Sat, February 10, 2018 Turnagain Pass Avalanche Forecast

Sat, February 10th, 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. Dangerous avalanche conditions.	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

Moderate to Strong Easterly ridgetop winds have been transporting snow and adding stress to the snowpack. Overnight Sunburst weather station recorded winds in 15-30mph range and Seattle Ridge was stronger, 20-40mph. Isolated pockets of newly formed wind slab may be easy to trigger, but more concerning will be drifting snow adding stress to a persistent slab and making the snow more cohesive. Several weak layers are buried within the top 1-2’ of the snowpack, including buried surface hoar and near surface facets. With our current snowpack structure these two avalanche problems (wind slab and persistent slab) are closely related. Basically they are both strong snow over weaker snow below. Slabs may range from small to large, soft or hard, and have the potential to break above you once committed to a steep slope. Identify smooth pillowed surfaces and evaluate the consequences of the terrain before committing to a slope.

Easterly winds yesterday loading West facing slopes of Pyramid Peak. Winds can also cross load gullies and terrain features on all aspects.

January 21 buried surface hoar sits below 1-2′ of snow and is widespread across our region. This is one of several persistent weak layers including near surface facets that may be lurking below wind drifted snow.

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

Weak snow can still be found near the ground and within deeper layers of the snowpack, above 3,000′. Although triggering a Deep Persistent Slab is less likely, it is worth keeping in mind that poor structure does exist in places with a generally thinner snowpack. Last Saturday (Feb.3) a large human triggered avalanche occurred on the East face of Twin Peaks and failed on weak snow (buried surface hoar and facets) sitting on a hard bed surface. This was a high consequence, hard to trigger avalanche where the 9th and10th skiers on a skin track found just the right trigger spot, a very shallow part of the snowpack. Luckily no one was caught or injured in this avalanche. A conservative approach would be to avoid large steep terrain with shallow snow cover or exposed rock. Wind scouring from overnight may be creating more trigger spots in thin snow covered areas.

The Twin Peaks avalanche was triggered at the apex of the crown in a shallow spot only 20″ deep.

Weather

Sat, February 10th, 2018

Yesterday skis were clear with high clouds rolling in later in the evening. Ridge top winds at Seattle ridge were stronger than the rest of the area, but overnight winds increased at Sunburst weather station to 15-30mph. Seattle Ridge recorded several hours overnight with winds in the 30-40’s mph range. An inversion kept some valley bottoms in the single digits and the alpine averaged in the low 20F’s. Radiation from the sun is starting to affect daily temperatures swings. No precip was recorded.

Expect overcast skies today and Easterly ridge top winds in the 10-35mph range. Scattered snow showers are possible, but not much accumulation is expected. Temperatures may rise to the low 30F’s at sea level and mid to high 20F’s at higher elevations.

Snow showers are possible Sunday, with a few inches of new snow forecasted. Another storm is tracking towards Southcentral, Alaska for Monday bringing warm temperatures and a possibility of more precipitation. At this point precipitation amounts are type (rain/snow) are uncertain.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	25	0	0	62
Summit Lake (1400′)	11	0	0	20
Alyeska Mid (1700′)	25	0	0	51

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	21	ENE	12	40
Seattle Ridge (2400′)	23	SE	22	49

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
11/16/23	Turnagain	Observation: Tincan Common	William Finley
11/14/23	Turnagain	Observation: Sunburst/Magnum	Adrian Beebee
11/14/23	Turnagain	Observation: Seattle Ridge	Schauer/ Creighton/ Ellis Forecaster
11/13/23	Turnagain	Avalanche: Tincan Trees	Eeva Latosuo
11/12/23	Turnagain	Observation: Sunburst	Schauer/ Creighton/ Nave Forecaster
11/12/23	Turnagain	Avalanche: Goldpan – avalanche	Mike Records
11/11/23	Turnagain	Observation: Tincan Common	Andy Moderow
11/11/23	Turnagain	Observation: Taylor Pass – Sunburst	Wadsworth / Stwertka
11/10/23	Turnagain	Observation: Tincan	Andrew Schauer Forecaster
11/10/23	Turnagain	Observation: Tincan Trees	Michael Kerst

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