Avalanche Forecast | Hatcher Pass Avalanche CenterAvalanche Forecast | Hatcher Pass Avalanche Center 12/05/2020

Sat, December 5th, 2020

Upper Elevation

Above 3,500'

2 - Moderate

Mid Elevation

2,500'-3,500'

2 - Moderate

Low Elevation

Below 2,500'

1 - Low

0 - No Rating

1 - Low

2 - Moderate

3 - Considerable

4 - High

5 - Extreme


Travel Advice	Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.	Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.	Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding, and conservative decision-making essential.	Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.	Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain.
Likelihood of Avalanches	Natural and human-triggered avalanches unlikely.	Natural avalanches unlikely; human-triggered avalanches possible.	Natural avalanches possible; human-triggered avalanches likely.	Natural avalanches likely; human-triggered avalanches very likely.	Natural and human-triggered avalanches certain.
Avalanche Size and Distribution	Small avalanches in isolated areas or extreme terrain.	Small avalanches in specific areas; or large avalanches in isolated areas.	Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas.	Large avalanches in many areas; or very large avalanches in specific areas.	Very large avalanches in many areas.

Recent Avalanches

Two human triggered avalanches occurred on 12/3 on SW aspects on Marmot. One was human triggered from the ridge at approx. 4000′ and failed on persistent grains and was large enough to bury, injure of kill a person. The other was a D1.5 and was remotely triggered at approx. 3200′ from 30′ away and also failed on persistent grains.

Marmot, SW, 4000′

12/3 remotely triggered avalanche on Marmot, SW, 3200′. Note: This location is very close to the standard untrack, where people build kickers, kid go sledding, and often multiple people are seen skiing/riding at the same time. *ZOOM in to see people for scale

Investigating 12/3 remotely triggered avalanche on Marmot, SW, 3200′

A D2 slab avalanche was observed in Rae Wallace and believed to have occurred during the wind event 12/2-12/3. If you have information about this avalanche, please contact us. Most of the avalanches in this photo (below) are old. The one to the left is new.

Rae Wallace chutes with new slab on the lookers left. 4500′ NW

Avalanche Problem 1

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

Persistent slabs will continue to be a problem this weekend on all aspects, at the mid to upper elevations. They will be possible to human trigger, small to large in size, up to 3 feet deep, on slopes 35 ° and steeper. Remotely triggered avalanches will be possible from the flats below a steeper slope, from adjacent terrain, or from a distance. Hard slabs will allow you to get out onto a slope before failing above you, making escape very difficult to impossible, and ski cuts ineffective.

Persistent slabs will be unlikely to human trigger at low elevation, but may be possible in very isolated locations. Most of the snowpack at low elevation is comprised of sugary, faceted snow and lacking a slab component.

This problem is very difficult to predict and carries a high level of uncertainty. Due to a generally shallow snowpack, being caught and carried in any avalanche also carries the added risk of rock hazards.

There are two different persistent slabs that currently exist in the snowpack.

One problem is shallow in the snowpack, up to a 1 foot deep sitting on an old layer of near surface facets. This slab formed from the early December wind event, and is generally located on West to North aspects.

The deeper problem, up to 3 feet deep, is a persistent slab sitting on an old, weak layer of facets near the ground which were formed in October. This problem can be found on all aspects and elevations.

Considering the lingering, unreliable and uncertain nature of this problem, we recommend conservative terrain choices, lacking any terrain traps. Recent avalanches, whumphing, collapsing and shooting cracks are all red flags for this problem. If you encounter stiff snow, expect that it us sitting on weaker layers below and therefore suspect. We recommend testing the snow with pole tests, hand shears, and instability tests. Spot your partners, ski/ride one at a time, and use appropriate safe zones out of the way of any potential avalanche.