Fri, December 14, 2018 Turnagain Pass Avalanche Forecast

Fri, December 14th, 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. Dangerous avalanche conditions.	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

Overnight ridgetop winds have picked up into the teens with some gust in the 20’s mph. In the alpine there’s over a foot of low-density snow available for transport and another 3-6” of new snow in the forecast today. Triggering an isolated windslab on leeward or cross-loaded features will be possible, especially if you see blowing snow. Otherwise mild weather and cold temps have been keeping all this new snow as light dry powder. Should you go into steep terrain, pay attention to how much new snow is falling and if winds are moving it around. Shooting cracks will be an obvious clue windslabs are tender. Feel for punchy or upside down snow and keep in mind the consequences of the terrain should even a small rug get pulled out from underneath you. In areas where winds aren’t an issue loose surface snow could move faster and farther than expected.

Shooting crack on a wind loaded terrain feature on a NW aspect of Tenderfoot yesterday at 2500′.

Some larger fast moving point release sluffs were observed yesterday in steep terrain on Sunburst.

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

No new avalanche activity has been reported in our forecast zone since a wet and windy storm ended last weekend. Yesterday dozens of tracks could be seen in steep terrain which is a good sign of increasing stability. With that said there remains some uncertainty around weak snow within older layers of the snowpack, especially in Summit Lake, in Crow Creek Valley and the Southern end of Turnagain Pass where a thinner snowpack exists. The mid-elevation band is also more suspect where the snow quickly transitions to shallower depths. Observations this week have found a facet/crust combo 1-2’ below the surface in the mid-elevations (2000’ – 2700’.) Whumpfing and reactive stability tests were observed on Tuesday on Magnum’s NW shoulder. Rotten faceted snow near the ground in Summit Lake is also a concern especially with more snow expected over the weekend. Evaluated the snowpack and terrain as you travel and be aware that obvious clues like whumpfing or recent avalanche may not be present.

Snowpit on Tenderfoot shows a thinner snowpack where weak snow is sitting on the ground.

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 and another on the SW face of Tincan Proper. 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.

Weather

Fri, December 14th, 2018

Yesterday: Skies were clear and sunny. Temperatures were in the single digits (F) in the upper elevations and teens (F) near 1000′. Winds were light and picked up in the evening from the East 10-15mph with gusts in the 20s mph. An inch of new snow fell in Turnagain Pass overnight and a trace in Girdwood.

Today: Temperatures will gradually increase throughout the day into the mid-20’s at 1000′. Snow showers will start this afternoon with 3-6 € of snow possible and another 4-5 € overnight. East ridgetop winds are expected to be 10-15mph and build into the 20’s mph overnight.

Tomorrow: Temperatures will continue to increase into the mid-30’s (F) at sea level. Rain/snowline may be around 500′. Another storm is expected Saturday evening through Sunday morning. An active weather pattern is expected to persist into early next week.

*Seattle Ridge weather station anemometer has been 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′)	13	1	0.1	31
Summit Lake (1400′)	7	0	0	9
Alyeska Mid (1700′)	14	trace	0.07	17

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	4	ENE	7	31
Seattle Ridge (2400′)	12	*NA	*NA	*NA

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
11/16/23	Turnagain	Observation: Tincan Common	William Finley
11/14/23	Turnagain	Observation: Sunburst/Magnum	Adrian Beebee
11/14/23	Turnagain	Observation: Seattle Ridge	Schauer/ Creighton/ Ellis Forecaster
11/13/23	Turnagain	Avalanche: Tincan Trees	Eeva Latosuo
11/12/23	Turnagain	Observation: Sunburst	Schauer/ Creighton/ Nave Forecaster
11/12/23	Turnagain	Avalanche: Goldpan – avalanche	Mike Records
11/11/23	Turnagain	Observation: Tincan Common	Andy Moderow
11/11/23	Turnagain	Observation: Taylor Pass – Sunburst	Wadsworth / Stwertka
11/10/23	Turnagain	Observation: Tincan	Andrew Schauer Forecaster
11/10/23	Turnagain	Observation: Tincan Trees	Michael Kerst

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