Sat, December 24, 2022 Turnagain Pass Avalanche Forecast

Sat, December 24th, 2022

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

Active wind loading near Alyeska yesterday was producing large natural wind slab avalanches 2-3′ deep. Otherwise it has been about 9 days since we have observed avalanche activity on the layer of facets buried about 1.5-2′ deep.

Avalanche Problem 1

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

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.

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

It has been about 9 days since the last significant snowfall in our forecast region, but yesterday on Notch Mountain in Girdwood we were still able to trigger a large collapse in a weak layer of facets above the Thanksgiving melt freeze crust. This is pretty surprising since it has been a long time since that layer has been stressed with additional snow load and is an indication that human triggered avalanches 2-4′ deep are still possible on this persistent weak layer. The Girdwood valley and the southern end of Turnagain Pass seem to have weaker snowpack structure and be producing more concerning test results on the layer of facets compared to the rest of the forecast area. However, this weak layer exists across the forecast area.

Managing persistent slab avalanche problems requires patience and careful terrain management. These layers can easily last for weeks and even though the likelihood of triggering an avalanche is relatively low the consequences could be high because of the potential for triggering a very large avalanche. We recommend paying close attention for any collapses or whumphing in the area you are travelling and sticking to smaller terrain features and lower angle slopes. Remote triggering is also possible with persistent slabs, which means you can trigger an avalanche from low angle terrain adjacent to steeper slopes. Snowpit tests can be unreliable for deeply buried persistent weak layers, so just because you get no results from a stability test doesn’t mean there is no chance of triggering an avalanche.

Snowpack structure in the area where we triggered the collapse. A thick layer of facets about 1.5′ deep collapsed on top of the very strong layer of melt freeze crust. Photo 12.23.22

Spacing out on the uphill and only exposing 1 person at a time to avalanche terrain is a great way to mitigate the risk from persistent slab avalanches. Photo 12.23.22

Avalanche Problem 2

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

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

Isolated portions of the forecast area are receiving strong NE outflow winds, with gusts reaching 50 mph at the Alyeska summit weather station yesterday and lots of active wind loading. These fresh wind slabs were releasing naturally yesterday and could be possible for a person to trigger 2-3′ deep today in areas that have received recent wind loading. Luckily our weather stations indicate that the winds did not impact the majority of the forecast region, but it is worth being aware of the potential for lingering wind slabs at upper elevations along ridgelines and cross loaded gullies.

Small wind slab on the SW aspect of Notch. Much larger wind slabs are possible at higher elevations where the wind transport was significant in some areas yesterday. Photo 12.23.22

Weather

Sat, December 24th, 2022

Yesterday: Mostly cloudy with occasional periods of partly clear skies. Localized NE winds with averages of 15-20 mph and gusts to 50 mph in favored areas (Alyeska). Trace of new snow at several weather stations from the past 24 hours but no real accumulation. Temperatures decreased throughout the day starting in the teens Friday morning and dropping to single digits during the day.

Today: Partly clouds with temperatures remaining in the negative and positive single digits. Mostly light winds in the 0-10 mph range are expected but some areas may experience higher wind speeds based on the terrain channeling of the NE outflow winds. Yesterday areas around Alyeska and Seward were strongly favored with higher wind speeds. Those outflow winds should start to diminish during the day today.

Tomorrow: Our next storm system is expected to start on Sunday and bring significant snowfall to the forecast area, but there is still a fair bit of uncertainty in exact timing and snowfall totals. From Sunday morning to Monday morning 8-12″ of new snow is expected for Turnagain Pass and Girdwood with higher totals in coastal areas. The snow line is expected to move up to 1300′ on Sunday evening and Monday with rain expected below that elevation. Temperatures should increase into the 20s at upper elevations. Strong E winds will accompany the snowfall with averages of 20-30 mph and gusts of 50 mph. Snowfall is expected to continue through early next week.

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

	Temp Avg (F)	Snow (in)	Snow Depth (in)
Center Ridge (1880′)	9	trace	39
Summit Lake (1400′)	1	trace	NA
Alyeska Mid (1700′)	9	trace	38
Bear Valley – Portage (132′)	12	trace	–

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	2	NE	6	22
Seattle Ridge (2400′)	4	NE	3	14

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
01/29/23	Turnagain	Observation: Tincan Backdoor	AAS-Level 1 1/27-1/30
01/28/23	Turnagain	Observation: Sunburst	Brooke Edwards
01/28/23	Turnagain	Avalanche: Seattle Ridge	W Wagner
01/28/23	Turnagain	Observation: Tincan Common	Tony Naciuk
01/27/23	Turnagain	Observation: Sunburst	John Sykes
01/27/23	Turnagain	Avalanche: Lynx Creek	Megan Guinn / W Wagner
01/25/23	Turnagain	Observation: Cornbiscuit	John Sykes Forecaster
01/22/23	Turnagain	Avalanche: Tincan	Schauer/ Guinn
01/21/23	Turnagain	Avalanche: Seattle Ridge	Elias Holt
01/21/23	Turnagain	Avalanche: Lynx drainage – avalanche	CNFAIC Staff

Riding Areas

Updated Fri, January 06th, 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	Open	Opened Dec 13th.
Placer River	Closed	Closed Jan 5th due to lack of snow (holiday storms rained away the snow at sea level).
Skookum Drainage	Closed	Closed Jan 5th due to lack of snow (holiday storms rained away the snow at sea level).
Turnagain Pass	Open	Opened Dec 13th.
Twentymile	Closed	Closed Jan 5th due to lack of snow (holiday storms rained away the snow at sea level).
Seward District
Carter Lake	Open	Opened Dec 13th.
Lost Lake Trail	Open	Opened Dec 13th.
Primrose Trail	Open	Opened Dec 13th.
Resurrection Pass Trail	Closed	Closed to motorized use for the 2022/23 winter season per Forest Plan. Open next season.
Snug Harbor	Open	Opened Dec 13th.
South Fork Snow River Corridor	Open	Opened Dec 13th.
Summit Lake	Open	Opened Dec 13th.