Hatcher Pass Avalanche Center

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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Sat, March 21st, 2020 - 7:00AM
Expires
Sun, March 22nd, 2020 - 7:00AM
Forecaster
HPAC Staff
The Bottom Line

 

Deep persistent slabs up to 5 ft deep will be stubborn, but possible to trigger today, most likely on E to W aspects at low and mid elevation up to 3800 feet. It will be possible to remotely trigger these large avalanches.

6-9″ of new snow forecasted for today will contribute to small wet-loose sluffs in the afternoon on E to SW aspects, at low and mid elevation, on slopes above 40º. Dry loose sluffs will be possible at upper elevation on steep northerly aspects in protected locations.

Variable conditions exist with pow in the most protected locations, upside down snow on firm 3″ sun crust, and sastrugi.  Although we always recommend spreading out in avalanche terrain…treat avalanche terrain like the virus and keep your distance!

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Sat, March 21st, 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

Hatcher Pass received approximately 6″ of new snow on 3/17-3/18 with light to moderate winds this week at 3500′.

Dry loose, wet loose, and persistent slab avalanches were observed this week.

Dry loose were observed in specific locations, on steep northerly aspects last weekend and early in the week.

Small to large natural and human triggered wet-loose sluffs were observed on E to W aspects, mostly on SE to SW at all elevations on 3/17 and 3/18.

Wet-loose activity on south aspects of Skyscraper , likely 3/17 when temps at 3500′ were almost 40deg. F

Wind slabs were observed but we do not have confirmation of any avalanche activity.

A few persistent slab avalanches were observed this week.

3/14 Snowmachine triggered or remotely triggered. Lucky Shot Mine area, Willow side of HP.

3/15 Natural or remote trigger. Divide Ridge 2700′

Avalanche Problem 1
  • Deep Persistent Slabs
    Deep Persistent Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.

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

Deep persistent slabs up to 5 feet deep still linger in the snowpack. Although we haven’t seen activity on this layer since March 9th and 15th, there is still concern for this low probability/high consequence problem. It will be possible to trigger deep persistent slabs this weekend, on all aspects, at all elevations and most likely on E to W aspects up to 3800′.

A significant amount of slope testing has been observed without incident in the past 5 days. Let’s remember that it will take finding a shallow, weak spot on the slope or a large load to trigger an avalanche on this layer.

Larger loads like snowmachines and cornice failure will have higher chances of triggering an avalanche. Lighter loads such as a person on foot will be able to trigger this problem in specific locations where the slab component is thinner and the buried weak layer is closer to the surface.

Poor structure in snowpack will continue to be a problem through this season. With additional daylight hours, solar gain and increased temperatures, this weak layer may keep rearing its head.

3/20 Facets and Depth hoar at the bottom of the snowpack.

3/20 Slope testing without incident up Willow Creek. This problem is still untrustworthy.

 

Avalanche Problem 2
  • Normal Caution
    Normal Caution
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
Normal Caution
Normal Caution means triggering an avalanche is unlikely but not impossible.

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

Wet-loose

New snow today will contribute to small wet-loose sluffs that will be possible to human trigger on E to SW aspects,at low and mid elevation, on steep slopes above 40º in the afternoon. Any additional new snow today will increase the size of the sluff. If you see rollerballs, it’s time to move to shadier and cooler terrain.

Dry-Loose

Additional new snow today will make it possible to trigger a dry loose at upper elevation in steep northerly protected terrain. Snow that accumulates today combined with 3/17 -3/18 snow has the potential to increase the volume and size of the 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 and remain a hazard. Although cornices are known to be unpredictable, we can expect more cornice failure as days lengthen and temperatures increase. Please give cornices a wide wide berth and reduce exposure underneath on up tracks or adjacent terrain. Cornices are so large that they could be a perfect trigger for the deep persistent slab problem, especially as we move forward into Spring.

Weather
Sat, March 21st, 2020

NWS Rec Forecast here.

NWS point forecast here.

State Parks Snow Report and Motorized Access information here.

Observations
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