Tue, January 09, 2018 Turnagain Pass Avalanche Forecast

Tue, January 9th, 2018

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

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

Above ~2,000′ our concern is the New Year’s storm snow that has now settled into a slab, around 12 – 20″ at Turnagain Pass and 20-30″ in the Girdwood Valley. We have the buried surface hoar that was the cause for last week’s avalanche activity that sits right under this New Year’s snow. Could this slab/weak layer combination be triggered by the weight of a skier or snowmachiner? The answer is still YES. The buried surface hoar layer is slowly gaining strength and the slab is becoming more stubborn to trigger. However, snow pit results are still showing this layer to be reactive and to propagate – meaning human triggered slabs are possible if one hits the wrong spot on the slope. In addition, we are also concerned about slopes that avalanched in early December which have a thinner snowpack and harbor facets under the slab. Observers yesterday dug in a thinner spot on Sunburst and found reactive facets near the ground.

On January 2nd it rained up to 2,000-2,300′, creating melt-freeze crusts in the upper snowpack at these elevations and below, making triggering an avalanche unlikely in lower elevation terrain.

For those riders and skiers headed out today:

Keep in mind that travel in the upper elevations is where triggering a slab is most likely. This is above 2,000′ where NO crusts exist in the top foot of the snowpack.
Remotely triggering a slab is possible, several tracks may be on the slope before a slab releases and no signs saying ‘the slope is unstable’ are likely to be present.
Larger slopes are more suspect as well and those with rocky features.
Safe travel habits are always key, but especially when dealing with persistent and deep persistent slab avalanche problems: expose only one person at a time, have an escape route planned, watch your buddies closely and view all slopes as avalanche paths. If the snow does slide where will it go? Avoid terrain traps.

Wendy’s video and photo from 3,000′ on Sunday illustrate our current buried surface hoar issue well:

Thin snowpack at 3400′ on Sunburst. Note the weak faceted snow at the base under the slab. Photo: Sam Galoob

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

Still reading… Deep Persistent Slabs have been in the forecast every day starting December 14th. Unfortunately it is a concern that we can’t ignore. At high elevations above 3,000’, human triggered large and dangerous deep slab avalanches are still possible. Weak sugary snow (basal facets) near the ground is creating a low probability/high consequence avalanche problem that is impossible to outsmart and will take a long time to heal. A big trigger like a snowmachine, more than one skier on the slope at the same time or a slab avalanche in the upper layers of the snowpack may be enough force to initiate a deep slab avalanche. Likely trigger spots will be in thinner areas of the snowpack that are connected to large, loaded slopes. Cautious route-finding is essential. This includes thinking about the remote trigger potential from below.

Weather

Tue, January 9th, 2018

Yesterday was mostly clear and sunny above the valley fog. Temperatures were in the low 20Fs at upper elevations and in the teens to single digits in the valley bottoms. Winds were mostly light and variable.

Today there will be patchy valley fog in the morning but that should dissipate as cold dry air pushes into the region. Winds will be northerly 5-15 mph with gusts into the 20s. Temperatures will be in the low 20s and high teens today, dropping into the low teens and single digits tonight.

Tomorrow will be partly cloudy with a chance of snow showers in the afternoon and temperatures in the high teens and low 20Fs. Winds will be from the SE 10-20mph. The weather looks to be unsettled into the weekend with snow showers and warming temperatures. Stay tuned!

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	24	0	0	43
Summit Lake (1400′)	6	0	0	15
Alyeska Mid (1700′)	18	0	0	36

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	22	variable	4	15
Seattle Ridge (2400′)	21	S	10	23

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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