Thu, December 13, 2018 Turnagain Pass Avalanche Forecast

Thu, December 13th, 2018

Alpine

Above 2,500'

2 - Moderate

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

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

Yesterday morning an unexpected sleeper storm was centered over Girdwood and dumped 6-12” of very low-density snow. Snow totals were greater in the alpine. Turnagain Pass received 3” along the road and up to 10” at higher elevations. Winds in the afternoon along ridgetops picked up for a few hours 10-15mph with some gusts in the 20s-40s mph. Triggering an isolated windslab on leeward and cross-loaded features will be possible. Look for smooth or pillow shaped features. Evaluate the snow and terrain as you travel and keep in mind the consequences should even a small rug get pulled out from underneath you.

In areas where winds didn’t change the surface expect new snow to be loose and unconsolidated. Don’t be surprised by ‘sluffing’ and loose snow moving faster than expected.

Low density loose snow was sluffing easily on steeper features yesterday at Tincan. Photo credit: Tully LaBelle-Hamer

Avalanche Problem 2

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

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 remains some uncertainty around weak snow within older layers of the snowpack. Observations this week have found a facet/crust combo 1-2’ below the surface in the mid-elevations (2000’ – 2700’.) Large collapses “whumpfs” were experienced on Magnum’s NW shoulder on Tuesday and stability tests were quite reactive in this zone. It’s unknown how intact this structure exists into the Alpine. On the Northern end of Turnagain Pass stability tests have been showing a strengthening snowpack at higher elevations. However keep in mind that we’ve seen a lot of snow over the last week and we don’t have a lot of snowpack info. It’s still early season. Areas with a thinner snowpack are more suspect for weaker and unstable snow including the Southern-end of Turnagain Pass, Summit Lake and Girdwood Valley. In general North and East aspects have a tendency to be thinner and there is a zone in the mid-elevations where the snowpack quickly transitions to shallower depths.

Be on the lookout for red flag warnings like ‘whumpfing’, shooting cracks, new avalanche activity or any changes in weather. Keep in mind there is a lot of snow available for transport. Any sign of increased winds could form reactive wind slabs or add stress to a persistent slab.

This is a good example of where the snow quickly transitions shallow and weaker snowpack in the mid elevations. This structure may be more widespread on the Southern end of Turnagain Pass and in Summit Lake.

Additional Concern

Glide Avalanches

Glide Avalanches

Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.

More info at Avalanche.org

The first glide cracks of the season were seen on Sunburst SW face under the weather station. A glide crack is the snowpack being pulled by gravity downhill along the ground. They can release at any moment without warning and are usually not associated with human triggers. The best way to manage this hazard is to avoid being on or beneath any slopes with cracks opening up.

Photo taken on Tues December 11, 2018 of new glide cracks on SW face of Sunburst

Weather

Thu, December 13th, 2018

Yesterday: Snow flurries were the most intense in the morning between 6am to 10am. Turnagain received 3 € of low-density snow along the road and Girdwood 6-10 € at valley bottom. Winds were light with the exception of a few hours along ridgetops where winds increased to 10-15mph with gusts in the 20-40s. Temps at sea level were in the low 20Fs and ridgetops dropped into the single digits.

Today: Temps will be similar with single digits at ridgetops and low-20Fs near sea level. Snow flurries will diminish becoming mostly sunny in the afternoon. Winds are expected to be light (5-10mph) from the West and shift to the East by this evening. Tonight snow flurries will return with a few inches possible overnight.

Tomorrow: An active winter weather pattern will persist tomorrow and into the weekend. A continuation of more snow showers, cool temps and light to moderate winds is expected.

*Seattle Ridge weather station anemometer is rimed and not recording wind data.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	18	4	0.1	33
Summit Lake (1400′)	15	3	0.3	10
Alyeska Mid (1700′)	17	7	0.11	20

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	8	ENE	5	30
Seattle Ridge (2400′)	14	*N/A	*N/A	*N/A

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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