely expose rocks at the ground surface.

Any dry loose avalanche risk is compounded in areas with terrain traps and shallowly buried hazards.

 

Weather
Sat, December 12th, 2020

New snow totals 12/6-12:

Independence Mine: 3-4″  (12/8)

Frostbite bottom: 1-2″  (12/8)

No new appreciable precipitation since 12/8.


Winds at 4500′ were light yesterday. Overnight winds remained light from the S/SE.

Temps at 3500′ were in the teens yesterday, warming slightly in the lower twenties this morning. Yesterday’s temps at 4500′ hovered around 20F.

Marmot Weather Station 12/10-12:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Frosty Bottom Weather Station:

Temps 12/10-12:

Snow water equivalent accumulation this week:

 


NWS Rec Forecast here.

NWS point forecast here.

State Parks Snow Report and Motorized Access information here.

 

 

 

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