Hatcher Pass Avalanche Center

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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Sat, January 2nd, 2021 - 7:00AM
Expires
Sun, January 3rd, 2021 - 7:00AM
Forecaster
Allie Barker
The Bottom Line

It’s the New Year, but the old persistent slab problem refuses to “turn a new leaf!”

Today it will be possible to human trigger 1-3′ deep, large persistent slabs at mid and upper elevation, on all aspects.

It will be possible to trigger small avalanches in isolated or extreme terrain at low elevation.

Remote triggering an avalanche is still possible.

A significant large, human triggered avalanche on 12/29 reminds us that persistent slabs continue to remain active days, weeks and sometimes months, after loading events.

Riding conditions remain marginal. Enjoy the sun, sledding, and possibly some soft snow in the low to mid elevations on low angle, wind protected terrain, or practicing your observation and rescue skills.

Special Announcements

This week’s forecast is sponsored by Mat Su Trails and Parks Foundation.

Happy New Year! Thank you for your support.

Thanks to our sponsors.
Sat, January 2nd, 2021
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

A large human triggered avalanche occurred on 12/29 on the south face of Microdot.

See more detailed info HERE.

No other avalanches have been observed or 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

Yep, you guessed it! Persistent slabs , 1-3´deep, will be possible to human trigger today on all aspects, at mid and upper elevation, on slopes steeper than 30º.

It will be possible to trigger small avalanches in isolated or extreme terrain at low elevation.

Anywhere there is stiff snow, there is almost certainly weaker snow below it and this poor structure and potential for propagation is the avalanche problem. This slab-weak layer combo is widespread.

In places where the snowpack is thick, such as northerly and westerly terrain that has been wind loaded through the season, triggering persistent slabs will be possible, but less likely than the opposing southerly aspects.  In places where the snowpack has remained thinner through the season, on southerly and easterly aspects, the snowpack consists of more developed facets and weaker snow grains. Wind events have cross loaded these aspects and deposited stiff snow (slab) in a patchy distribution on top of this very weak snow. To add more salt in the wound, a melt freeze crust is making for a slick bed surface. An example of this is the recent human triggered avalanche on Microdot on 12/29.

We continue to see persistent slabs propagate and fail in 3 different locations in the snowpack. The first layer is closer to the surface and consists of rounding facets. This layer is buried anywhere from 1-3’ from the surface. The second layer is near the ground and sits above a thin, supportable melt freeze crust. This layer consists of 1mm rounding facets and was buried on November 6th.  This sugary snow was responsible for the near miss on 12/29. The third layer is a layer of depth hoar that sits at the ground.

While we continue this monotonous pattern of low probability, high consequence gambling with the persistent slab problem lets remember how to hedge our bets to stay safe. Hard slabs may break above you and make it challenging to impossible to get off the slab. We recommend finding low angle, 30° or less, wind protected terrain where the best snow lies, and is only marginally affected by the wind. These areas also generally lack the slab component, but are hard to find. Maintain appropriate distance and S-P-R-E-A-D out when travelling through avalanche terrain. Pay attention to other parties locations especially since remote triggering is possible.  Stay out of harm’s way by skiing/riding one at a time, spotting partners, using appropriate safe zones, avoiding slopes with terrain traps, and using radio communication if possible. Lastly, pray that these conditions change with the New Year!

 

The video below shows our investigation of the hard slab conditions and attempts to explain our understanding of the spatial variability of the slab thickness as it relates to human triggering an avalanche.

 

Additional Concern
  • Cornice
    Cornice
Cornice
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
More info at Avalanche.org

Strong winds this season have been steadily building cornices along the upper elevation ridgelines on leeward aspects. Large, overhanging cornices are unreliable, difficult to predict failure, and pose a  significant threat to people. Use extra caution around and near cornices, as they can break very far back from the actual ridgeline, often surprising travelers.

A large cornice failure will have the ability to injure or kill a person, with the potential additional hazard of triggering persistent slabs below.

Large, overhanging cornice on Rae Wallace, northerly aspect of Marmot.

 

Cornice debris on slope after intentionally triggering. Cornices were mostly difficult to trigger on 12/29, but some large cornices were alarmingly easy to trigger.

Weather
Sat, January 2nd, 2021

New snow totals 12/27/20-1/2/21

Independence mine: less than 1″


Light winds at 4500′ since Thursday, with a max gust on Thursday of 15 mph and a max gust of 9mph overnight.  Temps at 3500′ and 4500′ hovered in the upper teens to 20°F. The next chance for snow looks to be Sunday evening into Monday, with little precipitation expected.

A stale weather pattern with


Marmot weather station 1/1-2

 

 

 

 

 

 

 

 

 

 

 


Independence Mine 1/1-2


NWS Rec Forecast here.

NWS point forecast here.

State Parks Snow Report and Motorized Access information here.

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