Mon, February 06, 2023 Turnagain Pass Avalanche Forecast

Mon, February 6th, 2023

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

3 - Considerable

Below Treeline

Below 1,000'

3 - Considerable

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

There were no new avalanches reported yesterday, but it is likely there was some natural activity with the new snow and strong winds. Hopefully visibility will improve enough to get a better eye on things today.

Avalanche Problem 1

Storm 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

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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

Yesterday’s storm brought 6-10″ low-density snow to the area, with easterly winds blowing 30-50 mph and gusts getting up to 75 mph. The system is moving out this morning and we should see fairly quiet conditions by the middle of the day, but all of that wind and new snow will make it likely a person can trigger a large avalanche even as the weather is calming down. If you are out traveling in the mountains today, keep a few things in mind:

Temperatures were warming throughout the storm, stacking up warmer, heavier snow on top of colder, drier snow. This ‘upside-down’ setup may make it a little easier to trigger an avalanche within the storm snow.
The strong winds have been building reactive wind slabs, especially at upper elevations. Even though we only got 6-10″ snow, we can expect these slabs to be 1-2′ deep or deeper. Be extra cautious around steep slopes just below ridgelines, convexities, and steep gullies.
At the mid and lower elevations, the storm snow buried a surface crust that went up to somewhere between 1500′ and 2000′. We’re not quite sure how that interface will behave today, but it should be a layer to watch out for until we have a better idea of how well the new snow is bonding.

Avalanches are most likely during and immediately after a storm, which is exactly where we will be today. If the snow and wind hang around longer than expected today, expect to see the avalanche danger rising. In addition to those problems within the new snow, remember we have a problem layer buried about 2-4′ deep now that has the potential to make even bigger avalanches. More on this in problem 2 below.

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

We’ve been concerned with a layer of buried surface hoar for several weeks now. After yesterday’s storm, the layer is now buried somewhere around 2-4′ deep. This is the first loading event the layer has seen in over two weeks, and though it’s not a huge dump, it is pushing the needle towards instability. We should be treating this layer with a little extra caution for now, until we get a better idea of how it is responding to the new load. This is another good reason to tread lightly today, avoiding spending time on or below steep terrain.

This avalanche in the Summit Lake area failed on a weak layer in the upper snowpack and was likely triggered from low on the slope. Although this is outside of our advisory area, it is similar to the kind of activity that we can expect at Turnagain Pass and Girdwood today. Photo: Ryan McLaughlin, 02.04.2023

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

We haven’t forgotten about the problematic facet/crust combination buried 4-8′ deep in the snowpack. It is unlikely the new snow from the past 24 hours will be enough to significantly impact this weak layer. It is still unlikely a person will be able to trigger an avalanche this deep in the snowpack, but if you were to find a spot, it would be in an area with a thinner snowpack, and it could propagate across multiple terrain features on multiple aspects. Because of the small likelihood of triggering one of these very large avalanches, the more concerning issues for now still lie in the upper 2-4′ of the snowpack, and that will be enough to keep us out of big terrain for today.

Weather

Mon, February 6th, 2023

Yesterday: We picked up 6-10” low density snow, with snow down to sea level for most of the storm, but rain sneaking up to around 300’ by early this morning. Winds have been blowing 30-50 mph out of the east with gusts reaching 65-75 mph. Temperatures climbed up from the mid 20’s F yesterday morning to the low 30’s F by the afternoon.

Today: Snowfall is tapering off and should finish late morning to early afternoon, with another 1-3” expected at Turnagain Pass and up to 6-9” possible in Girdwood and snow line dropping back to sea level. Winds should blow out of the east at 25-35 mph until later this morning when they shift more southerly and drop down to 5-15 mph. Skies will be cloudy with some breaks in the clouds this afternoon. Temperatures should hover in the upper 20’s to 30 F during the day, dropping into the mid to upper teens tonight.

Tomorrow: We should see some breaks in the clouds tomorrow during the day, with high temperatures in the low to mid 20’s F. Winds will be out of the east at 5-15 mph with gusts of 10-20 mph. Chances for precipitation pick up overnight and into Wednesday, with another inch or two of snow possible down to sea level.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	28	10 (est)	0.8	74 (est)
Summit Lake (1400′)	22	0	0	34
Alyeska Mid (1700′)	26	8	0.6	69
Bear Valley – Portage (132′)	35	rain	0.9	–

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	19	ENE	33	73
Seattle Ridge (2400′)	23	SE	13	28

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
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
04/29/24	Turnagain	Avalanche: Turnagain aerial obs	Tully Hamer
04/27/24	Turnagain	Observation: Johnson Pass	Noah Mery
04/23/24	Turnagain	Observation: Turnagain Sunny Side	Travis SMITH
04/21/24	Turnagain	Observation: Bertha Creek	Anonymous
04/20/24	Turnagain	Avalanche: Spokane Creek	Schauer/ Mailly Forecaster
04/16/24	Turnagain	Observation: Cornbiscuit	Krueger / Matthys Forecaster

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