Mon, February 18, 2019 Turnagain Pass Avalanche Forecast

Mon, February 18th, 2019

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

Triggering a 1-2′ thick slab avalanche on slopes 35 degrees and steeper is likely today. Wind loaded slopes are the most suspect. Yesterday the advisory area saw steady snowfall with an additional 4-10″ of snow (favoring Girdwood) and easterly wind loading throughout the day. Sunburst saw gusts into the 80s and Max’s into the 60s. Observers noted cracking and small test slopes being easily triggered later in the day. Visibility made Alpine observations difficult. The strong winds eased off in the evening and there was a break in the snowfall. Snow started again early this morning and is forecast to continue today throughout the advisory area. The old wind harden snow surface from before the storm and Saturday’s low-density snow will make it tough for the new snow to stick right away. ‘Upside-down’ snow was noted by observers yesterday. Today look for recent avalanches, shooting cracks and listen for whumpfs or hollow sounding snow. Choose terrain carefully. Cornices may be quite tender and should avoided.

Storm slab triggered on Sunburst at 1300′, 2-17-19. Photo: Elliot Gaddy

Storm slabs triggered on Tincan later in the day, 2-17-19. Photo: Ray Koleser.

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

New snow and wind loading have the potential to add stress to underlying weak layers. As we have been hammering home for weeks, in Turnagain Pass roughly 1-3′ below the snow surface sits a layer of buried surface hoar. Periphery zones such as Summit Lake and Johnson Pass harbor a poor overall snowpack structure with a variety of weak layers. Although these persistent weak layers have not been reactive lately, additional load may start to tip the balance. It is good to keep in mind that triggering an avalanche today could to step down into old weak layers and initiate a larger more dangerous slide. This is more likely in Summit Lake and the central Kenai mountains.

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 unpredictable, not associated with human triggers, and can release without warning at any time. The best way to manage this problem is to avoid traveling on slopes directly below glide cracks.

Weather

Mon, February 18th, 2019

Yesterday: Skies were obscured with snow falling throughout the day. Winds were easterly 20-40 mph gusting as high as 88 mph on Sunburst. Temperatures were in the 20Fs to mid 30Fs. Overnight skies were mostly cloudy and winds were easterly 5-15 mph gusting into the 20s.

Today: Mostly cloudy skies and snow showers, 3-8″ of snow forecast. Rain/snow line around 700′. Temperatures in the 20Fs to mid 30Fs. Winds will be southeasterly 5-15 mph with gusts into the 20s. Light snow showers continue overnight with temperatures cooling into the low 20Fs.

Tomorrow: Skies clearing in the early morning becoming mostly sunny. Winds shifting to the northwest and increasing to 20-30 mph with gusts into the 40s. Temperatures will be in the low to mid 20Fs. The next storm system is forecast to move into the area Wednesday afternoon.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	29	2	0.3	62
Summit Lake (1400′)	30	2	0.2	28
Alyeska Mid (1700′)	29	5	0.6	58

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	20	NE	20	88
Seattle Ridge (2400′)	25	E	13	38

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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