Sat, January 04, 2020 Turnagain Pass Avalanche Forecast

Sat, January 4th, 2020

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.

Recent Avalanches

Three snowmachine triggered avalanches were reported yesterday in the Turnagain area. These all likely failed on the Solstice buried surface hoar.

Main Bowl, East aspect ~2400’. Snowmachine triggered Friday morning. Possibly remote triggered. Photo: Travis Smith

G. Predeger investigates the buried surface hoar in the crown of the snowmachine triggered avalanche in Main Bowl, Turnagain Pass. Photo: Chris Yelverton

Main Bowl, SE Aspect. ~2300’. Snowmachine triggered, ~300’ wide. Debris 10-15’ deep. Photo: G. Predeger

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

Three snowmachine triggered avalanches were reported yesterday in Main Bowl on the backside of Seattle Ridge, with two occuring on slopes with previous tracks. Our layer of concern remains buried surface hoar from Solstice, which sits within the top 3′ of snowpack in many areas throughout the region. These were all triggered on slopes at elevations above the New Years Day rain crust. Where this crust ends is variable across the region. On Seattle Ridge it was observed ending around 2300′ and in Summit Lake around 2500′. Where the crust doesn’t exist the snow immediately above the buried surface hoar has formed a dense slab under the fresh unconsolidated surface snow. This is a formula for more human triggered avalanches and this trend could continue into the future. Signs of instability are unlikely at this point and it might be the first or the tenth person that triggers the avalanche. Travel one at a time in on steeper slopes, remain outside of the runout when observing others, and choose terrain carefully. If the slope does slide will you end up in a terrain trap?

This persistent weak layer is buried under a dense slab and remains a concern throughout the region. Photo: R. Van Luit

Avalanche Problem 2

Dry Loose
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

Dry Loose

Dry Loose avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.

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

Dry unconsolidated storm snow and cold temps have us thinking about sluff management. Clearing skies Thursday allowed observers to spot a handful of loose snow avalanches in steep terrain, including on the front face of Seattle Ridge. Be mindful of consequences if you were to get knocked off your feet by a fast moving sluff in steep terrain.

Additional Concern

Glide Avalanches

Glide Avalanches

Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.

More info at Avalanche.org

Glide cracks: Glide cracks are the precursor to glide avalanches which are unpredictable and can release at anytime. When possible, limit exposure near and under glide cracks. It can be difficult to see glide cracks when approached from above, especially in low light conditions.

Glide cracks observed in the Library near Kickstep yesterday. Photo: Andy Moderow

Cornices: Cornices have been steadily building throughout the region. Avoid travel on cornices, and limit your exposure when beneath them. Cornices can obscure the true ridgeline and may fail far back from the edge.

Corniced ridge above Sunburst’s SW face, 1.2.19. photo: Paul Wunnicke

Weather

Sat, January 4th, 2020

Yesterday: Mostly to partly cloudy skies with no precipitation in the region. Temperatures were mostly single digits. Winds were easterly 1-3 with gusts into the 10’s. Overnight temperatures near 0°F.

Today: Partly cloudy skies with a chance of patchy freezing fog. Temperatures will likely range from 10°F to -6°F. Winds expected to be light and easterly. Overnight temperatures will be mostly near or below zero from sea level to ridgetops.

Tomorrow: Sunny, with a high near 2°F. Northwest wind 5 to 10 mph becoming light in the afternoon.

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

	Temp Avg (F)	Snow Depth (in)
Center Ridge (1880′)	7	40
Summit Lake (1400′)	7	14
Alyeska Mid (1700′)	7	36

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	4	W	5	12
Seattle Ridge (2400′)	2	VAR	2	7

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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