Fri, December 11, 2020 Turnagain Pass Avalanche Forecast

Fri, December 11th, 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

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

It’s about to be a very nice sunny Friday out there in the mountains! The snowpack is showing promising signs of general stability, but we are not quite fully confident in some of those weak layers that are hanging out in the snowpack. People are venturing further from the road and into terrain not yet traveled this season. Along with that, there have been many folks out and only a few observations coming in. Does this mean stable snow is being found? I hope so. Let us know what you are seeing out there!

The two layers we are watching are below. If someone does trigger an avalanche in one of these, it is likely to be large and dangerous, hence our caution.

Mid-pack facets: Under the 2-3′ of snow from last Sunday/Monday sit near surface facets and surface hoar. At elevations below 2,500′ those facets sit on a crust. From what we know, this layer has stabilized in the heart of Turnagain Pass and even possibly in the Girdwood area, but other areas on the periphery are still a question.

Basal facets: At the base of the pack are those October facets we keep talking about. These have not caused a reported avalanche for a week now. The last known slides failing on this layer were on Tincan last Thursday and Silvertip last Saturday. Although this layer is unlikely to trigger, it is still worth keeping in the back of our minds, especially in shallow snow cover areas such as the south end of Turnagain Pass and Summit Lake.

What can we do about this uncertainty if we want to head to big and steep terrain today? We can watch for signs of instability. We can also know those signs may not exist and an avalanche could be lurking. We can watch our partners closely from true safe zones, expose one at a time and have escape routes planned.

Besides the questions above, there are the Normal Caution avalanche issues consistent with a LOW danger. These are keeping a lookout for those old wind slabs that could pop out on you in steep complex terrain, watching your sluff on long sustained steep slopes and traveling well away from a cornices‘s edge.

Snowpack in the Summit Lake area – to the south of our advisory area. The snowpack here was around 3′ deep and at 2,500′. You can clearly see those basal facets at the bottom of the pack. 12.9.20.

Weather

Fri, December 11th, 2020

Yesterday: Clear skies were over the region. Winds were westerly in the 5mph range along the higher ridgelines. Temperatures were in the teens at the mid and upper elevations and in the 20’sF at lower elevations.

Today: Another clear sky day in on tap today. Winds will swing around to the east later this morning and blow in the 5-10mph range before increasing to the 20-30mph range tonight into tomorrow. Temperatures will remain winter-like, in the teens in the mountains and the 20’sF at the low elevations.

Tomorrow: High clouds will start streaming in ahead of a low-pressure system spinning in the Gulf. Winds are forecast to increase into the 30 to even 40mph range through the day ahead of light snow flurries that should start late Saturday night and into Sunday. Only small accumulations are expected Sunday.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	17	o	o	64
Summit Lake (1400′)	7	0	0	24
Alyeska Mid (1700′)	20	0	0	58

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	17	W	4	9
Seattle Ridge (2400′)	17	N/A	N/A	N/A

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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