Fri, December 18, 2020 Turnagain Pass Avalanche Forecast

Fri, December 18th, 2020

Alpine

Above 2,500'

2 - Moderate

Treeline

1,000'-2,500'

1 - Low

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

Deep Persistent Slabs
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.

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

The mountains received a few inches of new snow yesterday, just enough to begin hiding some of the old tracks. Along with this, ridgetop winds bumped up slightly midday for a period, averaging in the 10-20mph from the east. This may have moved a little snow around along the ridgetops and a few shallow wind slabs could be found in exposed areas today. These are likely to be small and in the 6-8″ thick range at most. This is a ‘Normal Caution’ avalanche issue to look for, along with remembering to give cornices a wide berth and watching our sluff on steep sustained slopes.

What isn’t a Normal Caution avalanche issue is our Deep Persistent Slab avalanche concern. As recently as yesterday, we are still getting snowpit results pointing to the weak October snow near the ground failing. Granted, it’s taking a lot of force to fail, and it’s not failing in many areas, which are all good signs that this problem is slowly going away…. But, if a person or snowmachine hits just the wrong spot, it is not out of the question that a large dangerous avalanche could be triggered. The most concerning areas are: (1) steep slopes with an overall thinner snowpack, such as the south end of Turnagain Pass, Johnson Pass, Lynx Cr, Silvertip and in Summit Lake, and (2) a thin spot in the slab in steep rocky terrain and on unsupported slopes. If choosing to travel in this type of terrain, extra caution is recommended; know there could be a weak layer under you, expose only one person at a time, have your partners keep a close eye on you and have escape routes planned.

A classic ‘gray-bird’ Alaskan day yesterday. This photo shows some wind effect along the top of Manitoba Pk in the Summit Lake area. 12.17.20. Caleb Rauch.

Weather

Fri, December 18th, 2020

Yesterday: Overcast skies with light snow flurries were over the region. Most locations picked up around 2-3″ of very low density snow. Ridgetop winds increased into the 20’s mph during the day from the east with gusts to 32mph on Sunburst before quieting back down overnight. Temperatures were in the 20’sF at the mid and upper elevations.

Today: Mostly cloudy skies should remain over the region with a trace of new snow possible. Ridgetop winds have shifted to the northwest this morning and should blow 5-10 mph today before decreasing tonight. Temperatures will stay cool, in the teens along the upper elevations and upper 20’sF at sea level.

Tomorrow: A Special Weather Statement has been issued by the NWS for snowfall over much of Southcentral tomorrow. Only 3-6″ of snow is forecast from Girdwood to Seward, however it’s anticipated to effect vehicle traffic in many areas. Along with the snow, ridgetop winds should swing back around to the east and be relatively light, 10-20mph. Temperatures will remain cool, in the 20’sF at the lower elevations and teens at the higher elevations.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	21	2	0.2	61
Summit Lake (1400′)	21	1	0.1	25
Alyeska Mid (1700′)	19	2	0.2	61

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	18	E	12	32
Seattle Ridge (2400′)	19	SE	10	17

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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