Tue, December 18, 2018 Turnagain Pass Avalanche Forecast

Tue, December 18th, 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

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

Snowfall over the past several days continues to deepen our early winter snowpack and provide some excellent surface conditions. Girdwood Valley has been the most favored for snowfall and has seen over a foot in the past 48 hours, while Turnagain Pass has seen 6-10″. The Summit Lake zone has seen roughly 6-8″. Very little wind effect during this time has been observed and avalanche activity has been relegated to small pockets of storm slab, soft wind slab and loose snow sluffs. For today, another 1 to 3″ of light snow is forecast and will add to these ‘surface instabilities’.

If skies open enough for travel to the upper elevations today, keep an eye on the surface conditions. Watch for stiffer snow over softer snow, cracking in the snow and whumpfing (collapsing). The Girdwood Valley could harbor some foot thick pockets of storm slab or wind slab due to the higher recent snowfall amounts. Also, watch your sluff. Sluffs should start getting larger with the cooler temperatures.

A small storm/wind slab pocket was triggered yesterday in Tincan’s Common Bowl. Thank you to Drew Petrie for sending in his photo.

Avalanche Problem 2

Persistent Slabs

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.

More info at Avalanche.org

As we have been mentioning for a while now, in thinner snowpack zones such as Summit Lake and Crow Pass, we are tracking buried layers of facets, crusts and buried surface hoar that sit 1-3′ under the snow surface. These layers are most prevalent in the mid-elevations (2000’ – 2700’). Snow pit data and a lack of avalanche activity has been pointing to an unlikely chance for an avalanche releasing in these deeper layers. However, as we push out into more and more terrain, we need to keep in mind the snowpack could have some surprises lurking.

Snow pit on Tenderfoot, in the Summit Lake area, from Sunday shows buried surface hoar deep 2′ below the surface that will react with a lot of force.

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

Glide cracks are moving. We have had several reports that cracks are opening. Glide cracks we know about are on Sunburst’s SW face under the weather station, SW face of Tincan Proper, Gold Pan area (behind Cornbiscuit/Magnum) and a crack that did release in the Johnson Pass area. These cracks can release at any moment. They are not associated with human triggers and the best way to manage the hazard is to avoid being on or beneath slopes with cracks.

Weather

Tue, December 18th, 2018

Yesterday: Partly cloudy skies with some valley fog were over the region. Intermittent flurries overnight has added a trace to an inch of snow at Turnagain Pass and 1-2″ of snow in the Girdwood Valley. Ridgetop winds were easterly yesterday, 5-10mph, and have been light and variable overnight. Temperatures remain near 20F along ridgelines and the upper 20’sF at sea level.

Today: Mostly cloudy skies with a chance for 1 to 3″ of cold, low-density snow are on tap today. Ridgetop winds should be northerly in the 5-10mph range. Temperatures will remain winter-like, in the 20’sF at sea level and in the teens along ridgelines. Tonight, we may see a bump in snowfall rates with another 2-4″ of low-density snow possible with this light northerly flow.

Tomorrow: Light snow showers should cover the region Wednesday. Models are showing a low-pressure forming just south of Seward that looks to shift winds back to the east and pump in a bit of moisture. Snow numbers for tomorrow look to be in the 3-6″ range.

*Seattle Ridge anemometer (wind sensor) is rimed over and not reporting.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	21	trace	0	36
Summit Lake (1400′)	20	0	0	15
Alyeska Mid (1700′)	21	1-2″	.1 – .2	27

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	19	E	5	21
Seattle Ridge (2400′)	20	*no data	*no data	*no data

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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