Mon, December 16, 2019 Turnagain Pass Avalanche Forecast

Mon, December 16th, 2019

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.

Recent Avalanches

Avalanche in motion across the Arm observed from the Tesoro at 11:15 am, 12.15.19. Photo: Nancy Pfeiffer

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

Yesterday the mountains received another half an inch to close to an inch of water with precipitation favoring Turnagain Pass. Above around 1800′ this fell as snow. This added to the 1-3′ that fell at upper elevations over the past few days. The winds continued to be easterly 20-40mph, gusting into the 50s. Observers in the field noted wind transport and tender wind slab conditions in wind exposed terrain. Today wind speeds are forecast to slowly decrease and overall snowfall should be light but wind slabs will still be possible in steep leeward terrain. It will be important to look for signs of wind effect and pay attention to blowing snow. What direction is the drifting? Does the slope or gully look fat and pillowed? Feel for stiffer snow over softer snow, look for cracking and listen for hollow sounds. Is there cornice overhanging the slope you want to ski?

Cornices: Continue to develop – give them plenty of space to prevent triggering from above, and limit exposure when traveling in the runout.

Wind slab over a rime crust. 2700′ Tenderfoot. 12.15.19

Wind effect on Tenderfoot Ridge, 12.15.19

Triggering small wind slab and cracking on wind loaded test slope on Tenderfoot Ridge, 12.15.19

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

Stay on the lookout for glide cracks and glide avalanches. There was a glide cycle in the Girdwood Valley at the end of last week and a few cracks opening on Tincan. If you do come across glide cracks give them a wide berth and limit time underneath. Glide avalanches are highly unpredictable. Sometimes cooling temperatures are associated with release.

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

Keep in mind that in the Summit Lake area and in parts of the advisory area that have an overall shallower snowpack in the Alpine, we are concerned about weak faceted snow that sits under the new snow from the past week. This is above 3000′, where the snow is dryer. Before the storm cycles, faceted snow existed at the base of the snowpack and other layers of facets/crusts. We have limited recent data from upper elevation terrain.

Weather

Mon, December 16th, 2019

Yesterday: Cloudy skies with rain/snow showers throughout the day. Rain/snow line was around 1800′. Easterly winds were 20-40 mph gusting into the 50s. Temperatures were in the 40Fs at sea level and in the mid 20Fs at 3000′. Overnight skies remained cloudy with minimal precipitation and temperatures dropped slightly in the early morning.

Today: Cloudy skies and light rain/showers. Rain/snowline around 1700′ and 1-6″ of snow in the forecast. Temperatures will be in the high 30Fs at sea level and high 20Fs at ridgetops. Winds will be easterly 10-20 mph gusting into the 30s. Overnight temperatures are forecast to drop into the 20Fs, light snow showers will continue and winds will remain easterly and ease off.

Tomorrow: Continuing cloudy skies and rain/snow showers. Light east winds and temperatures in the low 30Fs to mid 20Fs. There is a cooling trend for the week with this quote in the National Weather Service long term discussion this morning, “At this point, Saturday looks brutally cold for Southcentral“. Stay tuned!

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	34	3	0.8	32
Summit Lake (1400′)	34	0	0.1	7
Alyeska Mid (1700′)	34	1	0.3	17

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	24	NE	26	60
Seattle Ridge (2400′)	NA*	NA*	NA*	NA*

*Seattle Ridge weather station is down and as soon as the weather clears we will get it up and running!

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
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
04/15/24	Turnagain	Observation: Taylor Pass	John Sykes Forecaster
04/15/24	Turnagain	Observation: Seward Highway – Tern Lake to Portage	Joel Curtis
04/14/24	Turnagain	Observation: Sunburst	T. Olsen-Lund, R. Sullivan I. Adams
04/14/24	Turnagain	Observation: Turnagain Pass Snomo	K. Gallaher
04/13/24	Turnagain	Observation: Spokane Creek	Everet Megli
04/11/24	Turnagain	Observation: Tincan	John Sykes Forecaster

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