Fri, December 24, 2021 Turnagain Pass Avalanche Forecast

Fri, December 24th, 2021

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

2 - Moderate

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.

Avalanche Problem 1

Wind Slabs
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.

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

Strong northwest outflow winds are still at work this morning building fresh wind slabs 1-2′ deep that will be easily triggered today. Winds have been blowing 20-35 mph since yesterday morning, with gusts as high as 76 mph near Girdwood, 56 mph near Turnagain Pass, and 68 mph near Summit Lakes. As of this morning weather stations are still showing sustained westerly winds at 15-20 mph, which are expected to continue for most of the day before calming this afternoon. Recently wind loaded slopes will remain touchy through the day.

There are a few nuances to consider with these strong northwest winds:

Although the general direction is out of the northwest, we tend to see channelized flow through local terrain, with lower level winds blowing up the Six Mile Creek drainage, then wrapping around the south end of Turnagain Pass. This usually leads to southerly winds on the skier side of the pass, with northerly winds over Seattle Ridge.
It is looking like Seattle Ridge and Summit Lakes will see the strongest winds today, which means it will be more likely to find sensitive wind slabs in these areas.
It will be possible to travel out into the middle of a stiff wind slab before triggering it, increasing the chance of getting caught if you do trigger something.

Today it will be important to identify and avoid recently wind loaded slopes. The most likely places to find these will be in gullies, below convexities, and below ridgelines. With the winds blowing as hard as they have been, keep in mind wind slabs are likely forming further below ridgelines than you might normally expect. Keep an eye out for the telltale signs of instability- shooting cracks, collapsing, and new avalanche activity. It is also important to remember that a relatively small wind slab avalanche could trigger a big avalanche on weak snow lurking below the surface. More on this in problem 2 below.

Snow blowing off the ridge at Taylor Pass yesterday, loading the east side of the pass. 12.23.2021

Avalanche Problem 2

Deep Persistent Slabs
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.

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 are slowly starting to gain some confidence in the weak snow buried 2-5′ deep in most areas in the forecast zone, but we aren’t in the clear just yet. This round of strong winds is putting a heavy load on some slopes, which adds stress to those weak layers and nudges the needle just a little higher up the likelihood meter for today. As mentioned previously, a relatively small wind slab triggered near the surface has the potential to step down to these deeper persistent weak layers.

The most likely places to run into trouble with a deep persistent weak layer are areas with a thinner snowpack. You can think about this on a regional scale, with thinner conditions on the south end of Turnagain Pass towards Lynx Creek, Silvertip, and Summit Lakes. You can also consider slope-scale differences, with likely trigger points at thin spots on a slope, maybe with some rocks sticking out. These avalanches are becoming more difficult to trigger, but I wouldn’t feel comfortable stepping out into bigger terrain just yet, especially on the heels of a big wind loading event.

Click here to view yesterday’s snowpack summary from Taylor Pass if the video doesn’t load in your browser.

Weather

Fri, December 24th, 2021

Yesterday: We were hit with a round of strong northwest winds, with sustained speeds of 15-35 mph and gusts to 76 mph near Girdwood and 56 mph near Turnagain Pass. High temperatures reached the low 20’s to low 30’s F under partly cloudy skies with no recorded precipitation.

Today: Winds continue to blow 15-20 mph out of the northwest and are expected to continue until this afternoon. High temperatures are expected in the upper teens to low 20’s F. Skies are expected to be mostly clear, with increasing cloud cover late this afternoon. No precipitation is expected today.

Tomorrow: We might see a trace of snow overnight, with low temperatures in the mid teens F. Highs tomorrow will be in the upper teens to low 20’s F, with light westerly winds and mostly cloudy skies. No precipitation is expected during the day tomorrow.

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

	Temp Avg (F)	Snow Depth (in)
Center Ridge (1880′)	27	69
Summit Lake (1400′)	23	25*
Alyeska Mid (1700′)	26	42

*Estimate. No data from the snow depth sensor at the Summit Lake station.

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	17	NW	17	56
Seattle Ridge (2400′)	23	NNW	19	46

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
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
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

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