Fri, February 16, 2018 Turnagain Pass Avalanche Forecast

Fri, February 16th, 2018

Alpine

Above 2,500'

2 - Moderate

Treeline

1,000'-2,500'

2 - Moderate

Below Treeline

Below 1,000'

1 - Low

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
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

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.

Likelihood of Avalanches

Terms such as "unlikely", "likely", and "certain" are used to define the scale, with the chance of triggering or observing avalanches increasing as we move up the scale. For our purposes, "Unlikely" means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. "Certain" means that humans will be able to trigger avalanches on many slopes, and natural avalanches are expected.

Size of Avalanches

Avalanche size is defined by the largest potential avalanche, or expected range of sizes related to the problem in question. Assigned size is a qualitative estimate based on the destructive classification system and requires specialists to estimate the harm avalanches may cause to hypothetical objects located in the avalanche track (AAA 2016, CAA 2014). Under this schema, "Small" avalanches are not large enough to bury humans and are relatively harmless unless they carry people over cliffs or through trees or rocks. Moving up the scale, avalanches become "Large" enough to bury, injure, or kill people. "Very Large" avalanches may bury or destroy vehicles or houses, and "Historic" avalanches are massive events capable of altering the landscape.

Signal Word	Size (D scale)	Simple Descriptor
Small	1	Unlikely to bury a person
Large	2	Can bury a person
Very Large	3	Can destroy a house
Historic	4 & 5	Can destroy part or all of a village

More info at Avalanche.org

Triggering a persistent slab avalanche 2′ deep remains possible across the region, and winds today may add additional stress. Several weak layers including widespread buried surface hoar (from Jan. 21) sits roughly 1-2′ below the surface. A person skiing or on a snowmachine may tip the balance and initiate a larger slab avalanche. Triggering a smaller wind slab or a cornice may also activate this layer. The snowpack is now at a point where no signs of instability are likely to be present before one of these avalanches is triggered. Assessing the terrain and the potential outcome of an avalanche breaking deeper in the pack is key. Are there terrain traps below you? Cliffs? Are your partners watching and rescue ready?

Winds this weak have loaded some slopes and scoured others as seen yesterday on Seattle Ridge. Triggering a soft or hard persistent slab is possible with our current snowpack.

Avalanche Problem 2

Wind Slabs
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

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.

Likelihood of Avalanches

Size of Avalanches

Signal Word	Size (D scale)	Simple Descriptor
Small	1	Unlikely to bury a person
Large	2	Can bury a person
Very Large	3	Can destroy a house
Historic	4 & 5	Can destroy part or all of a village

More info at Avalanche.org

Triggering an isolated wind slab is possible today and will be more likely in places where wind is actively transporting snow into steep terrain. Today Northwest winds may be moving loose snow along some ridgetops and forming new wind slabs on leeward features. Earlier in weak ridgetop winds loaded some slopes and scoured ridge lines. Pay attention to where the snow feels stiff, looks pillowed, sounds or feels hollow and watch for shooting cracks. Hard wind slabs tend to break when you are out onto the slope and often fracture above you. Be aware of wind loading patterns in the terrain, especially cross-loaded gullies. Sometimes Northwest winds can funnel in from the South near Sunburst and on the Southern end of Turnagain Pass.

Small wind slab triggered in the last few days on a wind loaded gully, SE aspect of Seattle Ridge at about 2300′.

Additional Concern

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

Deep Persistent Slab: Above 3,000′ in the Alpine zones, several old weak layers of facets and buried surface hoar sit near the ground and in the mid-pack. This structure is most pronounced in areas with a thin overall snow cover, such as the South end of Turnagain Pass and the Summit Lake area. As you plan your day, keep in mind that there are deeper persistent layers that could ‘wake up’ if you find the wrong spot.

Weather

Fri, February 16th, 2018

Yesterday was clear and sunny with no precipitation. Westerly winds became Easterly 5-15mph early afternoon. Temperatures along ridgetops were in the high teens (F) increasing to the low 20F’s during the heat of the day. Temps at sea level bumped into the high 20F’s and back into the teens F’s overnight. Patches of valley fog were present near Turnagain Arm.

Mostly cloudy skies are forecasted for today. Temperatures should average in the 20F’s and dip down into the teens (F) overnight. Ridge top winds will start out light from the East, but will transition to the Northwest and increase to 15-20mph. No precipitation is expected.

Clear skies are in the forecast for Saturday wth similar temperatures. Moderate Northwest winds will continue tomorrow, becoming light by late afternoon. Increasing clouds and warmer temperatures are expected Sunday evening into Monday with a possibility of a few scattered showers.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	26	0	0	64
Summit Lake (1400′)	17	0	0	24
Alyeska Mid (1700′)	23	0	0	58

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	18	W–E	7	22
Seattle Ridge (2400′)	21	W–E	10	25

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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