Sun, March 22, 2015 Turnagain Pass Avalanche Forecast

Sun, March 22nd, 2015

Alpine

Above 2,500'

2 - Moderate

Treeline

1,000'-2,500'

2 - Moderate

Below Treeline

Below 1,000'

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

Variability continues to be the theme of this weekend’s snowpack. Slabs 1-3 ft thick that formed over the past week rest on a variety of surfaces. In some places the slab is bonded well to what is below, while in others it sits on a layer of facets that still have the potential to propagate and avalanche.

Where do the pockets of facets exist? The complexity of the current situation is trying to figure this out. You could dig in one spot and 10 ft over have a completely different structure. Some areas avalanched during the Post St. Patty’s day storm and then were covered up by the next snow on Thursday hiding the evidence and adding to the tricky nature of this snowpack. The facets may have been blown away by winds before the storms higher on the slope yet part way down the run be present and reactive. Obvious clues may not be seen, however collapsing (whoomphing) does continue to be observed in certain areas and is a sure sign that facets exist in that location (photo below).

These conditions can be the type that allow multiple folks to travel down the slope before one hits the sour spot and triggers an avalanche. If you choose to go into avalanche terrain the potential of a slab/facet setup needs to considered and respected.

Use good travel practices: travel one at a time, have escape routes planned and avoid trigger points and terrain traps. Be cautious on slopes 35 degrees or steeper.

Avalanche Problem 2

Cornice

Cornice

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

More info at Avalanche.org

Cornices have grown significantly from the past week’s storms and are hazards in some areas.

Warming throughout the day can make these more likely to release naturally or under the weight of a traveler. Give them a wide berth while traveling below or next to and remember they often break farther back than you expect. The extra load of a falling cornice can trigger an avalanche if there is unstable snow where it lands.

Additional Concern

Wet Loose

Wet Loose

Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

More info at Avalanche.org

It’s springtime! By late in the day yesterday (~4-5pm) the snowpack below 2,000′ became wet and unsupportable while the upper elevations became damp; these surfaces have refrozen overnight. Warming should be expected again today and as the snow heats up, wet loose “push-a-lanches” will be possible on steep slopes. Although it’s more of an outlier, there is a chance for pulling out a wet slab in areas with poor snowpack structure basking in the sun.

Weather

Sun, March 22nd, 2015

It was very warm and mild yesterday as thin clouds filled the skies and lowered visibility to mostly “gray bird” conditions. Temperatures reached the low 40’s at 1,000′ and near 32F on the ridgetops. Winds were light and variable with no precipitation since Thursday.

Today, another warm day is in store with mostly sunny skies. Temperatures should reach the mid 30’sF on the ridgetops and low 40’s in the parking lots once again. Ridgetop winds will continue to be light and variable.

This mild spring-like weather should remain until Tuesday afternoon when a large North Pacific low develops and looks to usher in a warm, wet and windy storm.

“Greenhouse effect”? Yesterday we saw what avalanche practitioners call a greenhouse effect, or “greenhousing”. This is when there is a thin layer of clouds that let much of the solar radiation in but then also trap the longwave radiation, not allowing it to be released back into the atmosphere – essentially creating a blanket over the mountains which dampened the surface snow on all aspects. We may see this phenomenon again today if clouds filter in.

Today’s advisory is written with Aleph Johnston-Bloom, Executive Director of the Alaska Avlanche School.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	34	0	0	56
Summit Lake (1400′)	32	0	0	12
Alyeska Mid (1700′)	35	0	0	32

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	28	var	3	14
Seattle Ridge (2400′)	29	var	9	18

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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