Hatcher Pass Avalanche Center

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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Thu, December 31st, 2020 - 7:00AM
Expires
Fri, January 1st, 2021 - 7:00AM
Forecaster
Jake Kayes
Conditions Summary

Today’s avalanche problem will be persistent slab. Human triggered avalanches are possible at mid and upper elevations. Natural avalanches are unlikely. Hard stiff snow on top of weak sugary snow will be a sign that persistent slab problems are present. 

Winds from last week have created hard stiff slabs. These hard slabs will allow you to travel out onto a slope before fracturing above you. Remote triggering an avalanche will still be possible.

At low elevations human triggered avalanches are unlikely and natural avalanches are unlikely

This week we had a near miss involving a large persistent slab avalanche. This avalanche occurred on a south facing slope, continuing a pattern over the last month of human triggered avalanches on south facing slopes at the upper elevation.

Rocks and other hazards are still thinly buried, these hazards will increase the consequences of being caught in an avalanche. If you do get caught it is very likely that you will be swept through rocks and sustain traumatic injuries.

Riding conditions are still marginal with soft snow becoming more difficult to find.

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Thu, December 31st, 2020
Recent Avalanches

On 12/29 there was a near miss on Microdot S/SW 4400’. A solo rider triggered a persistent slab avalanche, the hard slab fractured above the person. Luckily the rider was able to out ski the slab and avoided hitting any rocks during the escape. Debris from this avalanche was up to 9’ deep in places.

Right next to this avalanche(same aspect and elevation), was a natural avalanche that occurred on 12/22. It is common when dealing with persistent slab problems, for slopes to remain sensitive to human triggers for days or even weeks after a loading event. This is the reason that you keep seeing persistent slab problems in the forecast and conditions summary. Click this link for more info on this avalanche.

Looking down the avalanche path with lots of exposed rocks. These rocks increase your chance of traumatic injury.

The debris was up to 9 feet deep in places.

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

Human triggered persistent slab avalanches are possible at mid and upper elevations and on all aspects, in terrain 30° or steeper. Natural avalanches are unlikely. These avalanches will be 1-4’ thick and will be large enough to bury, injure or kill a person. 

It will be easier to affect the weak layer where the snowpack is thin. One of these locations is on south aspects. In contrast,  North aspects tend to have a thicker snowpack so it will be harder to trigger an avalanche on leeward aspects. We believe that the small number of human triggered avalanches does not correlate to a stable snowpack. We have had very few users travel into steep terrain due to poor riding quality, exposed rocks, and the consequences of trigger persistent slabs. Remember persistent slab avalanches can be triggered days or weeks after a loading event.

There are several problem layers of weak sugary faceted snow sitting in the snowpack. The first layer is closer to the surface and consists of rounding facets. This layer is buried anywhere from 1-3’ from the surface. The second layer is near the ground and sits above a thin melt freeze crust. This layer consists of 1mm rounding facets and was buried on november 6th.  This sugary snow was responsible for the near miss on 12/29. The third layer is a layer of depth hoar that sits at the ground.

On top all these different layers of weak sugary snow, are hard stiff slabs. Hand pits, pole tests and instability tests will help you identify the poor structure of weak sugary snow sitting below these hard persistent slabs. Hard slabs can allow people to travel out onto a slope, before fracturing above them making escape extremely difficult. The avalanche on 12/29 allowed the rider to make several turns down the slope before fracturing above him.

Remote triggered avalanches are still possible. Use adequate spacing when traveling uphill, ride down one at a time, spot your partners, and choose safe zones that are out of harm’s way. Avoid any terrain that appears to be cross loaded and choose slopes with safe runouts that are not above any terrain traps.

Additional concern: Cornice fall

Cornices have grown very large on some ridgelines. Give these cornices a wide berth, cornices tend to fracture further back than expected. When in doubt take another step away from the cornice and make sure that you aren’t standing on top of one.

Crown profile of 12/29 human triggered avalanche

1mm rounding facets, this layer was responsible for the human triggered avalanche on 12/29

South Face of Skyscraper Ridge. Thin coverage and a persistent slab problem make for a dangerous recipe

A large cornice above the Ray Wallace chutes.

Signs of cross loading can be seen on lower Eldorado bowl

Weather
Thu, December 31st, 2020

new snow totals 12/27-12/30

Independence mine: less than 1″


With a storm system to the south Hatcher Pass experienced an increase in temperatures and strong winds this week with a trace amount of new snow. Temperatures at Independence Mine started out the week in the teens but by mid day on 12/27 began increasing into the 20s and peaking at 31°F by the 28th. On 12/27 Marmot weather station began reporting strong winds from the southeast that didn’t dissipate for 24 hours. A trace amount of new snow fell on the 28th at Independence Mine. Temperatures and winds increased on the 30th with Independence Mine 30°F and strong winds from the SE at Marmot station.


NWS Rec Forecast here.

NWS point forecast here.

State Parks Snow Report and Motorized Access information here.


Marmot 12/27-30


Frostbite bottom 12/27-30

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