Mon, December 17, 2018 Turnagain Pass Avalanche Forecast

Mon, December 17th, 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

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

Expect storm slabs in terrain 35 degrees and steeper today, the bigger the slope the more potential hazard. Look for signs of instability. Slabs maybe quite soft and manageable but it will be important to ease into terrain and use small test slopes to get a sense of reactivity and depth. Yesterday’s storm brought periods of heavy snow to the advisory area with observers reporting snowfall at an inch an hour in the afternoon. Storm totals were in the 6″ to 1′ foot range favoring Girdwood Valley. Expect deeper snow in the upper elevation terrain. Observers on Tincan and Notch Mountain reported cracking and reactive shallow storm slabs in the afternoon. The storm snow was a bit more cohesive yesterday than the snow that fell prior. Today this instability may settle fairly quickly but should be approached with caution. Easterly winds gusting into the 30s yesterday may have also increased slab depth and slab cohesion, especially in the Alpine.

Loose snow avalanches: In areas where the snow does not act as a slab the sluffs may be quite large and pushy in steep terrain. These could knock you off your feet and carry you. Choose terrain carefully.

Storm slab on Notch Mountain, 12-16-18. Photo: Andy Moderow

Slab depth in the mid-afternoon (snow continued through the night). Notch Mountain, 12-16-18. Photo: Andy Moderow

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

We are tracking buried layers of facets and crusts that sit 1-3′ under the snow surface. These layers are more 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 deeper in the pack. However, observers yesterday on Sunburst were able to get propagation on weak snow near the ground at 2100′. Additional snow load especially in the Summit area could start to tip the balance. Evaluate the snowpack and terrain as you travel. The thinner the snowpack the more suspect it is. Observers keep finding spots that whumpf on facet/crust combinations reminding us not to forget about this potential avalanche hazard.

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 beginning to open up in many places, including Sunburst’s SW face under the weather station, SW face of Tincan Proper, Gold Pan area (behind Cornbiscuit/Magnum) and there was one glide avalanche 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

Mon, December 17th, 2018

Yesterday: Skies were obscured and it snowed throughout the day with periods of heavy snow in the afternoon and into the night. Temperatures were in the 20Fs. Winds were easterly 5-15 mph with gusts into the 30s.

Today: Mostly cloudy skies and snow showers are forecast for the day. Winds will be SE 5-15mph with gusts into the 20s. Temperatures will be in the 20Fs to low 30Fs depending on elevation.

Tomorrow: Skies will be mostly cloudy with a chance for snow, light winds and temperatures in the 20Fs. Tomorrow afternoon there is an increased chance of snow into Wednesday as a small low circulates in the Gulf. The pattern continues to be active into the weekend.

*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′)	23	6	0.3	37
Summit Lake (1400′)	20	6	0.4	17
Alyeska Mid (1700′)	20	10	1.01	29

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	21	NE	9	35
Seattle Ridge (2400′)	24	*no data	*no data	*no data

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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