Sun, December 27, 2020 Turnagain Pass Avalanche Forecast

Sun, December 27th, 2020

Alpine
Above 2,500'

Considerable (3)

Treeline
1,000'-2,500'

Moderate (2)

Below Treeline
Below 1,000'

Low (1)

Alpine Above 2,500'		Considerable (3)
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 multiple wind slabs on test slopes in Pete’s North yesterday. While they were not deep (~6”), they did propagate fairly wide (50-150’). We can expect more of the same today.

Wind slab that fractured with a ski-cut. The slope was just not quite steep enough for the slab to run. 12.26.2020

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 are ramping up today ahead of a storm that will last through tonight and taper off tomorrow. As increasing winds through the day continue to drift snow into sensitive slabs, it will become likely that a person could trigger a wind slab avalanche 1-2’ deep. These avalanches will be the largest and most sensitive at elevations above 2500’, where wind speeds will be the highest and slopes the least protected. The most suspect slopes will be near ridges, below convex rollovers mid-slope, or in cross-loaded gullies (picture those facing the highway on Seattle Ridge, or on the steeper south-facing shots below Hippy Bowl). Be on the lookout for clear signs of decreasing stability, such as shooting cracks, collapsing, and recent avalanches. Pay attention to changing conditions as this next system moves in– with close to a foot of low-density snow accumulating over the past few days and more on the way, there is plenty of ammo for slab-building as winds pick up throughout the day.

Shooting cracks on a freshly wind-loaded slope. This crack propagated roughly 150′. Photo: Andy Moderow. 12.26.2020

Additional Concern

Deep Persistent Slabs

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.

More info at Avalanche.org

We have been tracking the crust that was formed on 12/1, after the rain level made it up to 2500’. At this point, the 12/1 crust is buried about 2-3′ deep. While we have yet to see any avalanches failing at this layer, we have noticed some faceting in the middle of the crust, which has been showing mixed results in stability tests. Yesterday, a group of skiers in Pete’s North reported a large collapse in this layer while skinning up through the trees. This observation was unexpected and it is the only red flag reported on this layer thus far. For now, we are keeping an eye on this layer as a potential problem if it continues to show signs of decreasing strength.

The Summit Lake region to the south of our forecast area has a thinner snowpack with faceted snow at the ground. This layer is gaining strength and it is becoming less and less likely to trigger an avalanche deep in the snowpack, However, we cannot entirely rule out the possibility of a person triggering a deep slab avalanche in, to quote Wendy, ‘just the wrong thin spot’. This is still a layer worth keeping in mind before trying to push into bigger terrain in the Summit Lake area.

Weather

Sun, December 27th, 2020

Yesterday: Skies cleared in the afternoon after a quick round of snow left 2-4”. Winds were light to calm for most of the day, with the exception of the south end of Turnagain Pass, where easterly winds blew 15-20 mph for a few hours mid-day. Easterly ridgetop winds picked across the advisory area yesterday evening, blowing 15-20 mph with gusts to 44 mph.

Today: Our next round of snow and wind moves in this afternoon, with 2-4” snow expected by this evening and another 6-12” by tomorrow morning. Easterly winds will continue to pick up ahead of and during the storm, with ridgetop speeds expected between 20-30 mph today and 30-40 mph tonight. Temperatures are expected to rise gradually through today and tonight, reaching the low 20’s F at upper elevations and low to mid- 30’s F at lower elevations. It looks like the rain level will stay down to 500 feet.

Tomorrow: Light snowfall will continue, with another 1-2″ expected during the day. High temperatures will be in the mid-20’s to upper 30’s F, with easterly ridgetop winds blowing 20-25 mph and gusts around 30 mph.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	27	2	0.2	77
Summit Lake (1400′)	22	1	0.1	31
Alyeska Mid (1700′)	28	2	0.2	78

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	20	NE	11	44
Seattle Ridge (2400′)	24	SE	9	18

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.