Wed, December 12, 2018 Turnagain Pass Avalanche Forecast

Wed, December 12th, 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

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

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

Incremental loading over the last three days and more observations have nudged the mid-elevation band to MODERATE danger for triggering a slab 1-3’ thick releasing on weak snow. Yesterday on Magnum Mountain widespread ‘whumpfing’ caused a group to turn around at 2300’. A reactive layer of facets between two crusts showed easy propagation and they had low confidence in continuing into steeper terrain. This is a good reminder that it is still early season and limited snowpack observations exist across the forecast zone. Areas with a thinner snowpack are more suspect for weaker and unstable snow including the Southern-end of Turnagain Pass, Summit Lake and Girdwood Valley.

In the Alpine on the Northern end of Turnagain Pass stability tests have been showing a strengthening snowpack. However keep in mind North and East aspects have a tendency to be thinner and there is a zone in the mid-elevations where the snowpack quickly transitions to shallower depths.

As always be on the lookout for red flag warnings like ‘whumpfing’, shooting cracks, new avalanche activity and any changes in weather. Today a 1-3″ inches of snow is expected and there is currently 4-8” of loose snow available for transport. Should ridgetop winds increase above the forecasted 5-15mph, keep an eye out for active wind loading. Any sign of increased winds could form wind slabs or add stress to a persistent slab.

One of three pits between 2000′ and 2300′ on Magnum’s NW shoulder that showed poor structure and propagation potential in stability tests

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

Yesterday several glide cracks were noted on the Southwest face of Sunburst below the weather station. 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.

These are the first glide cracks of the season and we hope the last. If you see any new glide cracks in other areas, snap a photo, and send us an observation HERE. This helps us provide better information in our forecasts.

Weather

Wed, December 12th, 2018

Yesterday: Snow showers ended yesterday morning and skies were partly cloudy. Temperatures fell throughout the day from the upper-20Fs into the low-20F’s. Overnight ridgetop temps have fallen into the teens F. Winds were light and variable.

Today: Light snow flurries started early this morning. A few inches of snow (1-3″) is possible today across our region at all elevations. Temperatures will remain in the low 20F’s at 1000′ and low-teens F near ridgetops. Light Westerly winds 5-10 mph are expected to increase 10-15mph later this evening.

Tomorrow: Temperature will remain in the low 20F’s to low teens. Partly sunny skies are in the forecast for tomorrow and light winds. Friday into Saturday looks like our next best chance for snow showers.

*Seattle Ridge weather station anemometer is 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′)	25	0	0	29
Summit Lake (1400′)	24	1	0.1	7
Alyeska Mid (1700′)	25	2	0.16	13

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	17	variable	2	8
Seattle Ridge (2400′)	23	*N/A	*N/A	*N/A

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
12/02/23	Turnagain	Observation: Seattle Ridge	Schauer / Keeler Forecaster
12/02/23	Turnagain	Observation: Tincan South Side	Anonymous
12/02/23	Turnagain	Observation: Eddies up track	Luc Mehl
12/01/23		Avalanche: Sunburst	John Sykes Forecaster
12/01/23	Turnagain	Observation: Eddie’s trees	Anonymous
12/01/23	Turnagain	Observation: Turnagain – God’s Country	Graham Predeger Forecaster
11/30/23	Turnagain	Avalanche: Tincan Trees	Kakiko Ramos-Leon
11/27/23	Turnagain	Avalanche: Tincan Ridge	Schauer/ Stiassny Forecaster
11/26/23	Turnagain	Observation: Road report: Slide with dirt on Repeat offender	Anonymous
11/26/23	Turnagain	Observation: Pete’s North	Ben Sullender

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