Sat, December 09, 2023 Turnagain Pass Avalanche Forecast

Sat, December 9th, 2023

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

2 - Moderate

Below Treeline

Below 1,000'

1 - Low

Sun, December 10th, 2023

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

3 - Considerable

Below Treeline

Below 1,000'

2 - Moderate

Sun, December 10th, 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

We received a report of an avalanche on the north side of Tincan Ridge yesterday, running into Todd’s Bowl. It was reported to have been recent and triggered by the winds. We also received a photo of a large slab avalanche on the backside of Seattle Ridge (between Main and Zero Bowls). This slab could be older, but it’s a deeper slab that may have released on the Thanksgiving crust. More on that in our ‘Additional Concerns’ below.

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

After a break in storms yesterday, another shot for a few inches of snow and wind is slated for today. Plus… an even larger storm is on the horizon for tomorrow into Monday. An active weather pattern is setting up for Southcentral over the next week and with that will come increasing avalanche danger.

For today, ridgetop winds are on the rise from the east (10-20mph with gusts as high as 40mph) and light snowfall should add up to 2-6″ by tonight. With a surface that has already seen varying degrees of wind effect, another round of wind slabs will be adding to the equation. Winds today should be enough that in exposed areas above treeline, and especially along the higher ridgelines, natural wind slab avalanches may occur. These are likely to be in the foot deep category and if they are fresh, should be easy for us to trigger. With low visibility conditions expected, sheltered areas in the mid elevation trees will be the best bet for not only avoiding these avalanches but also finding the softest snow.

Additional Concern

Persistent Slabs

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.

More info at Avalanche.org

As mentioned above, there was a large avalanche seen on the backside of Seattle Ridge on Thursday, 12/7, (photo below) . The slab appears to be 2-3′ deep and has the character of an avalanche releasing on a buried weak layer, as opposed to a typical wind slab for example. This is the first avalanche we have seen that looks like it could have released on the Thanksgiving crust. We do not know when it occurred, but best guesses are sometime in the last week.

Whether this avalanche is a precursor of things to come, we don’t know yet. However, we are keeping a close eye on the buried Thanksgiving crust. We have seen in snow pits that facets are developing on top of the crust. As time goes on these facets can continue to grow and become a weak layer, making the ingredients for a scary avalanche setup: lots of snow on top (big slab), weak layer (facets), over a uniform crust (bed surface).

What we believe is a natural slab avalanche between Main and Zero Bowl on the backside of Seattle Ridge. Thank you to Travis Smith for the photo taken Thursday, 12.07.23.

Weather

Sat, December 9th, 2023

Yesterday: Partly sunny skies were over the region yesterday after 4-8″ of light snow fell the evening before. Ridgetop winds were from the northwest in the 10-15mph range with gusts near 30mph. Temperatures were cold – single digits.

Today: Clouds have built in overnight and today a weak weather system moves in. Ridgetop winds have turned easterly and should increase through the day to the 15-25mph range with gusts near 40mph in some areas. Between 2-6″ of snow is expected through today before tapering off tonight.

Tomorrow: A more powerful storm is forecast for Sunday into Monday. Strong east winds with 12-18″ of snow could fall by Monday morning. The rain/snow line looks to climb to around 500′ with this event. A stormy weather pattern is setting up for the remainder of the work week.

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

	Temp Avg (F)	Snow Depth (in)
Center Ridge (1880′)	8	50
Summit Lake (1400′)	12	26
Alyeska Mid (1700′)	13	35
Bear Valley – Portage (132′)	15	–
Grouse Creek (700′)	9	26

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	4	W->E	12	35
Seattle Ridge (2400′)	8	SE	7	16

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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