Hatcher Pass Avalanche Center

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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Sat, November 14th, 2020 - 7:00AM
Expires
Sun, November 15th, 2020 - 7:00AM
Forecaster
Jed Workman
The Bottom Line

The avalanche hazard is Moderate at all elevations and on all aspects for Persistent Slab avalanches and Loose Dry.

It will be possible to human trigger large, full depth slab avalanches, generally 2-3 feet deep, that could bury, injure, or kill a person. Natural avalanches are unlikely.

Remotely triggered slab avalanches are especially dangerous, and will be possible. They are difficult to predict or anticipate. It will be possible to trigger avalanches from a long distance away, from adjacent terrain, or from the flats below a steeper slope.

Whumphing and shooting cracks are bulls-eye clues for the persistent slab problem.

While we have received a lot of snow recently and Hatcher is looking like a winterland, the coverage is still quite shallow. Rocks and hazards are lingering just below the snow surface.

The beginning of the season is always a good time to review the North American Avalanche Danger Scale, click HERE for a 2.5 minute informational video.

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Sat, November 14th, 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

The last reported avalanche was on 11/10.  This human triggered avalanche was reported to be 3 feet deep, 2oo feet wide and ran 40 vertical feet. The rider thinks they may have triggered it from the base of the slope after decent, as it was seen coming down after them. See observations HERE.

 

11/10/20 Hatch Common. Human triggered avalanche. North, 4200′, 35° slope

On 11/9 numerous natural avalanches were documented after the 11/7-8 storm (Election Dump) which deposited approximately 2 feet of snow over a persistent weak layer formed in October. Click HERE for more photos and information. This weak layer allowed for wide avalanches and crowns that wrapped around terrain features.

11/9 – Punk Spines, W, 4135´, Crown is visible just under the red line. Crown length approx. 1,500 feet.

 

11/7-8 – Martin Mine, East, 4200′. The crown on this avalanche wraps the gulley and pulls out snow on both sidewalls. We rated the destructive size at D2.5, capable of damaging a vehicle, and partially burying it.

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

The good news is the avalanche hazard has been slowly improving over time since the last natural avalanche cycle on 11/7-8. Natural avalanches are unlikely.

The bad news is that while it is becoming less likely to trigger an avalanche, it is still possible, and any avalanche triggered will likely be large in size, and able to bury, injure, or kill a person. The persistent slabs problem exists at all elevations and on all aspects.

Additionally, remotely triggered avalanches are possible. Remotely triggered avalanches are very difficult to predict and can be triggered from a long distance away, from the flats below a steeper slope, or in adjacent terrain. Pay close attention to other user groups in your area, as either party could unknowingly endanger the other.

The primary layer of concern is a persistent weak layer of faceted, sugary snow near the ground which was formed in October. This flaw in the snowpack will continue to be a problem with each subsequent loading event. Expect a spike in avalanche activity during and just after significant loading events, and for increasing stability to be slow thereafter.

More details:

The October facet layer was buried and reactive to human triggers after it was overloaded by approximately one foot of snow (at approx. 3000′) on 10/26 (Halloween Storm). Human triggered and remotely triggered avalanches occurred just after the storm.  See observations HERE.

After this storm cycle, the avalanche hazard slowly improved, but the weak layer did not go away. And its not going to anytime soon.

The Election Dump on 11/7-8 deposited approximately 24″ of new snow at 3000′. This overloaded the weak layer again which resulted in numerous natural avalanches and lingering instabilities after the storm. More info and pictures in observations HERE.

This is where we are now, 6 days after the Election Dump avalanche cycle, and seeing slow improvement. Note that the last human triggered avalanche was reported 4 days ago on 11/10, which was large in size, and may have been triggered from the base of the slope. This demonstrates the lingering persistence of the current avalanche problem.

Avalanches are more likely on slopes 35° and steeper, and deeper where previous wind loading has drifted snow, either in the mid pack and/or on the surface. A very general rule of thumb is that recent SE winds loaded West to North aspects at mid to upper elevations and these slopes may be more prone to avalanches. This should be taken with a heavy grain of salt, as we have seen the avalanche problem on all aspects.

While winds have loaded leeward slopes, and increased the depth of the snowpack in specific areas, its worth mentioning that no portion of the snowpack is so deep that your weight can not affect the buried weak layers.

In the lower elevations we are keeping the hazard elevated to moderate. Even though there is less snow, the weak layer is present and whumphing has been observed. Our concern here is that you may be able to trigger an avalanche remotely on a steep slope above you. An example would be traveling at the low/mid elevation boundary under marmot mountain. We would have some concern being exposed to large, steep slopes above you, and for the potential to trigger them given the propagation potential of the weak layer. We also think that there may be isolated locations where the wind has stiffened the snow and created a slab capable of an avalanche. It’s still early in the season, we could use more data to increase the forecast certainty.

How we will reduce risk in avalanche terrain this weekend:

We will be playing it safe and choosing conservative terrain choices this weekend. We will avoid slopes with terrain traps in the runnout. We will be choosing lower angle slopes in the mid elevation band that have a grass or tundra base and lack stiff snow overlying weaker snow. We will also be avoiding obvious areas of previous wind loading and drifting where stiffer snow overlies the weak layer. We will be probing the snow to identify this structure and avoid it.

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

Loose dry avalanches and sluffing will be possible on steep slopes, 40 degrees and steeper, at mid to upper elevations on all aspects, in wind protected areas. Even a small loose avalanche poses a significant risk with shallowly buried rock hazards.

Weather
Sat, November 14th, 2020

NWS Rec Forecast here.

NWS point forecast here.

State Parks Snow Report and Motorized Access information here.

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