Hatcher Pass Avalanche Center

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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, March 7th, 2020 - 7:00AM
Expires
Sun, March 8th, 2020 - 7:00AM
Forecaster
HPAC Staff
Avalanche risk The Bottom Line

It will be possible to trigger a large persistent slab 1-4 feet deep today on all aspects at all elevations.

Small dry loose sluffs will be possible to trigger on steep slopes, on all aspects, at upper elevation in protected locations, and increasing in size throughout the day.

Cornices continue to grow and demand respect as they teeter over most upper elevation leeward aspects. Cornice fall has the potential to trigger a large slab, increasing the size and consequence of any avalanche.

***A Winter Weather Advisory is in effect until 6am Monday and could bring up to 12″ of new snow to HP . Significant new snow and/or wind could overload the persistent weak layer and increase the avalanche danger by Sunday.

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Sat, March 7th, 2020
Upper Elevation
Above 3,500'
2 - Moderate
Avalanche risk
Mid Elevation
2,500'-3,500'
2 - Moderate
Avalanche risk
Low Elevation
Below 2,500'
2 - Moderate
Avalanche risk
0 - No Rating
1 - Low
2 - Moderate
3 - Considerable
4 - High
5 - Extreme
Avalanche risk Avalanche risk Avalanche risk Avalanche risk Avalanche risk
Travel Advice Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features. Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern. Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding, and conservative decision-making essential. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended. Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain.
Likelihood of Avalanches Natural and human-triggered avalanches unlikely. Natural avalanches unlikely; human-triggered avalanches possible. Natural avalanches possible; human-triggered avalanches likely. Natural avalanches likely; human-triggered avalanches very likely. Natural and human-triggered avalanches certain.
Avalanche Size and Distribution Small avalanches in isolated areas or extreme terrain. Small avalanches in specific areas; or large avalanches in isolated areas. Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas. Large avalanches in many areas; or very large avalanches in specific areas. Very large avalanches in many areas.
Recent Avalanches

We hypothesize that the SE 9-28 mph winds with gusts 20-53 mph at 4500′ knocked down widespread 3-7 mm surface hoar prior to new snowfall this week.

3/1-3/2  HP received about 12-16″ of new snow and 1.3″SWE.

3/3 D2 Large natural storm slab on S aspect of Sunnyside of Hatch 3500′ . Debris ran all the way to the gully at the bottom of the slope.

 

3/3? D2.5  Large natural persistent slab avalanche on NW aspect of Punk Spines across from Stairstep 3500′. The bed surface looked like the old January facet layer. Debris ran to the gully and then continued downhill towards the lower alders.

 

3/3? D2 Large natural persistent slab avalanche on W aspect of Punk Spines at 3100′.

 

Several small natural wind slabs were observed up Pinnacle Valley and Archangel Valley.

 

Several small D1-D1.5 Dry Loose sluffs were observed on steep slopes above 40° on all aspects at mid and upper elevation.

Avalanche Problem 1
  • Persistent Slabs
    Persistent Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    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

You might have thought this problem just disappeared (or at least want it to), but surprise…its still lingering in some cases more than a meter deep. The persistent slab will be challenging to find and stubborn to trigger today, up to D2.5 and 1-4 feet deep. This problem exists on all aspects and at all elevations.

This avalanche problem 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.

12″ of new snow early in the week 3/1-3/2 ,with some wind, was enough to stress the deep buried January facets, resulting in two sizable natural avalanches on NW and S aspects both at about 3500′. One in particular, on the Punk Spines was D2.5 in size, crown estimated at 3.5 feet deep, with debris running over 1000′. These avalanches likely occurred on 3/3.

Many observers continue to report whumphing at mid elevation specifically, but no cracking or avalanches. The last human triggered avalanche occurred on 3/4. See obs here.  We are still dealing with this low probability/high consequence problem.

Use safe travel protocol to 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 run-out of avalanches from above are all techniques that will increase your safety factor.

Going, going, almost gone!

 

Heads up! Another 6-12″ of new snow if forecasted for HP through Monday morning, with the main brunt of the storm on Sunday. It will be possible for this additional snow to overload the persistent weak layer, increasing the avalanche danger by Sunday/Monday.

 

 

Avalanche Problem 2
  • Dry Loose
    Dry Loose
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    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

More than a foot of new snow early this week (3/1-3/2) combined with colder temperatures will make it possible to trigger a small dry loose sluff on steep slopes above 40°, at mid and upper elevation, on all aspects today, although most likely in wind protected terrain.

Sluff management skills will be key to avoiding getting sluffed somewhere you dont want to go. Skiing one at a time, using safe zones, and clear communication will be aids in mitigating this hazard.

4″ of new snow since 6am with 6-12″ of new snow is forecasted today through Monday morning and could increase the avalanche danger and size for dry loose sluffs. Watch your sluff!

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

Cornices continue to grow with the ample snow and wind HP has received over the past several weeks.  Cornices are teetering over leeward aspects, most notable in Rae Wallace where entry onto slopes can be challenging. We recommend limiting exposure under corniced terrain and giving cornices a wide wide berth at the top. Cornice fall has the potential to trigger the larger persistent slab, increasing the size and consequence of any avalanche.

Large cornices loom over leeward aspects and could break further back than anticipated.

Weather
Sat, March 7th, 2020

NWS Rec Forecast here.

NWS point forecast here.

State Parks Snow Report and Motorized Access information here.

A Winter Weather Advisory is in effect from 6am Sat until 6am Monday morning

5-12″ of new snow are expected through Monday morning.

Observations
Recent Observations for Hatcher Pass