Thu, December 10, 2020 Turnagain Pass Avalanche Forecast

Thu, December 10th, 2020

Alpine

Above 2,500'

2 - Moderate

Treeline

1,000'-2,500'

2 - Moderate

Below Treeline

Below 1,000'

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.

Avalanche Problem 1

Wind Slabs
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

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

With decent visibility yesterday it was evident that winds during the storm moved snow around. There were small winds slabs in many of the front side gullies on Seattle Ridge, and lots of cross-loading and scouring. Cornices had grown and there was stiff snow in wind exposed terrain. Today lingering wind slabs are still a concern. It will be important to pay attention to terrain to with stiff hollow snow, especially steep unsupported slopes and gully features, and look for shooting cracks.

There is a bit of question mark today as well. Are the storm slabs from the Sunday/Monday storm becoming a new persistent slab issue or not? There are signs pointing to a stabilizing snowpack but there is still some uncertainty and with that potential for large human triggered avalanches. As you head out here are few things to keep in mind about this question:

Much of the snowpack in the forecast area remains untested after this last storm.
The new snow fell on surface snow that had weakened during our last clear period and below 2500′ this set-up sits on a rain crust.
During the storm on Monday observers found storm slabs easily sliding on the weak snow on the crust. Tuesday, two parties of skiers, one on Sunburst and one on Magnum, reported turning around due to large whumpfs in the mid elevation band where the old rain crust exists.

On the flip side, the Tuesday observers on Seattle Ridge found the new snow bonding well to the old snow and crust and no signs of instability. This was similar for observers on Sunburst yesterday and skiers and snowmachiners ventured out a bit more with no reported incidents. This seems promising but is still somewhat of an unknown as folks spread out testing more terrain. As we move into the next couple days of clear skies it is very important to ease into terrain, use good travel protocol, think about consequences and look for signs of instability.

Cornices: We know cornices have grown. Give them a wide berth as they can now easily break further back than we might expect. As seen last week, they can also trigger an avalanche below, creating a bigger problem if a person is involved.

Wind textured surface snow on Sunburst. 12.9.20. Pay attention to stiff hollow snow and look for shooting cracks. Note the scouring and cross loading on the north side of Magnum as well.

Will the new storm slab stick to the weak snow on top of 12.1 rain crust? That is still a bit of a question mark. Sunburst snowpack at 2200′, 12.9.20.

Avalanche Problem 2

Deep Persistent Slabs
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.

Likelihood of Avalanches

Size of Avalanches

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

In the Alpine, hitting a thin area in the snowpack or traveling in areas with an overall shallower snowpack (around 3-4′deep) is still concerning for triggering a large slab that fails in the old October snow. The southern end of Turnagain Pass through Johnson Pass, Silvertip and Summit Lake all have slopes without as much snow as northern end of Turnagain or Girdwood.

Weak snow at the base of the snowpack on Sunburst at 3200′. 12.9.20.

Weather

Thu, December 10th, 2020

Yesterday: Skies were overcast to partly cloudy. Winds were light and easterly and temperatures were in the teens to mid 20°Fs. Overnight skies were partly cloudy, winds remained light and temperatures stayed in the teens to mid 20°Fs.

Today: Skies will be mostly sunny. Winds will be light and northwesterly and temperatures will be in the teens to mid 20°Fs. Overnight skies will mostly clear, winds will be light and variable and temperatures will be in the teens and single digits.

Tomorrow: Mostly sunny skies, calm winds and temperatures in the teens and low 20°Fs. The mostly clear skies look to continue through Saturday.

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

	Temp Avg (F)	Snow Depth (in)
Center Ridge (1880′)	26	65
Summit Lake (1400′)	15	25
Alyeska Mid (1700′)	27	59

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	17	NE	8	29
Seattle Ridge (2400′)	22	E	4	14

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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