Sat, December 15, 2012 Turnagain Pass Avalanche Forecast

Sat, December 15th, 2012

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.

Avalanche Problem 1

Wind Slabs

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.

More info at Avalanche.org

Recent wind slab from Thursday is the primary concern. The worst areas will have a stiff layer of surface snow on top of very weak facets. This means above treeline in wind loaded zones including east facing slopes or pockets of wind slab in specific cross loaded terrain. The most obvious examples we have of this are the aspects easily visible from the highway of Seattle ridge in Turnagain Pass and Fresno ridge near Summit Lake. Both of these areas had large natural avalanches caused by wind loading. As time goes on, the likelihood of triggering similar slopes is diminishing, but the rate of stabilizing is slow due to the persistent nature of our weak layers.

There is no doubt in my mind that avalanches can be triggered today in the right terrain. Watch out for slopes steeper than 35 degrees and signs of recent wind loading. Avalanche size and destructive potential will be directly correlated to the size and consequences of the terrain.

Avalanche Problem 2

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

The only change in the persistant weak layers in the last week is they are now buried by 1-2 feet of new snow. Which means that we now have heavier snow sitting on the extremely weak snow from the early season. When you add the wind loading component to the mix, it becomes a dangerous recipe. Yesterday we found frequent whumphing (collapsing) of the weak layers, telling us that they are tenuously carrying the stress of recent snowfall. That collapse would be the initiation of an avalanche if the slope was steep enough…

It’s good to remember that we don’t often deal with weak layers on the scale we currently see them. Our typical strong maritime snowpack has been replaced by a weak and shallow continental snowpack you would more often find in Colorado. This means that problems don’t diminish for days or weeks following a storm. The only way to ensure safe mountain travel when you find abnormally dangerous conditions is to stay in mellower terrain. We can’t go the same places we could in our normal stable snowpack and expect a positive outcome.

Check out this video from our colleagues in Utah for some good theory on terrain management.

Weather

Sat, December 15th, 2012

Mostly sunny this weekend with colder temperatures. We may see some stiff northwest wind in the mountains today. The next chance of snowfall is still a few days away.

The temperature inversion that gave us comfortable temperatures above treeline yesterday seems to be breaking down this morning. The coldest areas are showing lower single digit temperatures.

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).

Wendy will issue the next advisory Sunday morning, December 16th.

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
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
04/16/24	Turnagain	Observation: Cornbiscuit	Krueger / Matthys Forecaster

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