Fri, January 05, 2018 Turnagain Pass Avalanche Forecast

Fri, January 5th, 2018

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

3 - Considerable

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

Persistent Slabs
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

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

Several weak layers are sitting under new snow from the New Years storm. Slab depths range from 15” – 30” with deeper amounts in the alpine, on the Northern end of Turnagain Pass, and in Girdwood. This poor set-up plus good visibility is the perfect recipe for human triggered avalanches today. Please understand there are several different dragons lurking in the snowpack right now. 1) a widespread layer of buried surface hoar that is showing signs of being reactive. 2.) weak facets near the ground in slide paths that released during the early December storm cycle, for example the SW face of Sunburst 3.) weak snow (basal facets) near the ground in the upper elevation (above 3,000’).

Rain fell during the end of the New Year’s storm in the lower and mid elevation zones, which has now formed crusts in the top foot of the snow pack up to 2000’. Trigging an avalanche is more likely in upper portion of the Treeline zone where the snow quality quickly improves. Keep in mind today:

Human triggered avalanches 1-3’ thick are likely above 2,000′
Several tracks may be on a slope before someone finds a trigger point and the whole slope release
Remote triggering from the top, side or bottom of a slope is possible
Avalanche size is expected to be large enough to bury and kill a person
The larger the slope the larger the avalanche

Additionaly, obvious signs of instability like “whumpfing” and shooting cracks may or may not be present. If winds increase above the forecasted 20mph, and you observe any blowing snow, this could form new wind slabs and add stress to the persistent slab problem.

Two different people traveling near our pit location at 2750′ experienced “whumpfing” due the buried surface hoar collapsing under the new snow.

–

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

In the Alpine, above 3,000’, weak sugary snow (basal facets) sits near the ground. The storms over the past few days have added additional load to slopes that already have a hard slab, 3-5+ feet thick. In the upper elevations, a human triggered, large and dangerous deep slab avalanches is still possible. This is a high consequence avalanche problem that is impossible to outsmart and can take a long time to heal. Keep this in mind if clear skies allow for travel into the Alpine. It is really important to remember that triggering an avalanche in the upper layers of the snowpack may then initiate a deep slab avalanche. Cautious route-finding is essential. This includes thinking about the remote trigger potential from below.

Weather

Fri, January 5th, 2018

Yesterday skies were mostly cloudy in Turnagain Pass with clear skies in Girdwood. Temperatures along ridgetops were in the low 20’s (F) and upper teens yesterday and overnight. Temps at sea level were in the high 20F’s overnight. Winds were light and variable and no precipitation was recorded.

Clear skies are expected today along with temperatures in the 20F’s at ridgtops and low 30F’s at sea level. Periods of valley fog are possible. Easterly ridge top winds could range from 5-20mph and no precipitation is expected.

Snow showers are possible this weekend, but not a lot of accumulation is expected. Temperatures could range from the low 20F’s at ridge tops to mid 30F’s at sea level. Easterly winds may bump up to moderate at times, 10-25mph on Saturday.

*Seattle Ridge anemometer is rimed.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	23	0	0	43
Summit Lake (1400′)	13	0	0	16
Alyeska Mid (1700′)	23	0	0	37

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	17	Variable	4	13
Seattle Ridge (2400′)	19	*n/a	*n/a	*n/a

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
04/16/24	Turnagain	Observation: Cornbiscuit	Krueger / Matthys Forecaster
04/15/24	Turnagain	Observation: Taylor Pass	John Sykes Forecaster
04/15/24	Turnagain	Observation: Seward Highway – Tern Lake to Portage	Joel Curtis
04/14/24	Turnagain	Observation: Sunburst	T. Olsen-Lund, R. Sullivan I. Adams
04/14/24	Turnagain	Observation: Turnagain Pass Snomo	K. Gallaher
04/13/24	Turnagain	Observation: Spokane Creek	Everet Megli
04/11/24	Turnagain	Observation: Tincan	John Sykes Forecaster
04/10/24	Turnagain	Avalanche: Seattle Ridge	Schauer/ Keeler Forecaster
04/10/24	Turnagain	Observation: Cornbiscuit South Face	Todd Kelsey
04/10/24	Turnagain	Avalanche: Tincan	N. Dumont J. Carlberg

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