Thu, December 02, 2021 Turnagain Pass Avalanche Forecast

Thu, December 2nd, 2021

Alpine

Above 2,500'

2 - Moderate

Treeline

1,000'-2,500'

2 - Moderate

Below Treeline

Below 1,000'

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

No known recent avalanches

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

The skies should trend towards clearing today with light winds and cold temperatures creating a mighty temptation to venture into bigger terrain, but dragons are lurking in our snowpack. Throughout the advisory area we have weak sugary persistent weak layers underneath 2-3′ of snow that fell over the past 10 days. As we get further away from the most recent storm the probability of human triggered avalanches is likely decreasing but the consequences if one of these persistent slabs releases could be high. To completely avoid this avalanche problem stay on lower angle slopes, smaller features, and be aware of terrain above you that could be triggered remotely.

There have not been many folks travelling in the mountains since the latest storm to test the reactivity of our weak layers, so we have some uncertainty about how sensitive they are to triggering. It is important to remember that persistent slabs are notoriously unpredictable and don’t always release with the first person on the slope. We continue to recommend careful evaluation of the snowpack by being alert to collapsing or shooting cracks and using snowpits to identify and test weak layers (small collapse on Sunburst 12.1.21). The most common weak layer setup we have seen recently is a layer of weak and sugary facets on top of a stout melt freeze crust (example here).

Motorized areas recently opened in Chugach National Forest: We have very limited information from the areas that opened to motorized access yesterday (Placer River, Johnson Pass, Lynx Creek, Lost Lake, etc). We expect that the persistent weak layers exist throughout the advisory area and recommend a conservative mindset while venturing into new terrain. If you travel into a new area please consider sending in an observation so we can start to fill in our knowledge gaps and get a better picture of the conditions throughout the area.

Wind slabs and cornices: Watch for old wind slabs and cornices that built up during the last storm, especially at higher elevations and along ridgelines.

Small avalanche in a wind loaded area on the E face of Seattle Ridge from sometime in the past two days. 12.1.21

Cornice fall in Hippy Bowl on Tincan Common from sometime during the last storm. Photo: Anonymous 12.1.21

Additional Concern

Glide Avalanches

Glide Avalanches

Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.

More info at Avalanche.org

Glide activity has continued to decline and many of the lingering glide cracks are slowly getting filled in by the recent storms. They can still be a hazard to fall into and have to potential to spontaneously release. Minimize time underneath open glide cracks to avoid this unlikely but catastrophic avalanche hazard.

Weather

Thu, December 2nd, 2021

Yesterday: Cloud cover was variable yesterday, with good visibility in the morning and mostly cloudy skies otherwise. Winds were light at less than 5 mph with gusts up to 20 out of the east in the morning before shifting to north and then west throughout the day. Temperatures were in the teens throughout the day and dropped into the low teens to single digits overnight. No new precipitation was recorded.

Today: Cloud cover should diminish throughout the day trending towards partly sunny skies. Chance of snow showers in the morning with trace amounts of accumulation. Winds will remain light out of the west to northwest from 5-10 mph. Temperatures will be cold again staying in the single digits throughout the day.

Tomorrow: Cold and calm weather looks to remain in the advisory area through the next few days, before another low pressure system starts to move in on Saturday night into Sunday. Temperatures will remain in the positive and negative single digits. Winds will remain light out of the west until the next system moves in when they will shift to the east.

PRECIPITATION 24-hour data (6am – 6am)

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	16	0″	0″	60″
Summit Lake (1400′)	13	0″	0″	12″
Alyeska Mid (1700′)	17	1″	0.1″	N/A

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	13	E then N	2	6
Seattle Ridge (2400′)	9	E then W	3	18

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
10/27/24	Turnagain	Observation: Tincan	Michael Kerst
10/21/24	Turnagain	Observation: Turnagain Pass Road Observation	Trevor Clayton
10/19/24	Turnagain	Observation: Tincan – Below Todds Run	Andy Moderow
10/18/24	Turnagain	Observation: Taylor Pass	Eli Neuffer
10/15/24	Turnagain	Observation: Tincan Common	John Sykes Forecaster
10/14/24	Turnagain	Avalanche: Tincan	CNFAC Staff
05/13/24	Turnagain	Observation: Eddie’s, Sunburst, Seattle, Cornbiscuit, Pete’s South	H Thamm
05/13/24	Turnagain	Observation: Turnagain Pass non-motorized side	Amy Holman
05/12/24	Turnagain	Observation: Warm up Bowl	Tony Naciuk
05/07/24	Turnagain	Observation: Turnagain Pass Wet Slabs	A S

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