Sat, December 12, 2020 Turnagain Pass Avalanche Forecast

Sat, December 12th, 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)

Avalanche Problem 1

Wind Slabs
N E S W NW NE SE SW

Aspect/Elevation
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.

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

I hate to say it, but it looks like a wind event is on the doorstep. Weather models are forecasting easterly ridgetop winds today to be in the 30-40mph range with stronger gusts and even increase into the 50’s mph tomorrow. If this verifies, we can expect fresh wind slabs to be forming. Winds also have a chance at making it into the trees, but we can cross our fingers it doesn’t ruin too much of the good riding conditions. Interestingly enough, only a few inches of snow at most should accompany the winds, so new slabs will primarily be composed of the existing loose snow on the surface.

For today- watch for active wind loading and any stiff snow over softer snow. Cracking in the snow around you is a sure bet you’ve found a slab. It should be relatively easy to suss out any new wind slabs as conditions should be changing in front of our eyes through the day. Areas spared from the winds have a generally LOW avalanche danger.

Winds transporting snow along the top of Tincan’s Hippy Bowl yesterday. 12.11.20. Photo: Eric Roberts

Plenty of loose snow on the surface to blow into slabs… The good news, the new crop of surface hoar will hopefully get blown over ahead of the next snowfall event. 12.11.20. Photo: Alan Abel

Avalanche Problem 2

Deep Persistent Slabs
N E S W NW NE SE SW

Aspect/Elevation
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.

Aspect/Elevation of the Avalanche Problem

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

In the higher Alpine elevations on the southern end of Turnagain Pass and in the Summit Lake area, we are still concerned about the weak facets on the ground. These areas have a much shallower and weaker snowpack than the core of Turnagain Pass. In order for an avalanche to be triggered, we need a slab as well as a weak layer. Elevations below 2,500′ the pack is littered with mid-pack crusts and is lacking that ‘slab’, at the higher elevations it’s not necessarily. Hence, if traveling in the higher terrain be suspect of a hard shallow snowpack over weak snow where a deep slab could be lurking.

Regarding the mid-pack faceted layer that sits 1-3′ below the surface (below the last storm snow from Dec 6-7), this continues to show bonding and good stability. This is a layer we were watching for the past few days.

Weather

Sat, December 12th, 2020

Yesterday: Mostly blue skies were over the region. Winds along the ridgelines were from the east in the 10-20mph range with gusts into the 30’s mph after sunset. Temperature were in the single digits in creek bottoms and warmed to near 20F at 1500′ above the inversion.

Today: Clouds, wind and some slow flurries are moving in as a front passes over associated with a low-pressure system in the Gulf. Ridgetop winds are expected to increase from the east up to 40-50mph with stronger gusts. Only a meager amount of snow is expected, up to an inch at most today and a couple inches tonight. Temperatures should remain just cool enough for snow to sea level.

Tomorrow: Strong easterly winds are expected to remain through tomorrow with little chance for precipitation. Skies do have a chance for some clearing along with the strong winds. A better chance for snow is headed in on Monday.

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

	Temp Avg (F)	Snow Depth (in)
Center Ridge (1880′)	19	62
Summit Lake (1400′)	10	24
Alyeska Mid (1700′)	21	57

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	17	E	15	37
Seattle Ridge (2400′)	19	SE	13	26

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.