Thu, February 15, 2018 Turnagain Pass Avalanche Forecast

Thu, February 15th, 2018

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

Persistent Slabs
Almost Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic

Very Large

Large

Small

Size

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.

Likelihood of Avalanches

This graphic depicts how likely you are to trigger avalanches or encounter natural avalanches while traveling on avalanche prone slopes. Unlikely means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. The chance of triggering or observing avalanches increases as we move up the scale. Certain means that humans will be able to trigger avalanches on many slopes, and natural avalanches should be expected.

Size of Avalanches

This graphic depicts the potential size and destructive force of expected avalanches. 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, large enough to bury or destroy vehicles and break a few trees, and large enough to destroy railway cars, buildings, or a substantial amount of forest. Historic avalanches are massive events capable of destroying villages and gouging or altering the landscape.

More info at Avalanche.org

Triggering a large persistent slab avalanche up to 2′ or more in depth remains possible across the region today. Below the storm snow and recent wind crusts/slabs sits the Jan 21st buried surface hoar that we have been talking about for a few weeks. This layer is roughly 1-2′ below the surface. With the recent wind and snowfall adding stress, a person skiing or on a machine may tip the balance and initiate a larger slab avalanche. Triggering a smaller wind slab or a cornice fall may also activate this layer. The snowpack is now at a point where no signs of instability are likely to be present before one of these avalanches is triggered. Assessing the terrain and the potential outcome of an avalanche breaking deeper in the pack is key. Are there terrain traps below you? Cliffs? Are your partners watching and rescue ready?

Snowpit near the Johnson Pass area that clearly shows the January 21st buried surface hoar.

Snowmachine triggered avalanche observed in the Johnson Pass area that likely occurred in the last week.

Avalanche Problem 2

Wind Slabs
Almost Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic

Very Large

Large

Small

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

Size of Avalanches

More info at Avalanche.org

Monday night and Tuesday morning, North and West winds wreaked havoc on much of the terrain around Turnagain, Summit Lake, Portage and the Girdwood Valley. Wind scalloped many snow surfaces, scoured some ridgelines to the rocks and sastrugi was reported on Tincan. Along with the winds, no natural avalanche activity was seen in the Turnagain area, but the Summit Lake region saw several shallow natural wind slab avalanches along with one on Fresno that appeared to step down into an older weak layer. Watch for wind slabs lurking intermixed with the variable surface conditions on the steeper slopes. Pay attention to where the snow feels stiff, looks pillowed, sounds or feels hollow and watch for shooting cracks. Hard wind slabs tend to break when you are out onto the slope and often fracture above you. Be aware of wind loading patterns in the terrain, especially cross-loaded gullies.

Wind effect and a wind slab on Maynard Mountain in Portage observed Tuesday.

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

Above 3,000′ in the Alpine zones, several old weak layers of facets and buried surface hoar sit near the ground and in the mid-pack. This structure is most pronounced in areas with a thin overall snow cover, such as the South end of Turnagain Pass and the Summit Lake area. As you plan your day, keep in mind that there are deeper persistent layers that could ‘wake up’ if you find the wrong spot.

Weather

Thu, February 15th, 2018

Yesterday was partly cloudy with very light evening snow showers in Girdwood. Temperatures were in the 20Fs and winds were easterly 5-15 mph with a few gusts into the 20s and 30s. Overnight temperatures were in the low 20Fs.

Today will be mostly sunny with temperatures in the low to mid 20Fs. There may be some valley fog. Winds will be northerly 5-15 mph with gusts into the 20s. Temperatures will drop into the teens overnight and skies will become partly cloudy.

Tomorrow will be partly to mostly cloudy. It is forecast to clear again for Saturday and then cloud up again Sunday. There is uncertainty about what next week will bring with either high pressure or a chance for snow in the forecast!

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	32	0	0	64
Summit Lake (1400′)	25	0	0	25
Alyeska Mid (1700′)	28	trace	0.02	59

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	22	E	5	23
Seattle Ridge (2400′)	24	SE	8	32

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
12/10/19	Turnagain	Avalanche: Tincan and Sunburst from the air	CNFAIC Staff
12/10/19	Turnagain	Observation: Tincan	Nancy Pfeiffer
12/08/19	Turnagain	Avalanche: Tincan	Ryan Van Luit Forecaster
12/06/19	Turnagain	Avalanche: Sunburst	Billy Finley
12/04/19	Turnagain	Observation: Sunburst	A.Johnston-Bloom/ W.Wagner/ R.Van Luit Forecaster
12/03/19	Turnagain	Observation: Hippy Bowl	Nick Langowski
12/01/19	Turnagain	Observation: Tincan, All elevations	Eric Roberts
12/01/19	Turnagain	Observation: Tincan	Andy Moderow
11/30/19	Turnagain	Observation: Tincan Treeline Plateau/ Common Bowl/ Ridge	Eric Roberts
11/29/19	Turnagain	Observation: Sunburst Ob #2	Aleph Johnston-Bloom Forecaster

Riding Areas

Updated Wed, December 11th, 2019

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.
Placer River	Closed	Closed.
Skookum Drainage	Closed	Closed.
Turnagain Pass	Closed	Closed.
Twentymile	Closed	Closed.
Seward District
Carter Lake	Closed	Closed.
Lost Lake Trail	Closed	Closed.
Primrose Trail	Closed	Closed.
Resurrection Pass Trail	Closed	Closed. Will be open for the 2019/20 season pending adequate snow cover.
Snug Harbor	Closed	Closed.
South Fork Snow River Corridor	Closed	Closed.
Summit Lake	Closed	Closed.