Tue, December 17, 2019 Turnagain Pass Avalanche Forecast

Tue, December 17th, 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

One glide avalanche reported in the Girdwood Valley yesterday.

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

Moderate winds have been consistent for the past several days, and will likely continue today at 10-20mph from the east. These wind speeds in combination with new snow in the alpine over the past three days may create wind slabs in the lee of ridges and gullies. Pay close attention to which direction the wind is blowing, and to signs indicating which direction the wind has blown in the past. Whumphing, cracking in the snow around you, and hollow sounds are some indicators you could be on a windslab.

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

Sunburst weather station data from the past week of windy weather.

Avalanche Problem 2

Glide Avalanches
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood

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

More info at Avalanche.org

Glide avalanches are unpredictable and not triggered by people. Yet there are several cracks in the Girdwood Valley and a few on Tincan in Turnagain Pass. Often, when one occurs that can be sign there could be more. There was a glide avalanche cycle in the Girdwood Valley at the end of last week and a few cracks have opened on Tincan. If you see signs of glide cracks give them a wide berth and limit time in their runout.

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

In the Summit Lake area and in parts of the advisory area that have an overall shallower snowpack in the Alpine, there could be suspect layers of weak faceted snow that sit either in the mid-snowpack or base of the pack. This is above 3000′, where the snow is dryer. We have limited data from upper elevation terrain and will looking to gain more information on this.

Weather

Tue, December 17th, 2019

Yesterday: Mostly to partly cloudy skies were over the region. Light rain in the morning and evening added between .2 – .4″ of rain below 1,500′ and 2-4″ of snow above this and into the Alpine. Ridgetop winds remained easterly and averaged in the 15-20 mph range. Temperatures have cooled slightly overnight and are sitting in the mid 30’sF at sea level and the mid 20’s F along the ridgelines.

Today: Again partly cloudy to cloudy skies are forecast with a chance for light rain showers below 1,400′ and snow above this in some areas. Favored locations could see 2-3″ of snow by tonight at the mid and upper elevations, other areas could see clearing skies. Ridgetop winds should stay in the 10-20 mph range from an easterly direction. Temperatures look to continue on a cooling trend with the snow line creeping down to just below 1,000′ overnight where light precipitation could bring another 2″ of snow.

Tomorrow: Intermittent snow showers (with a snow line around 1,000′) should continue through tomorrow until skies clear and we head into a cold clear period that extends through the weekend.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	33	2	0.3	32
Summit Lake (1400′)	33	0	0	7
Alyeska Mid (1700′)	33	2	0.36	17

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	24	NE	16	39
Seattle Ridge (2400′)	N/A*	N/A*	N/A*	N/A*

*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
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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