Sun, December 09, 2012 Turnagain Pass Avalanche Forecast

Sun, December 9th, 2012

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

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

High winds yesterday afternoon through midnight, sustained around 40mph with gusts over 70mph, raked the mountains, scouring windward slopes and depositing a combination of new snow and existing snow onto leeward slopes and catchment zones. Even the mid and lower elevation snow saw substantial wind transport. There was likely a peak in natural wind slab acivity and HIGH danger at the tail end of these strong winds overnight.

Wind slabs that were just starting to be formed yesterday were 4-10” thick, very sensitive and could be cracked out and released with the slightest touch. Because these slabs have formed on very weak sugary snow they are not likely to bond anytime soon.

Steering clear of ANY wind deposited snow on slopes over 30 degrees will be prudent. These areas may not be obvious as 1-3” of new snow forecast with little wind today could mask these monsters. Remotely triggering a slope (i.e., from below or a flat area) is possible. Collapsing and cracking should be widespread – if you are on a slope steep enough it will probably avalanche.

Avalanche Problem 2

Storm Slabs

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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

Though the forecast for 2 feet of snow by this morning was a bust and yesterday evening’s quick and dirty onslaught was finished around midnight, the 8-12” of new dense snow above treeline is highly unstable. Not only did the storm come in cold and leave warm (creating an unstable upside down storm layer), it fell on extremely weak faceted snow – ball bearings – that will have little chance of allowing the new snow to stick to the sides of the mountains. Hence, areas lacking a wind slab will still be likely to avalanche with just the new layer of storm snow.

Weather

Sun, December 9th, 2012

Snowfall numbers (storm total as of 6am 12/9):
Turnagain Pass SNOTEL 1880′: 9 € snow, .7 € water
Summit Lake SNOTEL 1400′: 3 € snow, .3 € water
Alyeska mid mtn: 10 € snow, .8 € water (1.2 € water at 2800′)

The leading edge of the first “real” storm system for over a month moved through quickly yesterday evening with strong easterly winds (sustained 40mph, gusting 70’s) and heavy snowfall in Eastern Turnagain Arm. As the storm weakens today there is a chance for another 1-3 € as well as a few more inches for Monday. The easterly winds have decreased substantially since midnight and should be in the 10-20mph range on the ridgetops today. Temperatures around treeline increased from ~20F yesterday to the upper 20’s today where they should remain. There is a light rain/snow mix this morning at sea level along Turnagain arm with slippery roads.

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).

Kevin will issue the next advisory Monday morning, December 10th.

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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