Thu, December 31, 2020 Turnagain Pass Avalanche Forecast

Thu, December 31st, 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

A group of skiers skinning along the Cornbiscuit ridgline remote triggered a small avalanche on the north side. From the observation, ‘Debris, though, shallow, ran most the way to the valley floor.’

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

As skies clear and venturing into more terrain becomes easier, it is important to remember that yesterday the winds continued to move snow around. Observers reported gusty ridgetop winds and blowing snow flagging at times. The wind slab avalanche on Cornbiscuit was touchy enough that it was triggered remotely from above on the ridge. The winds eased off overnight. Today wind slabs remain the primary concern. These wind slabs could be 1-2′ deep and may be a bit more stubborn to trigger. These will found in wind exposed terrain on steep unsupported slopes and cross-loaded gullies. You may be able to travel out onto the slab before it breaks above you as is often the nature of hard wind slabs. Looking for signs of wind effect and wind slab habitat will be key to safe travel today.

Classic wind slab signs:

Rounded and pillow-like surfaces (wind deposited snow)
Hollow feeling or drum-like feeling on the snow due to stiff snow over softer snow
Cracks in the snow that shoot out from your machine, skis or board
Any collapsing in the snow or ‘whumpfing’

In addition, there are couple of ‘Normal Caution’ avalanche issues to pay attention to today. On steep slopes out of the winds where loose surface snow is found, keep an eye on your sluff. And, as always, give cornices a wide berth and limit time under them.

Wind effect along the Tenderfoot ridgeline, 12.30.20

Scoured ridges and cross-loaded gullies on Magnum north side, 12.30.20. Photo: Emily Sullivan

Avalanche Problem 2

Deep Persistent Slabs
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

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.

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

A month ago it rained up to around 2500′ and then snow fell on top. This formed a crust that is now buried 3-5′ below the surface. The snow around the crust is showing signs of forming a reactive weak layer (facets) in some parts of the forecast area, but not all which is tricky. Triggering an avalanche on this layer is unlikely but could have bad consequences due to the depth of the layer and the connectivity of crust across terrain. Unfortunately signs of instability will not likely be present with this type of avalanche problem and stability tests could give unreliable data. Steep mid-elevation slopes (1,300′ – 2,500′) should be considered suspect and there may be multiple tracks on the slope before it fails. We are tracking the evolution of the snowpack structure around the crust and are concerned about the developing weak layer and deep slab potential.

The Summit Lake region to the south of our forecast area also has the December 1st crust/facet combo below 2500′. The overall snowpack is thinner and the weak snow and crust are only buried 1-2′ deep. Observers found this to be reactive in snowpack tests yesterday. In addition, there is weak snow at the ground that is a concern above 2500′. Both these weak layers are worth keeping in mind before trying to push into bigger terrain in the Summit Lake area.

Extended column test that failed (propagated) with 13 taps at 1950′ on Tenderfoot. 12.30.20. Photo: Eric Roberts

Weather

Thu, December 31st, 2020

Yesterday: Skies were mostly cloudy in the morning and became partly cloudy in the afternoon. Temperatures were in the 30°Fs at sea level and 20°Fs at the ridge top weather stations. Winds were easterly 5-15 mph with gusts into the 20s and 30s. Overnight skies were partly cloudy with temperatures in the 20°Fs and light and easterly winds.

Today: Partly cloudy skies in the morning with a chance of scattered snow showers becoming mostly sunny by the afternoon. Light easterly winds and temperatures in the 20°Fs. Overnight partly cloudy skies becoming mostly clear with temperatures in the teens and 20°Fs and light variable winds.

New Year’s Day: Mostly sunny skies with calm winds and temperatures in the teens to mid 20°Fs.

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

	Temp Avg (F)	Snow Depth (in)
Center Ridge (1880′)	29	79
Summit Lake (1400′)	27	31
Alyeska Mid (1700′)	29	82

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	21	NE	12	40
Seattle Ridge (2400′)	25	E	7	17

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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