Tue, November 20, 2012 Turnagain Pass Avalanche Forecast

Tue, November 20th, 2012

Alpine

Above 2,500'

3 - Considerable

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.

Avalanche Problem 1

Persistent Slabs

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.

More info at Avalanche.org

Rotten snow can be found nearly everywhere right now, and in some cases those weak facets will collapse when stressed. The recipe that caused the recent skier triggered avalanches is relatively simple to understand. All the October snow sat around during cold temperatures and turned to sugary facets before it got buried by subsequent storms. Shallow, weak snow is easily triggered by the weight of a person. If you are interested in investigating snow pits, now is a great time to look at textbook instability. Which makes it a bad time to explore farther into the backcountry.

On our backcountry patrols we’ve been quite conservative in our route and slope choices. Skiing “lightly” to avoid hitting submerged rocks, and avoiding the steeper slopes is a good idea until our situation changes. Examples of recent avalanches can be found HERE and HERE.

Sunburst crown detail

Besides the bulls-eye information of skier triggered avalanches, our pit tests are showing a collapsible weak layer and a tendency to propagate. Most stability tests are showing low to moderate strength, and will propagate across an extended column when the underlying weak layer collapses. This is very pertinent information! When a professional guide sees evidence like this, he/she will very deliberately alter travel plans to stay on conservative lower angle terrain.

The nature of avalanches breaking to the ground in our shallow snow situation is another good reason to stay conservative. Taking a ride through the rocks increases the chance of major injury.

Lower elevation slopes below treeline are more stable than up high due to a slightly better snowpack structure. Shallow snow cover in the trees will make travel challenging however, until we get more snow.

Weather

Tue, November 20th, 2012

It’s been mostly clear and cold over the last couple days. Minimal precipitation has not contributed much to the avalanche equation. The jet stream pattern is bringing colder arctic air across Alaska and currently diverting most of the Pacific moisture to Washington and British Columbia. Light wind and a strong temperature inversion is happening this morning between Girdwood and Summit. The outlook for a pattern change doesn’t look good until sometime next week. Don’t expect much precipitation in the next few days. The cold temperatures and shallow snow are forming a strong temperature gradient that continues to weaken the snow we already have on the ground…

This is a general backcountry avalanche advisory issued for Turnagain Arm with Turnagain Pass as the core advisory area (this advisory does not apply to highways, railroads, or operating ski areas).

I will issue the next advisory on Thanksgiving morning, Thursday November 22nd.

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
04/16/24	Turnagain	Observation: Cornbiscuit	Krueger / Matthys Forecaster
04/15/24	Turnagain	Observation: Taylor Pass	John Sykes Forecaster
04/15/24	Turnagain	Observation: Seward Highway – Tern Lake to Portage	Joel Curtis
04/14/24	Turnagain	Observation: Sunburst	T. Olsen-Lund, R. Sullivan I. Adams
04/14/24	Turnagain	Observation: Turnagain Pass Snomo	K. Gallaher
04/13/24	Turnagain	Observation: Spokane Creek	Everet Megli
04/11/24	Turnagain	Observation: Tincan	John Sykes Forecaster
04/10/24	Turnagain	Avalanche: Seattle Ridge	Schauer/ Keeler Forecaster
04/10/24	Turnagain	Observation: Cornbiscuit South Face	Todd Kelsey
04/10/24	Turnagain	Avalanche: Tincan	N. Dumont J. Carlberg

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