Mon, December 25, 2023 Turnagain Pass Avalanche Forecast

Mon, December 25th, 2023

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

3 - Considerable

Below Treeline

Below 1,000'

3 - Considerable

Tue, December 26th, 2023

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

2 - Moderate

Below Treeline

Below 1,000'

2 - Moderate

Tue, December 26th, 2023

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

2 - Moderate

Below Treeline

Below 1,000'

2 - Moderate

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

There was a widespread avalanche cycle yesterday, with multiple natural and artillery-triggered avalanches from Girdwood down to Kenai Lake. These avalanches were very large, running from alpine start zones all the way to valley bottoms. This included multiple avalanches that hit the road, and one that involved a vehicle, luckily with no injuries. See this article from Anchorage Daily News for more details.

Artillery-triggered avalanche in motion near Peterson Creek just south of Girdwood during hazard reduction work yesterday. 12.24.2023

Debris from a large natural avalanche in the Crow Creek area yesterday. Photo: George Creighton, 12.24.2023

Avalanche Problem 1

Storm 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

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.

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

Yesterday we saw an impressive cycle of very large natural and human-triggered avalanches from Girdwood to Moose Pass. This included multiple avalanches that were running full-path to valley bottoms, two of which reached the Seward Highway. No doubt about it, conditions were very dangerous. While things will likely be a bit more subtle, we are actually more concerned with the potential for an avalanche accident today. This scenario of a day of quiet weather immediately after a storm cycle on a reactive snowpack is the period when people get caught in avalanches.

Most of our advisory area received 2-3′ snow between Friday night and Sunday morning. This stormy period started cold and finished warm, which left a stiff layer of snow sitting on top of a low-density weaker storm layer. All of that snow buried a layer of surface hoar that we observed from Turnagain Pass to Carter Lake, making things even more sensitive. In most places that layer of surface hoar was forming on top of really soft snow, which will hopefully mean it can heal quickly. For now, that surface hoar layer may make it possible to trigger an avalanche remotely– from flat terrain connected to steeper terrain. Today, there is a lot of uncertainty with how reactive all of that new snow will be. Based on the extent of activity we saw yesterday, we have good reason to believe conditions are still dangerous today. If you’re planning to get out to enjoy all of this new snow before Christmas dinner, be sure to keep that high uncertainty and the potential for big avalanches in mind. Use a little extra caution today, avoiding steep terrain while we give all of that snow another day to settle out.

Crown lines from two avalanches near Girdwood yesterday. We’re concerned that similar activity will be likely today with more people getting out to enjoy all of this new snow. 12.24.2023

It was feeling more and more like the Nightmare Before Christmas yesterday with each new big avalanche we got word of.

Avalanche Problem 2

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

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

That new snow equaled 2.5-3.5″ snow water equivalent (SWE), which is adding a lot of weight to the layer of facets that we have been paying close attention to. We are not yet sure if any of the avalanches that failed during or just after the storm impacted this weak layer, so we’re still treating it with caution. The layer is now buried 4-6′ deep on average and will be very difficult to assess, adding another layer of uncertainty to the already challenging snowpack. We should be able to get a better idea of how reactive this layer is over the next few days, but for now it is another good reason to avoid traveling in steep terrain. If you do get out today, please let us know what you see.

Additional Concern

Glide Avalanches

Glide Avalanches

Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.

More info at Avalanche.org

We’ve seen a lot of glide activity over the past week, and it is possible we will see more today. Glide avalanches have been reported from Summit Lake to Girdwood, with some glide cracks opening up abovehigh-traffic areas like the motorized uptrack on Seattle Ridge or the common skin track in Magnum’s PMS bowl, to name a few. Glide releases are unpredictable and very large and destructive, so be sure to limit the time you spend under glide cracks.

Weather

Mon, December 25th, 2023

Yesterday: The storm tapered off through the day yesterday, bringing a trace to 3” snow through the day with easterly winds of 10-20 mph with gusts of 25-40 mph. The rain made it to just above 1000’ before dropping back down to sea level during the lingering snow showers. Temperatures were warmest yesterday morning in the high 20’s to mid 30’s F, and have since dropped to the single digits to high teens F.

Today: We should see quiet and cold weather today, with temperatures hanging in the single digits to low teens F through the day into tonight. Skies should be mostly clear with lingering low-level clouds and increasing sun later in the day. Winds will be light out of the west for most of the area, but common gaps like Whittier and Seward may see slightly stronger winds at 15-20 mph.

Tomorrow: The weather should remain fairly quiet tomorrow, with some scattered snow showers potentially bringing a trace to an inch of snow in the afternoon. Temperatures will remain in the single digits to low teens F during the day and into tomorrow night, with easterly winds ramping up to 10-20 mph through the day.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	21	0	0	85
Summit Lake (1400′)	18	0.1	1	n/a
Alyeska Mid (1700′)	21	2	0.17	75
Bear Valley – Portage (132′)	28	tr	0.45	–

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	13	ENE	8	42
Seattle Ridge (2400′)	14	n/a*	n/a*	n/a*

*Seattle Ridge anemometer is rimed an 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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