Avalanche Forecast | Hatcher Pass Avalanche CenterAvalanche Forecast | Hatcher Pass Avalanche Center 12/16/2021

Thu, December 16th, 2021

Upper Elevation

Above 3,500'

2 - Moderate

Mid Elevation

2,500'-3,500'

2 - Moderate

Low Elevation

Below 2,500'

2 - Moderate

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

There were two human triggered avalanches reported this week. Both were on the north side of 4068.

This avalanche is somewhat of an outlier. This avalanche occurred where an old glide crack had previously released. The snowpack was much shallower in this location than other areas.

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

Human-triggered persistent slab avalanches are possible at all elevations, predominately on SW-NE aspects. Natural avalanches are unlikely. These avalanches will be stubborn to trigger, small in size, with large avalanches in isolated areas. Persistent slab avalanches will be possible on slopes that are steeper than 30°.

There is a lot of spatial variability in the snowpack right now; knife hard slabs and knee deep faceted snow can be found in a relatively small area. Spatial variability also means that it may be possible to trigger a deeper avalanche from a thin portion of the slab, resulting in a larger avalanche than expected.

Any new snow will make visually identifying this persistent slab problem difficult. Use hand pits and poles/probes to identify stiff hard snow sitting on top of weak sugary facets. Whumping and shooting cracks will be red flags for this avalanche problem.

Stay disciplined with your travel techniques when out in the mountains. Space out while ascending slopes, ride one at a time, and make sure you stop out of harm’s way. Hard slabs tend to break above you making escape difficult. It will be possible for multiple people to travel on the same terrain before someone triggers an avalanche.The lack of recent avalanches is not representative of the stability. Few people have gotten out in the past two weeks. Forecasters have observed cracking, collapsing, and small avalanches this week.

Even a small avalanche can have big consequences if you’re traveling above a terrain trap.

Large Depth Hoar 6-8mm in size. these grains have been found in multiple areas of the forecast zone.

Marmot with signs of previous wind loading.

Weather

Thu, December 16th, 2021

NWS AVG Forecast here.

NWS Rec Forecast here.

NWS point forecast here.

State Parks Snow Report and Motorized Access information here.

Observations

Recent Observations for Hatcher Pass