Tue, December 15, 2020 Turnagain Pass Avalanche Forecast

Tue, December 15th, 2020

Alpine
Above 2,500'

Moderate (2)

Treeline
1,000'-2,500'

Low (1)

Below Treeline
Below 1,000'

Low (1)

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

Danger Scale:

No Rating (0)

Low (1)

Moderate (2)

Considerable (3)

High (4)

Extreme (5)

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

Wind slabs that have formed over the past two days are becoming more stubborn and difficult to trigger, but today it will still be possible to trigger an avalanche 1-2’ deep on a wind-loaded slope, especially near ridgetops and in cross-loaded gullies. Winds are expected to shift and start blowing out of the west during the day today, and although we are not expecting to see any remarkable speeds, this change in direction may redistribute pockets of snow that have previously been sheltered from the generally easterly winds for the past 48 hours. A few inches of light snow this morning is not expected to increase the danger significantly. While the avalanche danger is low below 2500’, there is still an unlikely chance that you could trigger a small wind slab in isolated pockets at the mid-elevations (1000’-2500’). Be wary of indicators of instability such as shooting cracks and whumpfing, and seek out slopes that have not been hit by the wind. They will have the best riding conditions anyway.

Cornices: Yesterday we noted debris from a cornice fall near Magnum, and similar activity will be possible today as the cornices adjust to the recent wind events. 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.

Cornbiscuit. Chunks of wind slab breaking on a small test slope. 12.14.2020

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

The weak snow at the ground continues to be a concern at elevations above 2500’, particularly in areas with shallower snowpack. Yesterday we got unstable test results in this layer (more info), and it is still on our radar for now. This is a tricky problem, because although it is unlikely you will trigger an avalanche like this, it could be very large if you do. Avoid steep and rocky terrain, and be extra cautious as you head farther south from Turnagain pass, where the snowpack is shallower and these avalanches will be easier to trigger.

If you find this type of problem particularly challenging and full of uncertainty, you are not alone! Tune in to our latest Forecaster Chat tonight to hear Aleph share some insight about deep persistent slab avalanches.

Weak facets like these at the bottom of the snowpack continue to give us cause for concern. Photo: Eric Roberts. 12.14.2020

Weather

Tue, December 15th, 2020

Yesterday: Easterly ridgetop winds were blowing 20-30 mph until they calmed down to 5-10 mph around 1 p.m. The mountains near Girdwood received 1-3” snow, while Turnagain Pass got a trace. Rain line rose to about 700 ft yesterday afternoon, with snow to sea level this morning. Temperatures reached the upper 20’s F at high elevations, and upper 30’s F at lower elevations.

Today: We will see periods of light snow throughout the day, with 1-2” expected by the end of the day. The rain line is expected to rise to 300 ft. Light westerly winds are expected, with speeds around 5 mph at the ridgetops. Mountain temperatures will reach into the upper 20’s F, with highs expected in the mid 30’s F at lower elevations.

Tomorrow: Light snowfall is expected to taper off through tonight, with only a trace expected by tomorrow morning. Temperatures will be in the upper teens F tonight at upper elevations, and in the low- to mid- 20’s F at middle and lower elevations, and will drop to the low teens to low 20’s F tomorrow. Light westerly ridgetop winds around 5-10 mph are expected tonight and tomorrow, with decreasing clouds through the day.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	32	2	0.1	59
Summit Lake (1400′)	29	0	0	24
Alyeska Mid (1700′)	32	3	0.4	61

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	23	E	12	49
Seattle Ridge (2400′)	27	E	7	22

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 Mon, May 15th, 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	Open	Chugach NF staff will be assessing daily after May 15th. Please keep machines on the snow and avoid brown spots to keep this area open as long as possible.
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.