Sun, March 16, 2014 Turnagain Pass Avalanche Forecast

Sun, March 16th, 2014

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

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

Yesterday was a classic sunny Saturday after a storm cycle and many folks were out enjoying Winter Part III. Along with this were reports of a handful of large human triggered avalanches. Luckily, no one was reported caught as they were 3-5’+ deep, large and dangerous. Details are limited, but here is what we know:

– Widow Maker Face (West facing, 2,500′ – Seattle Creek drainage). This avalanche was 5-6′ deep and likely remotely triggered from the ridgetop by a group of riders. More details HERE.

– Four deep pockets in Tincan’s Hippy Bowl (Southwest facing, 3,500/3,000). These slides are suspected to have been remotely triggered from an adjacent sub-ridge. More details HERE.

– Palmer Creek (South of Hope). At least one large snowmachine triggered avalanche was reported in this drainage – very little information at this time.

Photo below: Widow Maker avalanche – photo was taken by a person looking North on Seattle Ridge.

The avalanche activity noted above is suspected to be failing at the old snow/new snow interface. The new snow consists of the combined 3/10-3/14 storm snow that has settled into a very strong slab around 30″ to 4′ thick (thicker in wind loaded areas). The weak layer is suspected to be a thin 1-2cm layer of old weak snow and buried surface hoar sitting above the late February crust.

Lowering likelihood – high consequences:
The slab is thick enough that triggering the weak layer is difficult and the weak layer is showing signs of adjusting to the new load. However, this set up deserves our respect regardless. The sole reason is the sheer size and destructive power of a potential slide. If you haven’t noticed from the reports of yesterday’s activity, these are deep and dangerous avalanches – the kind that if you are caught up in, are likely not getting out of. Conservative terrain management is key once again for a safe day in the backcountry.

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

Ridgetop winds are forecast to pick up to ~20mph from the East by noon today and continue to increase into the 40-50mph range by tonight. Although only around an inch of new snow is expected this afternoon, there is upwards of 4-10″ of loose snow already on the surface available for the wind to transport. With these factors in mind, watch for winds to form slabs near ridgelines, on rollovers or by cross-loading in gullies. These slabs will likely be in the foot deep category and sensitive to human triggers.

Additionally, any wind loading and/or wind slabs triggered could stress buried weak layers and aid in triggering a deep slab.

Weather

Sun, March 16th, 2014

Yesterday was our first clear sky day since last Sunday; between Monday and Friday two storms rolled through bringing 4-5′ of dense snow to the Eastern Turnagain Arm mountains. During the past 24-hours temperatures have averaged in the teens from 1,000′ to 4,000′ and ridgetop winds have been light, 5-10mph, out of the East. Overnight, temperatures have dropped to the single digits above 1,000′ and clouds have begun to stream in.

Today, cloud cover and light snow showers will increase through the day due to a front sliding through from the West. Around an inch of snow is expected by this afternoon with another 3-5″ of low density snow possible overnight. Temperatures will remain plenty cold for snow to sea level – teens at 1,000′. Winds from the East are slated to pick up this afternoon and by midnight blow in the 50mph range.

Tuesday, light snowfall is expected to continue with up to another 2-4″ accumulating. Winds look to remain strong from the East and temperatures cold enough for snow to sea level.

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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