Hatcher Pass Avalanche Center

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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Sat, February 22nd, 2020 - 7:00AM
Expires
Sun, February 23rd, 2020 - 7:00AM
Forecaster
HPAC Staff
The Bottom Line

Human triggering wind slabs (winds 2/18-21) on leeward aspects, West to North, at upper elevations, may be possible today. With limited data from a week of stormy weather, it will be prudent to thoroughly assess this hazard before stepping it up in steep terrain at the upper elevations.

Persistent slabs continue to linger and will be possible to human trigger at mid to upper elevations. Avalanches will be larger on West to North aspects, 2-5 feet deep, where the persistent weak layers are buried deeper. Remotely triggering avalanches may be possible in isolated locations.

Loose dry avalanches, up to a foot deep, will be possible to trigger in wind protected areas, and on West to North aspects, at all elevations, on slopes 40° and steeper.

Cornices at upper elevations are large and potentially unstable. Any cornice failure may trigger other avalanche problems compounding the risk.

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Sat, February 22nd, 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

Due to stormy conditions Sunday through Wednesday, limited information is available for observed avalanches this week.  Cracking and collapsing were observed at low elevation during those days. New snow and wind likely obscured evidence of avalanches. We believe that human triggered and natural avalanches were possible on 2/17-2/19 with new snow and wind. However, it appears that the storm snow bonded better than anticipated. No natural avalanche evidence was observed at any elevation on Friday, also a marginal visibility day.

On Friday, only one test slope, NW, 4000′, 35deg, produced an small skier cut avalanche, 4″ deep x 10 feet wide x 20 feet vertical run, failing within the storm snow.

Storm tracking is as follows:

2/17 7″ new snow with winds SE G23-50 mhp

2/18 5″ new snow with low winds

2/19 7″ new snow

2/19- 2/20 winds increase SE G20-25 mph

2/21 Winds gusted SE 17-25 mph for 9 hours

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

SE wind gusts reached a max of 78 mph on 2/18, and gusts were SE/E 35-79 mph from 2/18-2/20, with additional wind gusting SE 17-25 for 9 hours on 2/21.  These strong winds have formed wind slabs which may be possible to human trigger up to D2 in size, in specific leeward locations, at upper elevations on West to North aspects, on slopes above 35º today. Wind slabs are estimated to be up to 12″/30cm thick. Natural avalanches will be unlikely today.

Cracking and collapsing are red flags for this problem. Probe testing is a good way to identify the presence of wind slabs, revealing firmer, strong snow, sitting over softer, weaker snow. Expect any firm wind slabs to have the potential for breaking well above you, making escape difficult.

The wind slab problem should be short lived and heal within the next few days.

Avalanche Problem 2
  • 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

It will be difficult to predict exactly where triggering a persistent slab will be possible. In isolated locations, it may be possible to remotely trigger an avalanche. You may not observe warning signs, such as collapsing, whumphing or shooting cracks prior to triggering an avalanche.

The persistent slab problem will continue to linger, with natural avalanches unlikely, and human triggered possible at mid to upper elevations on all aspects.

Slab avalanches will be deeper, 2-5 feet deep, on West to North aspects, due to the predominant wind direction and wind loading on these slopes. In other areas the persistent slab is approximately 1-2 feet deep.

It may require numerous people, or a large load, such as a snow machine, to trigger an avalanche. It could just take finding a thin spot in a slab to transfer your weight to the buried weak layer and allow for a failure to propagate a large release.

Last week the persistent slab problem was also stubborn to trigger, but one snow machiner found a low 30’s° slope and triggered a persistent slab, pictured below. This same weak layer problem continues to persist, but is now buried deeper.

2/16 – Craigie Creek, SW, 4000′. Persistent slab avalanche. This same problem will linger through the weekend. In this case the rider was able to traverse off the slab to safety.

 

Avalanche Problem 3
  • 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

At all elevations, and in wind protected areas, generally West to North aspects, approximately a foot of loose snow may be possible to human trigger on slopes 40° and steeper.

Small dry loose avalanches will have the ability to catch and carry a person into terrain traps, compounding the hazard.

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

With a lot of new snow and strong winds over the last couple of weeks, cornices have grown in size. Large cornices are overhanging and potentially unstable in specific areas at upper elevations, generally, but not limited to, West to North aspects. Natural failures may be possible, but will be very difficult to predict. Human triggering will be possible today.

Be cautious of triggering cornices from above, onto people below. If you are approaching cornices from behind, use extreme caution, gain a safe viewpoint to identify safe entries onto slopes below, and do not step out on overhanging cornice features.

If traveling under cornices, limit your exposure.

Cornice failures may be large, and the mass of a cornice may be large enough to injure or kill a person, or trigger additional avalanche problems.

Weather
Sat, February 22nd, 2020

Here’s what we are seeing in the models for this weekend:

Temps for Saturday at 5000′, 8-11°F cooling to 4-7°F, winds dropping to 0-7 mph, variable direction. Possible trace of new snow today, with a chance for 1-3″ tonight.

Cloudy skies, with the chance for partly cloudy skies today. More clouds this evening and overnight, with a break in the clouds and clearing skies Sunday afternoon and into Monday

Here’s a storm snap-shot of what we saw this week:

2/17 7″ new snow with winds SE G23-50 mhp

2/18 5″ new snow with low winds

2/19 7″ new snow

2/19- 2/20 winds increase SE G20-25 mph

2/21 Winds gusted SE 17-25 mph for 9 hours


NWS Rec Forecast here.

NWS point forecast here.

State Parks Snow Report and Motorized Access information here.

Observations
Recent Observations for Hatcher Pass