Hatcher Pass Avalanche Center

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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Sat, March 20th, 2021 - 7:00AM
Expires
Sun, March 21st, 2021 - 7:00AM
Forecaster
Allie Barker
The Bottom Line

The avalanche hazard is MODERATE today for PERSISTENT SLABS.  Persistent slabs will be possible to human trigger today on E to W aspects, mostly likely on SE,S, SW aspects, at all elevations. Small avalanches in specific areas or large avalanches in isolated areas are possible today. 

Head’s up! Many avalanches are being triggered by the 4th or 5th rider on slope with previous tracks prior to triggering avalanches. 

Dry loose sluffs will be small and slow moving today.

Conditions are improving at Hatcher Pass thanks to an unseasonably cold March, despite several buried crusts of varying thickness and distribution.

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Sat, March 20th, 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'
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

Since Wednesday, March 17th, a few avalanches have been reported (see info below).

For more info on recent avalanches earlier in the week, look at the conditions summary HERE.

 

Human triggered avalanche on 3/18 SW/W aspect 4500′ (unverified info)

 

Same avalanche as above. Look how many times the slope was skied before triggering the slab! 4500′ SW/W aspect 3.18 (unverified info)

 

Bullion Mtn on the W side of Hatcher Pass
SW aspect , elevation and trigger unknown, 3/19

 

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

Small to Large Persistent slabs will be stubborn but possible to human trigger today, on E to W aspects, mostly likely on SE,S, SW aspects where slabs up to 12″ thick will be possible, at all elevations. Small avalanches in specific areas or large avalanches in isolated areas will be possible this weekend. This week’s unusually cold weather has been driving the faceting process in the snowpack which has made this problem harder to find but it still exists.  A significant amount of skiing and riding has occurred this week without incident.

Let’s not forget that HP had a close call with a large avalanche this week on the Sunnyside of Hatch. This avalanche fits the persistent slab paradigm well for the following reasons. Persistent Slabs can remain dormant for weeks to months, re-activate with loading events, and cycle through dormancy and activation throughout the entire season. Persistent slabs are commonly low probability/high consequence events and can be hard to predict.  Lastly, persistent slabs can be widely distributed, or quite isolated, like we are observing now due to the lack of a slab in many locations.

Remember that tracks on a slope are NOT an indicator of stability. Many people skied or rode lines this week on the  the same features where avalanches were triggered on the 4th or 5th lap. This is a red flag! The lack of obvious clues is a good reason to dig in the pack to assess instability, choose aspects wisely and with reason, and use good travel protocol.  This low probability but high consequence hazard reminds us to be disciplined in implementing proper travel protocol in avalanche terrain. Ride one at a time, use appropriate safe zones, use communication, choose skin tracks and routes appropriately according to the danger of the day.

Compare the two photos below. A striking similarity is the number of tracks on the slope prior to triggering the avalanche.

Sunnyside of Hatch SSW 3500′ 3/13

 

Unknown location, SW/W aspect 4500′ 3/18 (unverified)

 

 

 

Avalanche Problem 2
  • 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

Small, slow moving, dry loose sluffs will be possible to human trigger today on steep slopes above 40º. Knowing how to manage your sluff is always a good skill.

Human triggered dry loose, Lodge Run SW 4500′ 3.19

Weather
Sat, March 20th, 2021

Cold temperatures have persisted all week. Temperatures at 3000′ will be 6-13ºF today with calm winds from the South up to 7mph.

No significant storms or new snow this week.

Temps and Snow Depth at Independence Mine 3550′ (below)

Temperature at Marmot 4500′ (below)

Winds at Marmot 4500′ (below)

 


 

NWS Rec Forecast here.

NWS point forecast here.

State Parks Snow Report and Motorized Access information here.

Observations
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