Hatcher Pass Avalanche Center
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Issued
Thu, December 3rd, 2020 - 7:00AM
Expires
Fri, December 4th, 2020 - 7:00AM
Forecaster
Jake Kayes
Conditions Summary

Today’s avalanche problem will be persistent slab. Human triggered persistent slabs will be possible today.

Strong winds and warmer temperatures have made their mark. Coverage is still thin and has deteriorated at mid elevations. Shooting cracks, whumping, and smooth pillow like snow will all be red flags.

Coverage is thin at upper elevations. Triggering avalanches will have higher consequences due to being swept through thinly buried rocks and other hazards. Riding conditions have deteriorated due to widespread wind effect.

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

Human triggered persistent slab avalanches are possible and natural avalanches will be unlikely. This avalanche problem will be found on all elevations and on all aspects on terrain 35° or steeper. 

Strong, consistent winds this week have transported snow onto leeward aspects(southwest through north facing slopes) and have further buried old weak sugar snow(11/20 facets). This wind event started at noon Nov 30th and began to taper by midday on Dec 2, with sustained winds of 25mph and gusts up to 46mph. Strong winds further buried the 11/20 facets. These persistent slabs will be small in size, range from soft to hard slab. Slabs will be a few inches thick to 18” thick, and sit on the old snow surface of weak, sugary, rounded near surface facets. 

Hard slabs will allow you to get out onto slope before failing above you, which will make escape difficult.

Deeper in the snowpack, approximately 1-3′ feet deep, lingers the October basal facets we have been dealing with since the beginning of the season. Loose sugary snow at the ground has continued to be reactive in snow pits. Human triggered avalanches will be possible in isolated areas and large in size. Natural avalanches are unlikely. Whumping and shooting cracks will be good bullseyes clues for this avalanche problem. Any areas with firm wind affected snow should be given caution, as there is likely loose sugary snow below, and this is the poor structure that has been failing. Pole or probe tests, snowmachine cuts on test slopes, hand pits and instability tests are good assessment and identification tools for this problem.

 

 

 

Wind scouring on Marmot Mid-Rib

 

Signs of strong winds, Marmot 4000′ SW

 

Pit from 12/1 showing both the Nov 22nd facets and the October basal facets

Weather
Thu, December 3rd, 2020

Last nights snow totals:

Independence Mine: 1″ of new snow

Frostbite: 1″ of new snow

The arrival of large storm systems in the gulf of Alaska have made their mark in Hatcher Pass. Starting Monday morning temperatures and winds speeds increased drastically. Strong to extreme winds from the southeast started in the morning on November 30th and lasted until midday Dec. 2nd. These winds have scoured slopes at both the mid and upper elevation and loaded leeward aspects. Warm temperatures also came with the winds, Independence Mine reached a high of 35 degrees on Wednesday afternoon. 2” inches of snow fell on Tuesday morning, with some mixed precipitation occurring midday Wednesday.

At Independence Mine one inch of new snow fell Wednesday night Thursday morning. Temperatures dropped drastically early Thursday morning, with temperatures plummeting to 6°F at Marmot station and 12°F at Independence mine. These colder temperatures should last into the weekend.

NWS Rec Forecast here.

NWS point forecast here.

State Parks Snow Report and Motorized Access information here.

 

Independence Mine:

11/29-12/02

 

Temperatures at Independence Mine

 

Marmot

11/29-12/02

 

Frostbite

11/29-12/02

 

 

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