Thu, November 22, 2012 Turnagain Pass Avalanche Forecast

Thu, November 22nd, 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

We found a big piece of the puzzle yesterday in the form of recent natural avalanches. The presence of naturals underscores the seriousness of the current snowpack. Sugary faceted snow crystals form the foundation of our snowpack everywhere right now. They are showing a tendency to collapse when stressed and will avalanche on steeper slopes.

Turnagain Pass got a small amount of snow on Sunday, which probably added enough stress to cause the avalanche in the picture below. This is a steep north facing slope on Seattle ridge, and is representative of most of the region. Similar problems have been found near Portage, Summit, and Eagle River.

Natural avalanche in Warmup Bowl, north face.

The cold weather combined with such a shallow snowpack is transforming all the snow into weaker faceted crystals. We are finding a strong temperature gradient everywhere which is made worse by sub-zero temperatures. In most places you won’t find much of a stiffer slab on top of the weaker snow. This lack of a slab is preventing a lot of slopes from avalanching for the time being. When we do get stiffer snow and more stress on top of the delicate foundation, avalanches will be much more likely and dangerous.

Avalanche Problem 2

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

Wind overnight hit gusts to 30mph at ridge tops. This is the strongest wind we’ve had in over a week. If the wind was strong enough to transport the loose surface snow, we could have enough slab to cause problems. Any stronger or denser snow on top of the super weak facets near the ground is likely to exacerbate our problems.

Weather

Thu, November 22nd, 2012

Wind overnight is the big news since yesterday. The last snowfall is now several days in the past, but was enough to cause some snow stability problems in the form of natural avalanches. The storm hitting Southeast Alaska today is not expected to have a significant effect on Southcentral. A slight chance of snow showers is possible today. Wind is expected to increase in some areas this afternoon.

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
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
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

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