Fri, April 07, 2023 Turnagain Pass Avalanche Forecast

Fri, April 7th, 2023

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

It has been 6 days since the last reported human-triggered avalanche near our advisory area, and 10 days since the last human-triggered avalanche involving the deep persistent weak layer we are concerned about.

Avalanche Problem 1

Deep Persistent Slabs
N E S W NW NE SE SW

Aspect/Elevation
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.

Aspect/Elevation of the Avalanche Problem

Specialists develop a graphic representation of the potential distribution of a particular avalanche problem across the topography. This aspect/elevation rose is used to indicate where the particular avalanche problem is thought to exist on all elevation aspects. Areas where the avalanche problem is thought to exist are colored grey, and it is less likely to be encountered in areas colored white.

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 day of weather expected today, our main avalanche concern lies within the weak layer of snow buried 3-6′ deep by all of the snow we got in the second half of March. As time moves on, this layer is slowly becoming more stubborn to trigger, but the possibility of triggering a very large avalanche on this layer remains. Deep persistent weak layers like the one we have been dealing with are very hard to assess, since the stability tests we typically use to investigate a weak layer are unreliable once the layer is buried deeper than 3′ or so.

Based on field observations from the past few days, we know the snowpack structure is still suspect, but it is starting to gain strength (more details in these observations from Bertha Creek and Pete’s North). This puts us in a challenging situation where the likelihood of triggering a massive avalanche is decreasing, but there is still a lingering chance of triggering an avalanche with fatal consequences. Because the consequences are so high, we are still avoiding traveling on or below big steep slopes. This problem layer is headed in the right direction, but we aren’t quite ready to trust it just yet.

Wind Slabs: As winds pick up later in the day we may see some small wind slabs forming in upper elevations. These should be small, but will be important to watch out for since they will be very sensitive as they are forming.

Skiers, splitboarders, and a dog on the north side of Sunburst crossing the aftermath of a very large natural avalanche that occurred about two weeks ago. Photo: Lance Breeding, 04.06.2023

Weather

Fri, April 7th, 2023

Yesterday: Skies were mostly cloudy with increasing sun in the afternoon. High temperatures were in the low 20’s F at upper elevations and mid 30’s F down low. Winds were light out of the west at 5-10 mph with gusts of 10-20 mph, with stations in Girdwood showing a brief stretch with slightly stronger winds yesterday afternoon. We did not record any precipitation yesterday.

Today: Increasing cloud cover today as a low pressure system in the gulf switches us back to a southeasterly flow. Winds should stay light at 5-15 mph out of the east, picking up slightly later in the day through tonight. High temperatures should be between 20 and 30 F, with lows in the mid teens to low 20’s F. We may see some light snow later in the day, but it is not likely to amount to any measurable accumulation.

Tomorrow: Skies will be mostly cloudy with some periods of light snowfall bringing a trace of new snow. Winds will start light out of the east before switching westerly and picking up to 10-15 mph with gusts of 15-20 mph. High temperatures will be in the low 20’s to 30 F with lows in the low to mid teens F.

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

	Temp Avg (F)	Snow Depth (in)
Center Ridge (1880′)	23	90
Summit Lake (1400′)	19	44
Alyeska Mid (1700′)	23	82
Bear Valley – Portage (132′)	26	–

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	12	WNW	4	17
Seattle Ridge (2400′)	17	NNW	3	12

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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