Thu, December 07, 2023 Turnagain Pass Avalanche Forecast

Thu, December 7th, 2023

Alpine

Above 2,500'

2 - Moderate

Treeline

1,000'-2,500'

2 - Moderate

Below Treeline

Below 1,000'

1 - Low

Fri, December 8th, 2023

Alpine

Above 2,500'

2 - Moderate

Treeline

1,000'-2,500'

2 - Moderate

Below Treeline

Below 1,000'

1 - Low

Fri, December 8th, 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

Strong NW Outflow winds occurred yesterday in many areas of Southcentral. Although much of Turnagain Pass looked to escape these winds, a group on Eddies noted the unique terrain channeling effect that can occur during a NW wind event. This is a band of southerly winds (yes southerly) that forms along the east side of Turnagain Pass (non-moto side). The southerly winds formed wind slabs, wind crusts, and a few shallow wind slab avalanches on west to north aspects yesterday. See their report HERE.

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

As of this morning, winds have quieted and temperatures have dropped; Turnagain Pass parking lots sit around 10F currently. The break in weather will be short lived as a quick hitting snow storm is forecast for this afternoon. Between 2 and 9pm 4-8″ of snow is expected with moderate easterly winds (10-20mph with gusts near 30). Snow should fall to sea level even though temperatures will rise a bit with the weather.

Wind Slabs: Lingering wind slabs formed yesterday and any new wind slabs that form this afternoon will be the main concern. They are likely to be relatively shallow, up to a foot deep. Watch for these on all aspects in exposed areas in the trees, and above the trees. The usual signs will be stiff snow over softer snow, cracks that shoot out from your machine, board, or skis. Keep in mind, new snow can cover up the older slabs making them a bit harder to see.

In the mountains after dark: The peak of the storm is forecast around 6pm (around 2 hours after sunset). So if anyone is out after dark, new snow sluffs in steep terrain may be seen in places that receive upwards of 6″ of new snow.

This NWS graphic shows 24 hour snowfall, yet for this storm it looks like it all happens between 2 and 9pm today, Thursday. These images can be found HERE.

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

If you have been reading this far into the forecast, you’ll know we are paying close attention to the Thanksgiving crust (buried 2-4′ deep). Crusts can develop weak snow around them over time and produce large avalanches down the road. How this one will evolve is still a question. We do not not know of any avalanches on it yet, but snowpits are starting to show signs it may be a future problem.

Additionally, at the upper reaches of the forecast zone (above 3,500′) a layer of facets at the ground, now buried 6-8′ deep, may still be weak enough to produce a large avalanche if a person finds the perfect thin trigger spot. This is becoming more and more unlikely with time but something to consider in high alpine steep and rocky zones.

Weather

Thu, December 7th, 2023

Yesterday: High clouds were over the region yesterday with no precipitation. Ridgetop winds were out of the northwest and blowing 5-15mph. However, a strong outflow wind event (NW winds) was seen along the Western Chugach Mtns and Southern Kenai.

Today: Increasing clouds, cold temperatures, and a burst of snow and wind this afternoon. This morning winds are light and temperatures sit between 0-15F. However, a front is forecast to pass through this afternoon/evening that will bring 10-20mph east winds and 4-8″ of snowfall (to sea level). Snow should start falling just after noon and peak around 5-6pm.

Tomorrow: Clearing skies, light winds, and chilly temperatures are expected tomorrow, Friday, before clouds and another chance of snow pushes in on Saturday.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	18	0	0	48
Summit Lake (1400′)	15	0	0	21
Alyeska Mid (1700′)	20	0	0	31
Bear Valley – Portage (132′)	–	–	–	–
Grouse Creek (700′)	28	0	0	–

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	11	NW	4	14
Seattle Ridge (2400′)	16	NW	8	18

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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