Hatcher Pass Avalanche Center

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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Thu, January 7th, 2021 - 7:00AM
Expires
Fri, January 8th, 2021 - 7:00AM
Forecaster
HPAC Staff
Conditions Summary

Today’s problems will be Persistent Slab and Wind Slab.

Large, human triggered persistent slabs, 1-4 feet deep, will be possible on all aspects.  Natural persistent slab avalanches are unlikely. Small, human triggered wind slabs, 4-6″ thick will be likely on leeward aspects at mid and upper elevations, and naturals will be possible. 

At low elevations, natural and human triggered avalanches are unlikely.

Remote triggering avalanches will still be possible.

Rocks and other hazards are lurking just below the surface, with generally shallow snow coverage. Triggering an avalanche will have a higher consequences due to the chance of being swept through rocks and other hazards.

Riding conditions have improved in isolated areas that are protected from the winds.

 

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Thu, January 7th, 2021
Recent Avalanches

No avalanches reported this week.

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 and upper elevations and on all aspects, in terrain 30° or steeper. Natural avalanches are unlikely. These avalanches will be 1-4’ thick and large enough to bury, injure or kill a person.

Remote triggering an avalanche is still possible. 

At low elevations natural and human triggered avalanches are unlikely.

We currently have very poor structure within the snowpack. Triggering an avalanche may catch you by surprise, as evidence of recent avalanches, and warnings such as whumphing, and shooting cracks may be lacking. However, this type of set-up will allow for large human triggered avalanches in specific, but hard to predict, locations. The depth of these avalanches combined with the generally shallow snowpack, will mean any persistent slab avalanche triggered will be large in size and carries the added consequences of colliding with rocks.

There are a few persistent weak layers within the snowpack that all contribute to the larger persistent slab problem (see picture below). Instability tests and hand hardness profiles will quickly identify the problem layers. We do think that the likelihood for triggering these types of avalanche has decreased over time, but because of the uncertainty surrounding this avalanche type, and the current conditions, we advise conservative decision making and terrain choices. We are also paying particular attention to the pattern of south facing human triggered persistent slab avalanches and identifying loading patterns on these aspects that are suspect.

A thin blanket of snow is obscuring visual clues for hard slabs sitting below the surface in many areas. Hand pits, pole tests and instability tests will help you identify the poor structure of weak sugary snow sitting below these hard persistent slabs. Hard slabs can allow people to travel out onto a slope, before fracturing above them making escape extremely difficult.

Make sure you use safe travel techniques EVERY time you are traveling on or near avalanche terrain. Increase your margin of safety with consistency. Spread out while ascending slopes, ride one at a time and choose safe zones that are out of harm’s way.  Avoid any terrain traps or features that will increase consequences if you get caught and carried. 

 

Avalanche Problem 2
  • 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

Strong winds from east and southeast for the last 12 hours, have transported snow and created wind slabs 4-6″ thick. Expect to find these wind slabs on southwest through north aspects near ridgetops. Human triggered winds slabs will be likely at upper elevations on slopes 35° and steeper, possible at mid elevation, and unlikely at low elevation, on southwest through north aspects.

Natural avalanches will be possible at upper elevations.

These avalanches will be small, but have the possibility of stepping down into the persistent slab problem. 

Use hand pits and pole tests to identify the presence of stiff pillows and drifted snow overlying weaker snow. Shooting cracks will be a red flags for this avalanche problem. Even a small wind slab avalanche could sweep you through rocks or other hazards compounding the hazard.

 

Weather
Thu, January 7th, 2021

New snow 1/3-1/7:

Independence Mine: 1″ of new snow


Weather this week started out cold and calm, but began to shift towards warmer temperatures with an increase in winds at ridgetops. On Sunday, temps were in the low teens at Independence mine and gradually increased throughout the day. 1” of new snow fell and was quickly transported from four hours of strong SE winds on Monday. Temperatures increased at all weather stations by midweek. At noon on Wednesday, winds from the east and southeast began to gust up to 32mph for most of the day, transporting snow. As these winds began to increase, independence mine reached 30°F.

Today’s Recreation Forecast is calling for a high of 30°F  and East winds 9-18mph at 3000′, no precipitation expected. At 5am this morning the Marmot Weather Station at 4500′ is reporting winds SE 16mph gusting 29.


Marmot Weather Station last 24 hours

 

Independence Mine Snotel last 24

 

 


NWS Rec Forecast here.

NWS point forecast here.

State Parks Snow Report and Motorized Access information here.

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