Mon, March 02, 2020 Turnagain Pass Avalanche Forecast

Mon, March 2nd, 2020

Alpine

Above 2,500'

2 - Moderate

Treeline

1,000'-2,500'

2 - Moderate

Below Treeline

Below 1,000'

2 - Moderate

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

Watch for wind slabs today on steep leeward slopes and gullies formed by the storm, Saturday into Sunday morning, that brought snow (6-12″) and steady east winds. There was weak snow on the surface prior and observers yesterday noted poor bonding in wind affected terrain. In addition, watch for an uptick in west/northwest winds today that may also transport snow and load opposite aspects. Look for cracking and collapsing and watch out on slopes that look pillowed or fat. Even a small wind slab in steep terrain can take you for a bad ride.

Storm slabs and Loose snow avalanches: On steep slopes protected from wind effect, watch for either very soft storm slabs or sluffing.

Cornices: Avoid travel on cornices and limit exposure under them.

Sun effect: It’s the time of year that sun can start to be an issue. If skies clear at all today watch for changing surface conditions on steep solar aspects.

Wind effect looking up towards Eddies proper and the low angle north bowl. 3.1.20. Photo: Andy Moderow

Storm snow failing in a shovel tilt test on the 2/29 buried surface hoar. 3.1.20. Photo: Andy Moderow

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

If you are headed out to the mountains today, don’t forget there is still a chance of triggering a slab that fails 3-6+ feet deep on the buried weak snow that formed in January. The likelihood is decreasing but the concern is lingering. The consequences of hitting the wrong spot are not to be taken lightly. As always it is important to use good travel protocol. Expose one person at a time, avoid terrain traps and look at slopes at avalanche paths. If the slope does slide where would you end up? We have been including the list below of things to keep in mind for many days now and you may have a bit of message fatigue but give it another look and respect the lurking deep slab dragon.

Things to keep in mind:

The snowpack can feel ‘stable’ and no signs of instability may be present before a deep slab releases.
The likelihood of triggering a large slab increases if you find shallower spots in the snowpack and near rocky areas.
Areas with little traffic so far this year are also more likely for triggering a deep slab.
Slopes may already have tracks on them and then still release.
These slabs can be triggered remotely, from the bottom, top or side.

Weak snow under the slab at 1600′. There is still a chance of triggering an avalanche that fails in the buried weak snow! 2.29.20. Photo: Andy Moderow

Weather

Mon, March 2nd, 2020

Yesterday: Skies were were partly to mostly cloudy with light snow showers starting in the evening continuing overnight with around an inch of accumulation. Winds were westerly 5-15 mph gusting into the 20s. Lower elevation temperatures were in the high 20°F to low 30°Fs and upper elevations were in the low 20°Fs to high teens during the day. Temperatures cooled slightly overnight.

Today: Skies are forecast to be mostly cloudy but there may be some clearing in the afternoon. There is a chance of snow showers. Winds will be westerly 5-15 mph gusting into the 20s and increasing in late afternoon gusting into the 30s. Winds will remain gusty overnight. Temperatures will be in the 20°Fs with a cooling trend during the day. Overnight temperatures will be in the single digits to below zero and skies will be partly cloudy.

Tomorrow: Mostly to partly cloudy skies. Temperatures in the teens and single digits with moderate northwest winds. Partly cloudy skies overnight clearing into Wednesday. Sunshine and cold temperatures are in the forecast through Friday.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	28	1	0.1	77
Summit Lake (1400′)	27	2	0.2	32
Alyeska Mid (1700′)	27	2	0.2	88

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	18	SW	8	27
Seattle Ridge (2400′)	18	*NA	*NA	*NA

*Seattle Ridge anemometer (wind sensor) is rimed over and not reporting.

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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