Tue, March 20, 2018 Turnagain Pass Avalanche Forecast

Tue, March 20th, 2018

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

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

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

Yesterday’s weather was all over the place. Snow and rain in the morning and then sunshine, strong winds and cooler air moving into the region. There were a few dry loose avalanches and small wind slabs observed. Today continued wind loading may trigger natural wind slabs and/or add stress to the snowpack. It is also important to keep in mind that triggering a deep slab avalanche 2-4+ feet thick remains possible. On Saturday a snowmachiner triggered slab in Main Bowl of Seattle Ridge, the slope had tracks on it from the day before and was remotely triggered from below. Remember that over the past week many large human triggered avalanches and/or natural avalanches have released from Girdwood to Lost Lake. Some of these avalanches have been remotely triggered and some with skier/snowmachiners on the slope. Widespread buried surface hoar and facets have been well documented at all elevations under a thick, connected slab and finding the wrong spot could be deadly.

*With a deep slab problem it is important to remember no signs of instability may be present before a slope releases. It may be the 10th person onto the slope that finds the trigger point and slopes may be triggered remotely. It is crucial to visualize the consequences if the slope does slide. Are there terrain traps below? Bigger slope = Bigger avalanche. Thin spots near rocks and along ridgelines are likely areas to find the trigger point.

Avalanche triggered on Sunburst last week illustrates the trigger point that had a shallower snowpack near a rocky ridge.

Deep slab structure: 3 feet of snow over a buried weak layer.

Avalanche Problem 2

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

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

Strong Northwest winds blew 25-35 mph yesterday and gusted into the 70s on Seattle Ridge. Winds are forecast to remain elevated today and ramp up even more this evening into tomorrow. Be aware of wind slabs on a variety of aspects due to unusual wind loading patterns and cross loading. Smooth supportable surfaces where the snow is hollow sounding are suspect, especially if the slope is unsupported. Look for cracking and identify terrain features with a pillow-shaped look where triggering a wind slab could break above you. A fresh wind slab could step down to older snow in the snowpack and create a much deeper and more dangerous avalanche.

Wind loading yesterday.

Additional Concern

Cornice

Cornice

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

More info at Avalanche.org

Cornices are large and looming and wind loading can add stress. Give these an extra wide berth and limit exposure underneath them. A cornice fall could trigger an avalanche on a slope below.

Weather

Tue, March 20th, 2018

Yesterday started out with snow and a little rain (at lower elevations) and then the wind started to blow from the Northwest. The skies cleared and the sun came out. Temperatures were in the mid 20Fs to mid 30Fs. The NW winds were blowing 25-35 mph with gusts into the 70s. Overnight temperatures dropped into the low 30Fs and low 20Fs/high teens. Winds continued from the NW gusting into the 30s and 40s.

Today will be partly cloudy with slight chance of snow showers. Winds will remain elevated from the NW gusting into 50s. Temperatures will be in the 20Fs to low 30Fs. Tonight skies will be clear and temperatures drop into the teens and single digits. The NW winds continue gusting into the 40s.

Wednesday is forecast to be clear and sunny. The winds may be even stronger. High pressure continues until the weekend and then there looks to be shift back to an active weather pattern.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	30	2	0.2	83
Summit Lake (1400′)	25	2	0.1	33
Alyeska Mid (1700′)	28	3	0.28	78

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	17	NW	10	47
Seattle Ridge (2400′)	24	WNW	30	70

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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