Mon, March 09, 2020 Turnagain Pass Avalanche Forecast

Mon, March 9th, 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

Winds today are forecast to be light and westerly and new wind slab formation is not likely during the day. The concern is triggering a wind slab that formed over the past couple days of easterly winds or an older one from the westerly outflow event last week. Wind slabs tend to heal up fairly soon after a wind loading event, especially with warmer temperatures. The question is whether this healing has happened or will these wind slabs actually stay reactive due to a buried layer of surface hoar and transition into a persistent slab issue. The wind slabs could be 1-2′ thick and will most likely be found in Alpine terrain on loaded slopes just off of the ridgelines and in gully features. As you travel in the mountains watch for signs of instability like cracking or collapsing. Steep terrain with hard wind affected snow over soft snow is the most suspect. Avoid terrain features with hard snow that look fat and loaded.

The question of the day is whether or not this weak interface on the buried surface hoar is an issue that heals up or lingers? 3.7. 20. Wolverine. Photo: Peter Wadsworth

The surface hoar that was buried on Leap Day.

Cornices: Avoid travel on or underneath cornices.

Loose snow avalanches: In areas that were protected from the wind, sluffs are possible in steep terrain.

Sun effect: If the sun pokes out today look for signs of sun effect on steep solar aspects: moist surface snow, small roller balls or loose snow avalanches in protected spots, especially below rocky areas.

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

There is nagging worry that someone will hit the wrong spot and trigger a very large avalanche on that fails on the weak faceted snow from January. This is still lurking 3 to 6 feet deep in the snowpack. The wind slab triggered in Lynx Creek last Thursday was a larger avalanche because it ‘stepped down’ into these old weak layers. As time passes the likelihood of triggering a deep slab is decreasing, i.e. is unlikely. However, the snowpack structure for this avalanche problem still exists and an outlier avalanche could be a nasty surprise. Areas that have a shallower snowpack are more concerning, places like Lynx Creek or Twin Peaks and south through Summit Lake. As always, use good travel protocol and as you choose your route consider the consequences if a deep slab was triggered.

Weather

Mon, March 9th, 2020

Yesterday: Skies were cloudy and there were light snow showers throughout the day with around 1-6″ accumulating, favoring upper elevation Girdwood. Temperatures were in the high teens to low 20°Fs in the Alpine and the high 20°Fs to high 30°Fs at lower elevations. Winds were easterly 5-15 mph with gusts into the 30s. Overnight snow showers stopped, temperatures dipped slightly and winds became light.

Today: Skies will be mostly cloudy with a chance of snow showers and/or a pocket or two of clearing in the afternoon. Temperatures will in the high teens to high 20°Fs depending on elevation. Winds will be light and westerly during the day. As skies clear overnight and cooler air moves in, northwest outflow winds will start to pick up. Overnight lows will be in the single digits.

Tomorrow: Mostly sunny skies and temperatures in the teens. Winds will be northwesterly 15-25 with gusts into the 30s and 40s. Winds look to peak during the day and then ease off overnight into Wednesday. Skies will be clear overnight with temperatures in the low single digits. The forecast is for clear and sunny with an overall warming trend through the spring break week! Time to bust out the sunglasses and sunscreen!

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	27	1	0.1	70
Summit Lake (1400′)	25	1	0.1	31
Alyeska Mid (1700′)	27	3	0.26	80

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	17	NE	10	33
Seattle Ridge (2400′)	22	E	7	19

Observations

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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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