Wed, February 26, 2020 Turnagain Pass Avalanche Forecast

Wed, February 26th, 2020

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

3 - Considerable

Below Treeline

Below 1,000'

3 - Considerable

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

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

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

Yesterday was the first day in over a week that there was no large (D2) or very large (D3) avalanches reported. There have been some impressive and scary avalanches triggered by snow and wind loading, explosives and remotely by skiers. If you have been following along with the forecast you know we are still concerned that someone will find the wrong spot on their skis or snowmachine, trigger one of these beasts and get caught. That worry continues today. It has snowed 6-12″ last night and winds were easterly 10-20 mph with gusts in to the 30s and 40s. Fresh wind slabs are likely in wind loaded terrain (more on that below in Avalanche Problem 2). There is also older wind hardened snow below. A person on skis or snowmachine could initiate a wind slab in the top 1-2′ of the snowpack that might ‘step down’ and trigger a much larger slide on a deep weak layer 3-8+ feet down. It doesn’t have to be an avalanche in the upper snowpack. It could also just be the 1st, 10th or 20th person on the slope. Because it is hard to know what will actually tip the balance with a deep persistent slab problem is is important to remember a natural avalanche could also occur today as snow and wind loading continue. The advice for the day with poor visibility is avoid travel in runout zones and don’t forget that avalanches have been triggered remotely, think about what the terrain you are on is connected to. Cautious route-finding and conservative decision making are essential. Use safe travel protocol and consider terrain consequences if a large avalanche were to occur. Where would you end up?

The January facet layer was 3′ down from the surface at 2500′ on Fresno in Summit Lake. 2.24.20.

Remote triggered avalanche above Grandview just south of Placer Valley. 2.24.20. Photo: Graham Predeger. This remote triggered slab is just up-ridge of the site of the double fatality ten years ago on Feb 13th, 2010. Link HERE.

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

As mentioned above, new snow and wind overnight have likely formed tender wind slabs in leeward terrain that will be easy to trigger. Pay attention to surface wind effect and watch for cracking. Is there harder snow over softer snow? Is there blowing snow and loading happening while you are out?

Storm slabs and loose snow avalanches: On slopes protected from the wind, soft snow may be cohesive enough to have formed shallow storm slabs that could easily slide in steep terrain or if still loose easily sluff in steep terrain.

Cornices: These have grown substantially over the past week and could be triggered today. Avoid travel on or underneath. Triggering a cornice fall could also initiate a deep slab on the slope below.

Weather

Wed, February 26th, 2020

Yesterday: Skies went from partly cloudy to mostly cloudy and snow showers started in the late afternoon. 6-12″ fell overnight favoring Portage. Winds were easterly 10-20 mph with gusts in the 30s and 40s. Temperatures started in the single digits and climbed into the 20°Fs.

Today: Snow continues today with another 2-8″ possible. Easterly winds will continue blowing 10-20 mph with gusts into the 30s and then diminish overnight. Temperatures will be in the 20°Fs to low 30°Fs. Light snow showers continue overnight with temperatures in the 20°Fs and hight teens.

Tomorrow: Snow showers in the morning becoming partly cloudy in the afternoon. Temperatures in the 20°Fs. Winds are mostly light and shift to the west with clearing skies. Temperatures drop into the the single digits overnight. There is a chance of another outflow event and mostly clear skies for Friday and Saturday.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	24	6	0.4	83
Summit Lake (1400′)	17	2	0.2	32
Alyeska Mid (1700′)	20	4	0.4	88

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	17	NE	17	47
Seattle Ridge (2400′)	19	*NA	*NA	*NA

*Seattle Ridge anemometer (wind sensor) is rimed over and not reporting.

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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