Sat, December 26, 2020 Turnagain Pass Avalanche Forecast

Sat, December 26th, 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.

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

With a quiet weather day on tap today (cloudy skies, an inch of new snow and light variable winds), our main avalanche concern lies with lingering wind slabs that either formed early yesterday morning or during the high winds on Tuesday. These lingering slabs could be hidden under a skiff of new snow and found lower on the slope than expected, for example below mid-slope rollovers or in cross-loaded gullies. They are also generally on the harder side, which could allow them to not break until a person is further out on the slope. The higher in elevation one goes, and into more exposed wind affected zones, the more likely wind slabs will be lurking. Although these were fairly stubborn yesterday and should be on their way to bonding more today, it is still something to keep a close eye out for.

If visibility is good enough to get to the steeper slopes, watch for the classic wind slab signs:

hollow, or drum like, feeling snow?
stiff snow over soft snow?
cracks in the snow around you that shoot out?

Very Chistmasy in the mountains yesterday! This photo shows significant wind effect on Peak 4940 that is across from the Johnson Pass trailhead and the furthest point of Seattle Ridge. 12.25.20

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

To the south of our forecast area in the Summit Lake region a much shallower snowpack exists compared with Turnagain Pass. Here there are old facets at the bottom of the snowpack and some old mid-pack weak layers. At elevations above 2500’ where there are no mid-pack crusts and the slab is composed of hard snow, these weak layers give us some pause. As the days go on and snowpit tests continue to show the layers loosing reactivity, the question is, can a dangerous avalanche be triggered if someone hits just the wrong thin spot? This is an outlier situation, but something I still have in the back of my mind.

Weather

Sat, December 26th, 2020

Yesterday: High clouds with some patches of blue sky filtered through yesterday. Ridgetop winds were easterly averaging 10-20mph with a few gusts into the 30’s. Temperatures were in the 20’s F at mid and upper elevations.

Today: Cloudy skies with light snow showers are forecast today. Only 1-3″ of accumulation is expected – from sea level and up. Ridgetop winds are light easterly (~5mph) this morning and should swing around to the west through the day and remain in the 5-10mph range. Temperatures should stay in the 20’s F at mid and upper elevations while sea level remains near 32F

Tomorrow: For Sunday, strong ridgetop easterly winds are forecast to develop ahead of a large low pressure system headed our way. Skies should be cloudy and precipitation associated with this front should begin Sunday night and extent through the early part of the week. This system is somewhat warm, but so far the rain/snow line looks to remain below 1,000′. Stay tuned!

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

	Temp Avg (F)	Snow Depth (in)
Center Ridge (1880′)	27	77
Summit Lake (1400′)	22	31
Alyeska Mid (1700′)	26	77

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	23	NE	14	39
Seattle Ridge (2400′)	23	*N/A	*N/A	*N/A

*Seattle Ridge anemometer is rimed over and not reporting wind 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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