Fri, December 02, 2022 Turnagain Pass Avalanche Forecast

Fri, December 2nd, 2022

Alpine

Above 2,500'

2 - Moderate

Treeline

1,000'-2,500'

1 - Low

Below Treeline

Below 1,000'

1 - Low

0 - No Rating

1 - Low

2 - Moderate

3 - Considerable

4 - High

5 - Extreme


Travel Advice	Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.	Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.	Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding, and conservative decision-making essential.	Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.	Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain.
Likelihood of Avalanches	Natural and human-triggered avalanches unlikely.	Natural avalanches unlikely; human-triggered avalanches possible.	Natural avalanches possible; human-triggered avalanches likely.	Natural avalanches likely; human-triggered avalanches very likely.	Natural and human-triggered avalanches certain.
Avalanche Size and Distribution	Small avalanches in isolated areas or extreme terrain.	Small avalanches in specific areas; or large avalanches in isolated areas.	Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas.	Large avalanches in many areas; or very large avalanches in specific areas.	Very large avalanches in many areas.

Recent Avalanches

The last known avalanche observations were natural wind slabs that occurred during last weekend’s outflow wind event on 11/25 to 11/26.

Avalanche Problem 1

Wind Slabs
N E S W NW NE SE SW

Aspect/Elevation
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

Lingering wind slabs that formed during yesterday’s NW outflow wind event are the primary concern for today. Those wind slabs likely formed up to 1′ deep on top of weak faceted surface snow, which can cause them to remain reactive for longer than normal. They are most likely to be found along upper elevation ridgelines, on convex features, and in cross loaded gullies. Keeping a close eye on the surface conditions around you and stepping out of the skin track to feel for stiff and hollow feeling wind transported snow is a good way to identify wind slabs in the terrain.

In addition to wind slabs, the surface snow has been loosing strength over the past week and turning into facets under the clear skies. We have observed some large skier triggered loose snow avalanches (aka sluffs) that could knock a person off balance in steeper terrain. Heads up to try and manage sluff by stopping and letting it go by you or skiing across the fall line to allow it to run beside you.

Mixture of weak surface snow (surface hoar and surface facets) that could be buried by a wind slab after yesterdays outflow wind event. Photo 11.30.22

Avalanche Problem 2

Persistent Slabs
N E S W NW NE SE SW

Aspect/Elevation
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

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

A layer of faceted snow on the ground at upper elevations continues to be on our radar, despite being over two weeks since the last avalanche activity on this layer. This persistent weak layer could produce avalanches 2-4′ deep, but it is currently unlikely for a person to trigger an avalanche on this layer. Part of the reason we are still concerned about this layer is that the snowpack depth at upper elevations remains thin and variable, with only 1-3′ of total snow depth in many areas. That means it would be relatively easy to find a spot where the weight of a person could initiate a failure on this persistent weak layer.

The other main reason is that we still don’t have a lot of information about the snowpack structure and general conditions from the higher alpine elevations of the forecast region. Some of our observations over the past week have indicated that this layer is gaining strength and others indicate that it is well preserved and just waiting for a new snow load to potentially become active again. We are seeking more information from high alpine areas to help fill our knowledge gaps, so please submit an observation if you are able to look for this layer at the base of the snowpack above 3000′!

Weather

Fri, December 2nd, 2022

Yesterday: NW outflow winds were the main weather feature yesterday. In areas exposed to gap winds, like along Turnagain Arm and higher elevations, wind speeds averaged in the 15-20 mph range with gusts up to 50 mph. However, most of the remote weather stations in the region saw much lighter winds, with averages of 5-15 mph and gusts to 20 mph, and were thankfully sheltered from the brunt of the outflow winds. Skies were clear and temperatures stayed in the 10-20 F range.

Today: Winds are expected to be light today, with averages in the 5-10 mph range. Skies will be mostly clear with some bands of high clouds possibly moving through the area. Another temperature inversion has set up, with current temperatures in the single digits at lower elevations and twenties at ridgelines. This inversion is expected to persist through Saturday.

Tomorrow: Saturday looks largely similar to Friday with light winds expected. Mostly clear skies with some bands of high clouds. Temperatures are expected to increase on Saturday to 15-25 F at lower elevations and 20-30 F at upper elevations. No precipitation is expected until Monday.

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

	Temp Avg (F)	Snow Depth (in)
Center Ridge (1880′)	13.5	23
Summit Lake (1400′)	6	15
Alyeska Mid (1700′)	13	20
Bear Valley (Portage) (132′)	12	–

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	17	WNW	9	20
Seattle Ridge (2400′)	15	N	4	13

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
10/27/24	Turnagain	Observation: Tincan	Michael Kerst
10/21/24	Turnagain	Observation: Turnagain Pass Road Observation	Trevor Clayton
10/19/24	Turnagain	Observation: Tincan – Below Todds Run	Andy Moderow
10/18/24	Turnagain	Observation: Taylor Pass	Eli Neuffer
10/15/24	Turnagain	Observation: Tincan Common	John Sykes Forecaster
10/14/24	Turnagain	Avalanche: Tincan	CNFAC Staff
05/13/24	Turnagain	Observation: Eddie’s, Sunburst, Seattle, Cornbiscuit, Pete’s South	H Thamm
05/13/24	Turnagain	Observation: Turnagain Pass non-motorized side	Amy Holman
05/12/24	Turnagain	Observation: Warm up Bowl	Tony Naciuk
05/07/24	Turnagain	Observation: Turnagain Pass Wet Slabs	A S

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