Sun, November 21, 2021 Turnagain Pass Avalanche Forecast

Sun, November 21st, 2021

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

No known avalanches reported yesterday.

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

Finding and triggering an older, or newer, wind slab on steep wind loaded slopes remains our main concern. Any wind slab found will likely be sitting on weak faceted snow, or even buried surface hoar, making them possible to trigger even several days after forming. The northwest outflow wind event last Wednesday (11.17) hit the mountains from one direction and yesterday a bump in easterly winds hit them from the opposite direction. Yesterday’s winds were 10-15mph with gusts near 30 for around 8 hours. Luckily, the heart of Turnagain Pass appears to have escaped much of the wind damage so far (as can be seen in the photo below), but not everywhere.

While the majority of the top 12-16″ of the snowpack is soft loose faceting snow, keep your eye out for areas that did see wind loading, for example where previous tracks are filled in or areas with smooth rounded and pillow-like surfaces. The classic signs of stiff snow over softer snow, hollow feeling snow, and cracks that shoot out from you should be easy to identify and hopefully avoid. Slabs could be up to a foot thick and most likely found along the higher exposed ridgelines.

Loose snow avalanches – sluffs: Watch your sluff. Skier triggered loose snow avalanches were noted yesterday on steep terrain without wind effect and sporting a loose snow surface.

Andrew assessing the upper two feet of the snowpack on Tincan around 3,000′ near the common up-track on Friday, 11.19.21. Varying degrees of soft and weak snow compromise the top 12-16″ of the pack. Under the soft snow sits the Halloween crust/dense snow. No wind slabs were found at this time in this location.

We are watching the development of the top layer of the snowpack because these cold and clear conditions are prime for not only surface hoar to grow on the surface, but for the faceting process to happen. Once we get another layer of new snow on top of this weak snow, we could be in for a change from relatively stable conditions seen so far this season, to unstable. When will this next snowfall hit us? Maybe over Thanksgiving, or even later according the weather model crystal ball.

Avalanche Problem 2

Glide Avalanches
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

Glide Avalanches

Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.

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

Despite the frigid temperatures, glide cracks are still oozing open and some have released into avalanches over the course of the last few days. The most recent known release in our forecast zone would be this one from around 2-3 days ago, pictured below on Seattle Ridge.

On the bright side, compared to a month ago, many cracks have slowed way down, have been buried by snowfall and look to be stopping. Staying cognizant of cracks and limiting exposure under them is key. We definitely do not want to be in the wrong place at the wrong time. These avalanches are extremely destructive, are not triggered by humans, and are completely unpredictable.

Glide avalanche on Seattle Ridge, released sometime late in the day on Thursday, 11.18 or early on Friday, 11.19.21.

Weather

Sun, November 21st, 2021

Yesterday: Cold and clear weather with mostly sunny skies was seen again yesterday. Winds bumped up from the east to the teens with gusts in the 30’s mph midday on Sunburst briefly before swinging around the west and blowing 5-10mph. Temperatures were between 0-15F along ridgetops and slightly colder in valley bottoms.

Today: Another cold and sunny day is on tap. Winds will remain west to northwest in the 5-10 mph range with gusts in the teens along ridgelines. An inversion is in place this morning with minus single digits in valley bottoms and 5-10F above along the ridgetops. Valleys should warm into the +0-10F midday.

Tomorrow: Cold and clear conditions look to persist tomorrow. The chance for snowfall is becoming slim to none as the low pressure to our south is moving quickly eastward. We may see some high clouds tomorrow and a small chance for snow Monday into the middle part of the week. It does look like temperatures will be on the rise, back to a more reasonable range in the +teens by Monday.

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

	Temp Avg (F)	Snow Depth (in)
Center Ridge (1880′)	7	41
Summit Lake (1400′)	-1	10
Alyeska Mid (1700′)	8	38

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	9	NE	13	33
Seattle Ridge (2400′)	10	var	6	15

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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