Sun, March 01, 2020 Turnagain Pass Avalanche Forecast

Sun, March 1st, 2020

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.

Avalanche Problem 1

Storm Slabs
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.

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

Temperatures increased yesterday afternoon to near 32°F at sea level as 4-8″ of new snow fell throughout the forecast region. With steady winds capable of transporting snow and gusting to 60mph, we’re likely to have new snow instabilities.

The new storm snow is falling on top of a layer of surface hoar formed 1.27-28. Although freshly formed storm slabs may be relatively shallow, if triggered they could take a rider or skier off guard and down slope. This may be a lower consequence avalanche concern, but keep in mind triggering a storm slab or wind slab could tip the balance and trigger our deep persistent slab concern discussed in problem 2.

Wind Slab: With moderate temperatures and new snow blowing around at the ideal wind speeds, formation of wind slabs on the lee aspects of ridges and gullies is likely. With buried surface hoar reported on all aspects and elevations, these could be touchy to trigger.

This surface hoar formed at all elevations and on all aspects on 2.27-28. This is now blown around or buried beneath yesterdays 4-8″ of new snow. 2.29.20 . Photo Andy Moderow

Cornices: As always, give cornices plenty of space and limit exposure under them.

Loose snow: “Sluffing” of loose snow is likely in steeper terrain and could entrain a rider.

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

With below freezing temperatures and incremental loads of snow, our deep persistent slab is slow to adjust. Weak faceted snow from January is still 3-6 feet below the surface. Triggering and becoming caught in a slab of this size would likely have fatal results. Resist complacency – now is a good time to make choices for yourself based on your own knowledge, risk tolerance, and skill set. 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 to trigger a large slab increases in shallower snowpack.
Areas with little traffic so far this year are also more likely for triggering a deep slab.
It can be the 10th or 15th person on a slope before it avalanches.
These slabs can be triggered remotely, from the bottom, top or side.
Sticking to the low angle terrain (slopes 30° and less) with nothing steep above are ways to avoid this hazard.

Weather

Sun, March 1st, 2020

Yesterday: Cloudy skies preceded a storm system producing 4-8″ of snow. Ridgetop winds increased to the 20-25mph range overnight with gusts to 60mph. Temperatures were in the teens to 20’s°F at most locations.

Today: Cloudy skies with snow accumulation of 1-3″ today. Temperatures are expected to range from teens to low 20’s °F. Winds are forecast to settle down from the west at 5 to 10 mph.

Tomorrow: Mostly cloudy skies with temperatures in the single digits to low 20’s °F. Winds expected from the west at 5 to 15 mph. Intermittent showers through the day with minimal accumulation expected.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	24	4	0.4	79
Summit Lake (1400′)	23	3	0.4	32
Alyeska Mid (1700′)	22	8	0.7	89

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	14	ENE	23	60
Seattle Ridge (2400′)	18	SE	15	34

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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