Hatcher Pass Avalanche Center

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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Sat, February 29th, 2020 - 7:00AM
Expires
Sun, March 1st, 2020 - 7:00AM
Forecaster
HPAC Staff
The Bottom Line

The persistent slab problem lingers with a low probability/high consequence risk. Persistent slabs are 1-4′ deep, on all aspects, and at all elevations.

Large loads, such as cornice falls, snow machines, and/or groups of people are more likely to trigger large persistent slab avalanches. However, it will also be possible for a single person to trigger this avalanche problem if they find a thin portion of the slab.

Cornices are large and unpredictable; limit your exposure under and around cornices.

***A significant storm beginning tonight could bring 1-2 feet of snow to HP. Significant snowfall will once again have the chance to overload the persistent weak layer and result in natural avalanches. Widespread surface hoar currently exists on all aspects and at all elevations. If preserved and buried by new snow, natural storm snow avalanche hazards will be likely.

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Sat, February 29th, 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'
Moderate (2)
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
Moderate (2)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
Recent Avalanches

Surface hoar is widespread on all aspects and at all elevations, 3-7mm in size, standing up on mostly soft surfaces. If winds do not break down these snow crystals and it is buried by significant new snowfall this weekend, the avalanche hazard will likely increase. Surface hoar pictured below:

2/28 – Widespread surface hoar

Additional winds, although less intense this week, contributed to a small avalanche cycle.  Winds blew ENE 18, gusting ENE 15-44 mph on 2/24-2/25 at 4500′ and gusted 15-28 mph at 3500′. The wind contributed to a few known small  D1 avalanches on the SE ridges and S aspects of Marmot.

2/24-25 Natural wind slab avalanches, Marmot, ~3300′, South

2-3″ of new snow fell on 2/22.

A cornice fall that triggered a large (D2) slab avalanche in Rae Wallace was estimated to have occurred on 2/21.

Approx. 2/21 Rae Wallace, NW, 4200′. HS/SS-NC-D2-O on 4F 1.5mm facets with rounding, 1-3 feet deep. 150′ wide, 500′ vertical run. Cornice naturally failed at ridgeline, bounced down slope, triggering this persistent slab mid-slope. Probed debris 75-100cm deep, mostly soft debris, with some 1F hard slab chucks and ice hard cornice chunks.

Idaho Peak, Divide Ridge – S, 2700′, a large, natural, persistent slab avalanche likely 2/21ish

Several natural slab avalanches were observed on 2/23, likely occurring (but challenging to see due to weather) on 2/21 after 2/17-19 precipitation and 2/19-21 wind. See obs here.

Numerous Arkose Ridge natural avalanches, likely 2/17-19

 

 

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

A low probability, but high consequence, persistent slab avalanche hazard continues to linger this weekend.

This pit represents the persistent avalanche problem throughout HP. In this pit the cohesive slab is approximately one foot thick sitting on a pile of weak facets. In many locations the slab portion reaches 4 feet thick.

The snowpack structure is poor, with cohesive slabs 1-4 feet deep sitting on a rotten sandbox of four finger to fist hard advanced facets (weak layer), generally 3 mm in size. Deeper and larger consequence avalanches are on upper elevation, west to north aspects where predominant winds have been building thick slabs since January. Recently ENE winds have loaded South to West aspects, increasing the potential depth of persistent slabs on these aspects. Evidence of this loading pattern is easy to identify with snow surface clues, such as wind pillows, scalloping, and feeling or probing for stiff snow over weaker snow.

Recent wind loading is visually evident:

The avalanche problem is stubborn to trigger and may fail on slopes as low as 30º in steepness. Either a large load, such as a cornice failure or a snow machine, or a lighter load on a thin spot in the slab, such as a single person on foot, may be able to trigger a large avalanche.

It will be difficult to predict exactly where a person may be able to trigger one of these avalanches. This avalanche problem will let you get well out on a slab before releasing above you, making escape near impossible.

Yesterday we dug a pit in a location that had previously avalanched on the early January facet layer. This slope has refilled with newer snow, and a new cohesive slab. In testing the slab was stubborn to trigger, but showed propagation potential. This means that even slopes that have previously slid, are capable of avalanches again on the same weak layer; repeat offenders.

Use safe travel protocol to hedge your bets and increase your safety margin. Traveling one at time, especially on wide open, broad terrain features, utilizing islands of safety, spotting each rider from a safe place, and staying out of the runnout of avalanches from above are all techniques that will increase your safety factor.

Whumphng, collapsing and shooting cracks have all been experienced lately by observers in isolated locations and will continue to be red flags for identifying this avalanche problem. Drummy, stiff snow, overlying softer, weak snow is also a good indicator of the problem, however in some cases the weak layer is so deep you probably wont know its there without probing or digging.

Looking forward into Sunday and Monday:

The additional problem of surface hoar, which is widespread on all aspects and elevations, could spike the avalanche hazard if it is preserved and buried under a big dump of snow. The combination of potential surface hoar instabilities with deeper persistent slab problems could make for very dangerous conditions. Again this will be determined by the rate, intensity, and quantity of new snowfall and/or wind loading in the coming storm system.

Avalanche Problem 2
  • Cornice
    Cornice
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
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.

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

With new snow last week and wind, cornices continue to build. A large cornice fall triggered a slab avalanche in Ray Wallace, likely Sunday. Cornices are unpredictable and should be avoided. We recommend limiting exposure underneath and giving cornices a wide berth on top. The current persistent slab problem will require a lot of force, or a cornice, to trigger deeper weak layers in the snowpack.

Cornice fall triggered persistent slab avalanche in Rae Wallace

Cornice crack in very large cornice creeping away from ridgeline on Marmot Mountain over the boot pack coulior in Rae Wallace.

Extremely large cornices at ridgelines.

 

Weather
Sat, February 29th, 2020

NWS Rec Forecast here.

NWS point forecast here.

State Parks Snow Report and Motorized Access information here.

 

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