Thu, December 03, 2020 Turnagain Pass Avalanche Forecast

Thu, December 3rd, 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

Paying attention to wind effect, from the strong winds during this last storm, is important if you are heading out into terrain above 2500′. In this elevation band, where the storm precipitation stayed all snow, look for cracking, collapsing and slopes that have stiff snow over soft snow. These are all signs of wind slab potential. The farther we get from the loading event these slabs may be more stubborn. Steep, unsupported slopes in leeward terrain and cross-loaded gullies will be the most suspect. Even a small wind slab in steep terrain can take you for a bad ride on skis or knock you off your machine in the wrong spot. In addition, give cornices a wide berth. These have gotten bigger and can break much farther back than expected. As you venture farther afield as skies clear it is prudent to ease into terrain, use safe travel protocol and look for signs of instability.

During this last warm, windy, wet storm the rain/snow line went as high as 2500′. Observers noted wet snow, rain runnels, saturated layers in the snowpack and surface crusts forming yesterday. As temperatures cool these crusts will become more stout and snowpack should continue to lockup, especially in the lower elevations. There are a couple of inches of new snow that fell yesterday on top of the crusts. As skies clear expect this snow to facet and potentially become our next weak layer. Stay tuned…

Stiffer layers of snow along the Tincan ridgeline. 12.2.20

Rain crust at 1000′ on Turnagain Pass. 12.2.20. This crust was found as high as 2500′.

Wet, saturated layers in the upper snowpack at 2200′ on Tincan. 12.2.20.

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

As visibility improves, folks will undoubtably get into more terrain. Keep in mind that in the Alpine (above 2500′), where the snow stayed dry, there is still a chance of triggering a slab that fails 4-6+ feet deep on the buried weak snow (that formed in October) at the bottom of the snowpack. The likelihood has significantly decreased but the concern is lingering, especially on the southern end of the pass and in the Summit Lake area where the overall snowpack is shallower. The consequences of hitting the wrong spot are not to be taken lightly. As always it is important to use good travel protocol. Expose one person at a time, avoid terrain traps and look at slopes as avalanche paths. If the slope does slide where would you end up?

Things to keep in mind:

The snowpack can feel ‘stable’ and no signs of instability may be present before a deep slab releases.
The likelihood of triggering a large slab increases if you find shallower spots in the snowpack and near rocky areas.
Slopes may already have tracks on them and then still release.
These slabs can be triggered remotely, from the bottom, top or side.

Weather

Thu, December 3rd, 2020

Yesterday: Skies were mostly overcast with light rain/snow showers in the morning becoming all snow showers in the afternoon. Temperatures started out with 40°Fs at sea level and hight 20°Fs in the Alpine and dropped to 20°Fs at sea level and teens/single digits in the Alpine overnight. Winds were light and variable. Skies stayed cloudy overnight.

Today: There is a chance of snow showers in the morning becoming partly sunny in the afternoon. Temperatures will be in the teens to low 20°Fs. Winds will be mostly calm. Overnight skies will be partly cloudy and temperatures will be in the single digits and low teens. Wind remain calm.

Tomorrow: Skies are forecast to be be mostly sunny with temperatures in the teens to low 20°Fs and light north winds. The clear skies look to continue into Saturday with the next snow producing storm system moving in Saturday night.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	29	3	0.4	60
Summit Lake (1400′)	24	1	0.1	26
Alyeska Mid (1700′)	25	2.5	0.2	58

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	20	NE	7	41
Seattle Ridge (2400′)	23	E	3	22

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
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
04/29/24	Turnagain	Avalanche: Turnagain aerial obs	Tully Hamer
04/27/24	Turnagain	Observation: Johnson Pass	Noah Mery
04/23/24	Turnagain	Observation: Turnagain Sunny Side	Travis SMITH
04/21/24	Turnagain	Observation: Bertha Creek	Anonymous
04/20/24	Turnagain	Avalanche: Spokane Creek	Schauer/ Mailly Forecaster

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