Sat, December 16, 2023 Turnagain Pass Avalanche Forecast

Sat, December 16th, 2023

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

3 - Considerable

Below Treeline

Below 1,000'

3 - Considerable

Sun, December 17th, 2023

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

3 - Considerable

Below Treeline

Below 1,000'

2 - Moderate

Sun, December 17th, 2023

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

3 - Considerable

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

During the last major dump of snow on Wednesday (~18″ in Turnagain Pass) there was widespread avalanche activity within the new storm snow in the forecast area (see obs from Tincan and Sunburst). We have not heard any reports of new avalanche activity on the facets above the Thanksgiving crust this week.

Avalanche Problem 1

Wind 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

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.

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

Another storm front is approaching the forecast area today, bringing strong winds averaging 30-45 mph with gusts to 60+ mph and 4-8″ of new snowfall. In more coastal areas, like Portage and Placer, we could see closer to 8-14″ of new snow today. Unfortunately, this storm front will also bring warming temperatures which are expected to reach the low 30s F at lower elevations and bring rain line up to 600-900′.

In the majority of the forecast area the most concerning avalanche problem today will be wind slabs 1-3′ deep which can form rapidly due to strong winds. These most commonly form along ridgelines and convex features at and above treeline. We recommend using small, steep test slopes to check whether wind slabs are reactive in the area you are travelling before committing to bigger terrain.

Given the low visibility and potential for rain at low elevations today it seems unlikely that many folks will venture into higher elevations in coastal areas, like Placer Valley. However, it is worth noting that due to the higher rates of snowfall expected there it is possible that avalanches could release within the new snow as storm slabs. Since this area just opened for motorized use yesterday we have very limited information and recommend approaching avalanche terrain cautiously.

Wet Loose: There is a lot of light dry snow on the surface at lower elevations right now, which could become unstable rapidly if rain starts to fall on dry snow. Wet loose avalanches are typically not large enough to bury a person but they can push you around if you get caught in one.

Avalanche Problem 2

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

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

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

We are still concerned about a weak layer of facets above the Thanksgiving crust that are now buried 3-5′ deep. Due to the potential for very large avalanches releasing on this weak layer we are continuing to monitor and test it throughout the forecast area. In the past week we have not seen any observations of natural or human triggered avalanches releasing on this layer, but it often takes awhile for stress to build on top of a layer like this before we see widespread avalanches. Digging a snowpit and checking out the snowpack structure in the area you are travelling can provide some information, but with the layer now buried roughly 3+’ deep our standard stability tests (Extended Column Test, Compression Test) struggle to provide consistent and accurate results. The only way to know for sure that you will not trigger a large avalanche on this weak layer is to be patient and stick to low angle terrain.

Snowpack structure near treeline in Lynx Creek from earlier this week. Photo 12.13.23

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

Glide avalanches continue to release in the forecast area, with the most recent being on Thursday on the southern end of Seattle Ridge. Staying out from underneath glide cracks is the only way to avoid these unpredictable avalanches.

Weather

Sat, December 16th, 2023

Yesterday: Cloudy skies and moderate to light snowfall with 1-4″ of new snow falling down to sea level. Winds picked up during the middle of the day with averages of 15-20 mph and gusts up to 36 mph. Temperatures stayed just cold enough to keep snow falling down to sea level.

Today: A storm front is approaching the forecast area today, bringing strong winds and snowfall starting in the late morning and becoming most intense in the afternoon and evening. Winds are expected to average 30-45 mph with gusts to 60 mph possible this afternoon. For snowfall Girdwood and Turnagain Pass should receive 4-8″ of new snow today, with Portage and Placer receiving closer to a 12″. Along with the strong winds and precipitation will come rising temperatures into the low 30s F at lower elevations. This will move rain line up to 600-900′ this afternoon.

Tomorrow: Light snowfall is expected to continue throughout the day on Sunday with 1-3″ of new snow and cloudy skies expected for most of the day. Temperatures will drop back down into the 20s F on Sunday, which will bring snowfall down to sea level again. Winds are expected to shift to the SW and remain elevated with averages of 20-30 mph and gusts of 40 mph.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	21	1	0.1	73
Summit Lake (1400′)	15	2	0.2	34
Alyeska Mid (1700′)	22	4	0.32	63
Bear Valley – Portage (132′)	26	3	0.28	–
Grouse Ck – Seward (700′)	21	4	0.3	36

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	15	ENE	9	36
Seattle Ridge (2400′)	18	SSE*	4*	18*

*Seattle Ridge weather station is rimed over with several hours overnight missing from 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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