Fri, December 15, 2023 Turnagain Pass Avalanche Forecast

Fri, December 15th, 2023

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

2 - Moderate

Below Treeline

Below 1,000'

2 - Moderate

Sat, December 16th, 2023

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

3 - Considerable

Below Treeline

Below 1,000'

2 - Moderate

Sat, December 16th, 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

The consistent snowy weather recently has made it difficult to assess how much avalanche activity has been associated with the recent snowfall events. A brief period of clearing yesterday showed some avalanche activity that occurred during the most recent storm, likely Wednesday afternoon or evening. Glide avalanche activity also continues, with a release observed along Seattle Ridge during the day yesterday. No new activity has been reported on the layer of facets above the Thanksgiving crust.

Silhouette of an avalanche that likely released during the storm on Wednesday and was subsequently buried by new snowfall. Photo Peter Wadsworth 12.14.23

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

Stormy weather resumes today, with 2-6″ of snow and cloudy skies expected throughout the day. The sneaky 18″ of low density snow that fell with very light winds in the forecast area on Wednesday will start to be redistributed by increasing SE winds today, averaging 15-25 mph with gusts to 35 mph. Wind slabs 1-3′ deep could form rapidly given the amount of soft snow on the surface available for transport.

Keep an eye out for active wind loading and hollow or stiff feeling snow on the surface as a sign of areas that are being wind affected. Jumping on small, steep test slopes can be a good way to evaluate how reactive wind slabs are before entering more consequential terrain. To avoid this issue look for wind sheltered areas where you can take advantage of the soft snow on the surface.

Dry Loose Avalanches are very likely in steep terrain that is sheltered from the winds. Be mindful of managing your sluff if you are travelling on steep slopes, especially if there is exposed terrain underneath you.

The blanket of soft snow on the mountains from Wednesday’s storm will be redistributed today with increasing wind speeds. Photo Peter Wadsworth 12.14.23

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

The layer of facets above the Thanksgiving crust has not shown any obvious signs of being active during the past couple of storm systems. However, we have limited information from the forecast area. An avalanche triggered on this 3-5′ deep weak layer could be very large and destructive so we are keeping it in the forecast as a reminder to check the snowpack in the areas you are travelling. If you find a layer of weak, faceted snow on top of the crust or get a concerning result in a stability test on this layer we recommend sticking to lower angle slopes in order to minimize your exposure to a potentially high consequence avalanche. This type of weak layer becomes more difficult to evaluate the deeper it gets buried so we will be keeping track of it to try and determine when it could become more likely for people to trigger.

Basic snowpack strucutre on Sunburst at ~3000′. Photo Peter Wadsworth 12.14.23

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

A glide avalanche released during the day yesterday on Seattle Ridge. These avalanches are very unpredictable and can release spontaneously regardless of weather conditions. Try to avoid spending time underneath a glide crack to reduce your exposure.

Glide release on the S end of Seattle Ridge on Wednesday 12.14.23. Photo Peter Wadsworth

Weather

Fri, December 15th, 2023

Yesterday: There was a brief break in the cloudy skies and stormy weather yesterday, with predominantly clear skies except for a layer of valley fog building in the afternoon. Temperatures were chilly, in the low to mid teens F. Winds averaged 5-10 mph in the alpine with gusts up to 20 mph picking up overnight. No significant new snowfall was recorded.

Today: Light snowfall is expected to resume again today, starting in the early morning hours and accumulating roughly 2-6″ during the day. Winds will shift to the SE as the snowfall starts and increase to averages of 15-25 mph and gusts to 35 mph at upper elevations. Temperatures will be in the low to mid 20s F, which will keep the precipitation falling as snow down to sea level. Cloud cover is expected to be overcast and obscured during the storm, with a possible period of clearing Friday night into Saturday.

Tomorrow: Saturday should be similar to Friday in regards to snowfall, with light snow throughout the day resulting in 2-6″ of additional accumulation. Temperatures will increase which could cause rain line to move up to 500-700′. Wind speeds are expected to increase on Saturday, but the timing and intensity of the winds are a question mark. Current weather models show winds up to averages of 45-65 mph starting around noon on Saturday.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	11	0	0	76
Summit Lake (1400′)	3	0	0	32
Alyeska Mid (1700′)	15	0	0.02	61
Bear Valley – Portage (132′)	14	1	0.12	–
Grouse Ck – Seward (700′)	14	0	0	32

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	12	W	5	20
Seattle Ridge (2400′)	13	SE*	-*	10*

*Seattle Ridge weather station is rimed over with only a few hours or data reporting for wind.

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
10/27/24	Turnagain	Observation: Tincan	Michael Kerst
10/21/24	Turnagain	Observation: Turnagain Pass Road Observation	Trevor Clayton
10/19/24	Turnagain	Observation: Tincan – Below Todds Run	Andy Moderow
10/18/24	Turnagain	Observation: Taylor Pass	Eli Neuffer
10/15/24	Turnagain	Observation: Tincan Common	John Sykes Forecaster
10/14/24	Turnagain	Avalanche: Tincan	CNFAC Staff
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

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