Wed, January 02, 2013 Turnagain Pass Avalanche Forecast

Wed, January 2nd, 2013

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

3 - Considerable

Below Treeline

Below 1,000'

3 - Considerable

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

Deep Persistent Slabs

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.

More info at Avalanche.org

This deep slab problem has already shown itself to be very active during the storm cycle. Now that the precip and wind has lessened, the likelihood of natural avalanche activity will decrease but human triggered avalanches will remain a serious problem. The longer we wait to jump onto steeper terrain, the less likely we will be to trigger a deep avalanche. As recently as yesterday we saw large avalanches triggered by the Department of Transportation. Since that time another 10 inches of snow has fallen in some areas. Some thoughts on the deep slab problem –

– whumpfing and shooting cracks may not be evident

– standard snowpit tests are not suitable for testing deeply buried weak layers

– a lack of observed avalanche activity is not a reliable indicator of potential hazard.

– Waiting 36-48 hours after significant weather changes will decrease the likelihood of triggering (but not eliminate it)

– In some places we have seen the same slope avalanche twice during the recent storm cycle

– Terrain management is the only reliable prevention tool when snow stability is a problem

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 slab may be a problem above treeline today. We haven’t been able to see avalanche activity within the new storm layers because we haven’t been able to access higher elevation areas in the past 8 days. Recent wind and forecasted wind today may be loading up lee oriented slopes.

Weather

Wed, January 2nd, 2013

It’s been a wet and wild week, and it isn’t finished yet. The graphs below show Turnagain Pass snow depth and wind from the recent weather events.

snow graph

wind graph

Today’s weather looks to be mild by comparison. Snow showers are expected today, with 2-3 inches of accumulation. Rain line will be 500 feet. Southeast wind from 30-40mph during daylight hours. Increased precip and wind is expected again tonight. The overall weather pattern is expected to remain active, although not as intense for the remainder of the week.

John will issue the next advisory on Thursday, December 3rd.

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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