Mon, February 08, 2016 Turnagain Pass Avalanche Forecast

Mon, February 8th, 2016

Alpine

Above 2,500'

2 - Moderate

Treeline

1,000'-2,500'

2 - Moderate

Below Treeline

Below 1,000'

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

After a short break between storms over the weekend, we have another small system over us today that is a bit more ‘blow than snow’. So far only a couple inches of snow has fallen at Turnagain Pass but up to 6″ in the Girdwood Valley; we are expecting another 2-5″ to fall through the day above 1,000′. Ridgetop winds are the big player today and have increased into the strong category from the East. Add these winds to the new snow, and the existing soft snow available for transport, and we have a wind slab avalanche problem.

If you are headed out today, watch for areas of wind drifted snow and/or slopes that the winds are actively depositing snow. Traveling over wind drifts on gentle slopes and watching for cracks that shoot out from you is a great way to assess if these wind slabs are not bonding well and could avalanche if the slope is steep enough. Quick hand pits in these areas (stiffer snow over softer snow) are good tools as well.

CORNICES:
Don’t forget, cornices are still falling and the weight of a person and/or snowmachine could help tip the balance. Give these guys a wider berth than you may think necessary along the ridgelines; many have grown to the point where it hard to determine where the ground ends and the cornice begins.

Photo Left: Northeast winds loading Southwest slopes late in the day yesterday, Magnum Ridge. Photo Right: Wind transport over Taylor Pass.

Avalanche Problem 2

Glide Avalanches
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

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.

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

Two new glide avalanches were seen yesterday in the forecast region. The first was on Pete’s South and the second was in Skookum Valley (side note: if you are not familiar with the names at Turnagain Pass check out this map – maps are found under the resource tab!). Avoiding and limiting time under glide cracks is essential as these avalanches will destroy anything in their path. Many many cracks still litter the mid-elevation band at Turnagain Pass and surrounding regions.

Pete’s South glide avalanche pictured below, before and after the crack released. Photos Tim Glassett. Additional photos can found HERE.
Left, Glide Crack 1:37pm Sat. 2/6 Right, Crack released and avalanched sometime before 11:40am Sun. 2/7

Skookum glide avalanche, this slide can easily be seen from the Seward Highway as you pass the Portage Valley road.

Additional Concern

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

We have been concerned about a layer of weak older snow that sits roughly 2′ below the surface. This older snow is essentially last week’s surface that became loose and started to facet before it was buried by 2-3′ of storm snow Feb 3-5th. Over the weekend, we have found that this old snow/new snow interface has bonded very well – great news. Report from yesterday’s investigation HERE. However, we have little to no data for areas South of the Pass, such as the high elevations above the Johnson Pass trail or Summit Lake. Hence, there may be areas that did not bond well and if this is the case large and ‘connected’ avalanches will be possible.

Weather

Mon, February 8th, 2016

Partly cloudy skies and light winds covered the region yesterday morning. By the early afternoon, a weak low-pressure system began to move in bringing cloud cover and a bump in Easterly winds (~25mph averages) along the ridgetops. Temperatures remained mild in the 20’sF at the mid-elevations.

Overnight, we have seen an inch of new snow at Turnagain Pass with up to 6″ in the Girdwood Valley. The rain/snow line is currently ~500′ and may rise to 1,000′ by later today. We are expecting an additional 2-5″ at the higher elevations by 6pm. Winds will continue in the 20-30mph range from the East along the ridgetops and temperatures are expected to rise to the mid 30’s at 1,000′ and the mid 20’s at 3,500′.

Tuesday looks to be a short break in weather before a much warmer and wetter system moves in from the South for Wednesday. We could see rain above 2,000′ with this next round of storms mid-week.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	28	1	0.1	109
Summit Lake (1400′)	29	0	0	32
Alyeska Mid (1700′)	30	6	0.5	87

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	22	NE	22	53
Seattle Ridge (2400′)	24	–	–	–

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
10/27/24	Turnagain	Observation: Tincan	Michael Kerst
10/21/24	Turnagain	Observation: Turnagain Pass Road Observation	Trevor Clayton
10/19/24	Turnagain	Observation: Tincan – Below Todds Run	Andy Moderow
10/18/24	Turnagain	Observation: Taylor Pass	Eli Neuffer
10/15/24	Turnagain	Observation: Tincan Common	John Sykes Forecaster
10/14/24	Turnagain	Avalanche: Tincan	CNFAC Staff
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

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