Wed, December 16, 2015 Turnagain Pass Avalanche Forecast

Wed, December 16th, 2015

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

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

Yesterday snowfall was concentrated near Turnigain Arm favoring the Northern side Turnagain Pass and Portage Valley with a total of 3-5” of new snow. Moderate ridgetop winds flipped from the Northeast to a Westerly direction for several hours during the most intense burst of snow flurries. Today another small shot of snow is expected along with moderate winds from the Northeast. Loose snow avalanches and tender wind slabs up to 8-10” thick are possible on steep convex terrain features and just below ridgelines, on all aspects. This unstable snow could have high consequences if triggered on exposed terrain features like cliffs, gullies, and large open slopes.

A total of 2-3’ of new snow has fallen since last Wednesday and has left a mostly right side up snow pack. Many layers of new snow exist as a result of incremental loading and moderate winds in the upper elevations. Visibility is expected to be limited today, but if a window opens up, assess snow conditions carefully at higher elevations.

Snow flurries intensified yesterday in the early afternoon making visibility difficult at Sunburst.

Additional Concern

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 have been actively tracking several layers of weak snow buried under 2-3’ of new snow over the past week. Data collected over the last 3 days (including heavy impact from motorized slope testers) is pointing towards good stability. Right now this problem is becoming an outlier, but we will continue to monitor these layers with any additional loading or stress to the snowpack.

Weather

Wed, December 16th, 2015

Yesterday snow showers intensified for several hours as a cold front left 3-5 € of snow in the upper elevations of Turnagain Pass and Portage Valley. Ridgetop winds were moderate from the Northeast, switching to a Westerly pattern in the afternoon through late evening. Temps were cool, high teens F in the alpine, and high 20F’s at sea level. Overnight winds were light from the Northeast.

Today expect more showery conditions with another 3-5 € of new snow in upper elevations. Easterly ridgetop winds will increase to 15-30mph this morning and remain moderate into the evening.

Tomorrow more snow showers are anticipated. Temps are expected to increase to freezing 32F at sea level causing mixed rain and snow. Winds are expected to be light to moderate from the East.

PRECIPITATION 24-hour data (6am – 6am)

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	24F	3	0.3	39
Summit Lake (1400′)	19F	1	0.1	15
Alyeska Mid (1700′)	25F	2	0.2	33

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	17F	Var.	12	54
Seattle Ridge (2400′)	19F	n/a	n/a	n/a

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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