Sat, December 23, 2017 Turnagain Pass Avalanche Forecast

Sat, December 23rd, 2017

Alpine

Above 2,500'

3 - Considerable

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

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

Large plumes of blowing snow were present yesterday along all the tallest peaks around our region and strong Easterly winds are expected to continue today. Wind loaded snow could be on all aspects due to top loading and cross loading. These slabs will be hard and supportable and have the classic smooth pillow shaped look. Wind slabs could easily be located further down slope than expected and have the potential to fracture above you. Triggering even a small wind slab could have very high consequences especially in the upper elevations where there is also a potential for initiating a much larger and more dangerous deep slab avalanche. More on this below. Should you go into the upper elevations maintain a conservative mind-set and avoid big objective slopes including their runout zones.

Active wind loading as seen yestereday on Big Chief in the Seattle Creek area.

Variable surfaces are preseent throughout the alpine. Note the crossloading pattern including the pillowed shaped snow loaded along gullies on the SE aspect of Seattle Ridge

Avalanche Problem 2

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

On December 20th, two skiers were approaching Pastoral and remotely triggered a very large avalanche from below. Luckily they were able to run from its path and avoid being caught. This is a very scary set-up that is likely not going away soon. Deep slabs are located in the upper elevations of our forecast zone, above 3000’, on slopes that did not avalanche during the early December storm cycle. The snowpack ranges from 3-5+’ thick and is sitting on weak basal facets. Observations over the last few weeks indicate this poor structure set-up is widespread across our region in the alpine elevations. This is a high consequence avalanche problem that is impossible to outsmart. Keep in mind:

Thinner areas of the snowpack (1-2’ thick) are likely trigger spots as well as scoured areas near rocks
Due to strong winds over the last month the snow depths are highly variable and there may be more trigger spots than we realize
Thicker areas (3-5+’ thick) will be difficult trigger and several tracks may be on a slope before someone finds a trigger point
It is possible to trigger this avalanche from below and it could run further than expected
Large snow covered slopes that do not have piles of old debris under them are all suspect

Photo taken on Dec.21, the day after the Pastoral avalanche. Approximate route of where the party triggered this avalanche and retreated out of its way.

Photo taken on Dec.21st when strong winds started impacting our region. Note theNorthern Chutes on lookers left side have not avalanche yet and are actively loading snow.

For more info on the snowpack from this incident click HERE and more details about this avalanche click HERE.

Weather

Sat, December 23rd, 2017

Yesterday skis were partly cloudy with strong Easterly winds averaging in the mid 20’s with gusts in the 60’s. Warm air was observed at lower elevations mid to low 30F’s and temps in the mid 20F’s along ridgetops. No precipitation was recorded.

Today expect skies to be mostly sunny with Easterly winds elevated averaging in the 20’s with gusts in the 40’s. Day time temps at 1000′ are expected to be in the mid to low 30F’s and dip down to mid 20F’s overnight. High clouds are expected to move into our region tonight with a few flakes possible overnight.

Scattered snow showers are possible tomorrow, but little to no accumulation is expected. Temperatures are expected to range from 20F-30F. Winds should decrease tomorrow morning and remain Light from the East.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	31	0	0	31
Summit Lake (1400′)	25	0	0	11
Alyeska Mid (1700′)	32	0	0	26

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	24	ENE	27	69
Seattle Ridge (2400′)	28	ESE	20	51

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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