Wed, December 05, 2018 Turnagain Pass Avalanche Forecast

Wed, December 5th, 2018

Alpine

Above 2,500'

2 - Moderate

Treeline

1,000'-2,500'

1 - Low

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
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

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

Sustained moderate winds from the east have been blowing for four days and will remain, to varying degrees, over the region again today. These winds have redistributed the snow and formed wind slabs and not-so-fun crusts is many areas; see Monday’s field report from Turnagain HERE. Wind slabs could be found in steep terrain and could be hard enough, and stubborn enough, to allow a person on to them before popping loose. Look for stiff, pillowed snow, cracking and listen for hollow, drum-like sounds.

Wind effect along Sunburst Ridge looking north toward Tincan Ridge.

Avalanche Problem 2

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

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

At the upper elevations we are keeping tabs on a thin layer of buried surface hoar sitting 1-3′ below the surface. An observer Sunday found this layer to still be reactive in a snowpit on Sunburst right around 2500′. Monday at 3100′ on Sunburst there were no results testing this layer but it was very easy to spot laid over in the snowpack. Side note, this layer was unreactive last week at 3,150′ on Tincan as well. The concern is finding a slope with buried surface hoar that is still intact, upright and reactive enough that it propagates into an avalanche. At this point obvious signs of instability may not be seen but some lingering suspicion is advised even as the likelihood decreases. As always use safe travel protocol and choose terrain with consequences in mind. For example, where is the avalanche path and where would I end up if the slope slides?

Pastoral Peak, looking east from Sunburst through Taylor Pass. Note the crown of a large avalanche under the rock band on Pastoral – earthquake triggered slab on Nov 30th. This avalanche likely released on the Nov. 23 buried surface hoar.

Surface conditions below 1,500′ at Turnagain Pass.

Weather

Wed, December 5th, 2018

Yesterday: Mostly cloudy to obscured skies were over the area. Ridgetop winds over the past 24 hours have been moderate (10-20mph) from the east with gusts up to 40mph. Temperatures have climbed overnight and sit in the mid 30’sF at 1,000′ and the upper 20’sF along ridgetops this morning.

Today: Partly cloudy to cloudy skies are expected with a chance for a few raindrops below 1,500′ and snowflakes above 1,500′. No measureable precip is forecast. Ridgetops winds will remain easterly in the 15-20mph range with gusts to the 40’s. Temperatures should remain warm, with daytime highs up to 38F at 1,000′ and 30F along ridgetops.

Tomorrow: Unsettled warm weather is forecast to continue with rain and snow showers picking up Thursday afternoon and intensifying on Friday. Our friends at the NWS said this about the upcoming system “The track of the storm favors western Prince William Sound/eastern Kenai Peninsula for some of the heaviest precipitation with strong east to southeast upslope flow. Warm air accompanying the low means most of this will be in the form of rain”. The rain/snow like for Friday looks to push into the 2,000′ plus range.

*Seattle Ridge wind sensor is rimed over. Alyeska Mid Wx Station and Summit Lake Snotel snow depth sensor are not functioning.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	33	1	0.1	13
Summit Lake (1400′)	29	0	0	5 (estimate)
Alyeska Mid (1700′)	32	trace	0.23	0.2

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	25	NE	15	42
Seattle Ridge (2400′)	28	*no data	*no data	*no data

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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