Sun, December 13, 2020 Turnagain Pass Avalanche Forecast

Sun, December 13th, 2020

Alpine
Above 2,500'

Moderate (2)

Treeline
1,000'-2,500'

Moderate (2)

Below Treeline
Below 1,000'

Low (1)

Alpine Above 2,500'		Moderate (2)
Treeline 1,000'-2,500'		Moderate (2)
Below Treeline Below 1,000'		Low (1)

Danger Scale:

No Rating (0)

Low (1)

Moderate (2)

Considerable (3)

High (4)

Extreme (5)

Recent Avalanches

Skiers triggered small wind slabs yesterday near Girdwood. It will be possible to trigger similar avalanches today, and they may be larger than yesterday as winds continue to blow snow into thicker slabs.

Avalanche Problem 1

Wind Slabs
Almost 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.

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

Winds have been at work building fresh and sensitive slabs since Saturday night, making it easy to trigger a wind slab avalanche 1-2’ deep at mid- and upper elevations. As of yesterday, we were already starting to see some avalanche activity, and as winds continue to move snow around today, we expect these slabs to get thicker and remain easy to trigger. Luckily, this type of avalanche is relatively easy to identify and avoid. Wind slabs may be recognized as being stiffer than the soft snow we’ve been enjoying for the past week. Here are some places you might expect to find fresh wind slabs today:

Below a ridge with a cornice on it. The same winds that are building the cornices will push snow farther down the slope and create a fresh slab.
Below convexities, or steep rollovers in the middle of a slope.
In gullies, which can get cross-loaded even when the rest of a slope is getting scoured.

You will also want to pay attention to cracks shooting out from your machine or skis, another clear indicator of unstable slabs. Wind slabs are usually easy to identify in a snow pit, and will usually show up in a stability test.

Cornices: We’ve seen debris from cornices falling during the most recent (12/6-12/8) storm. Continued winds will continue building cornices and push them near their breaking point. If you are traveling along a ridge, be sure to give them a wide berth. It is equally important to minimize the time you spend below them, as they can release naturally and unexpectedly.

Shooting cracks in a fresh wind slab yesterday. Photo: Brooke Edwards 12.12.2020

Avalanche Problem 2

Deep Persistent Slabs
Almost 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.

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 with a weak layer of facets on the ground. These deep persistent weak layers are tricky, and although it has been over a week since our last recorded avalanche on this layer, we still cannot rule it out entirely. This layer is most concerning in areas with thinner snowpack. We have been noting a thinner snowpack as you head south from Turnagain pass towards the Summit Lake area, and have also found thin coverage in some areas around Girdwood. This issue is less concerning at elevations below 2500’, where a rain crust makes it very unlikely a human could trigger an avalanche deep in the snowpack. However, it is still an issue for now at elevations above 2500’, where that buried crust is not present. Avoid travelling on steep slopes with thin coverage, and be aware that although unlikely, if you were to trigger an avalanche on this layer it could be big.

Weather

Sun, December 13th, 2020

Yesterday: Yesteday, we had partly cloudy skies with periods of ridgetop winds blowing 25-30 mph out of the east, with gusts to 47. We saw highs in the mid- 20’s to low- 30’s, with lows in the upper teens to mid- 20’s F, and no new snow.

Today: Easterly ridgetop winds have been blowing 25-30 mph since 3:00 this morning. Strong winds will continue through this morning, and (depending on the model you are looking at) may calm down this afternoon. Highs will be in the mid- to upper 20’s. Chance of precip will increase later today and into tonight, with only a trace to 2″ of new snow expected later this afternoon.

Tomorrow: Light snowfall overnight will taper off tomorrow, with 3-5″ expected in the mountains and snow levels staying down at sea level. Temperatures should stay in the mid-20’s to low 30’s tonight through tomorrow, with light winds out of the east.

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

	Temp Avg (F)	Snow (in)	Snow Depth (in)
Center Ridge (1880′)	32	0	61
Summit Lake (1400′)	28	0	24
Alyeska Mid (1700′)	29	tr	56

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	22	ENE	16	47
Seattle Ridge (2400′)	27	SE	11	28

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
05/22/23	Turnagain	Avalanche: Tincan	Nick D'Alessio
05/12/23	Turnagain	Avalanche: Tincan, Sunburst, Magnum, Cornbiscuit	Heather Thamm
05/07/23	Turnagain	Observation: Tincan – Bear Tracks	CNFAIC Staff
05/05/23	Turnagain	Observation: Seattle Ridge	AS/ WW Forecaster
05/02/23	Turnagain	Observation: Cornbiscuit	Schauer/ Sturgess Forecaster
05/02/23	Turnagain	Observation: Seward Hwy Turnagain Pass	Joel Curtis
04/30/23	Turnagain	Observation: Magnum	Ayla, Kit Crosby, Barton
04/29/23	Turnagain	Observation: Tincan	John Sykes
04/28/23	Turnagain	Avalanche: Taylor Pass/Pastoral	Schauer/ Creighton Forecaster
04/28/23	Turnagain	Avalanche: Tincan	Andy Moderow

Riding Areas

Updated Thu, June 01st, 2023

Status of riding areas across the Chugach NF is managed by the Glacier and Seward Ranger Districts, not avalanche center staff. Riding area information is posted as a public service to our users and updated based on snow depth and snow density to prevent resource damage at trailhead locations. Riding area questions contact: mailroom_r10_chugach@fs.fed.us

Area	Status	Weather & Riding Conditions
Glacier District
Johnson Pass	Closed	Closed May 15.
Placer River	Closed	Closed May 15.
Skookum Drainage	Closed	Will close to motorized use on April 1, 2023 per the National Forest Plan.
Turnagain Pass	Closed	Will close on June 1.
Twentymile	Closed	Scheduled closure May 1.
Seward District
Carter Lake	Closed	Scheduled closure May 1.
Lost Lake Trail	Closed	Scheduled closure May 1.
Primrose Trail	Closed	Scheduled closure May 1.
Resurrection Pass Trail	Closed	Closed to motorized use for the 2022/23 winter season per Forest Plan. Open next season.
Snug Harbor	Closed	Extended opening until May 15.
South Fork Snow River Corridor	Closed	Scheduled closure May 1.
Summit Lake	Closed	Scheduled closure May 1.