Mon, December 07, 2020 Turnagain Pass Avalanche Forecast

Mon, December 7th, 2020

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

3 - Considerable

Below Treeline

Below 1,000'

3 - Considerable

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

Storm Slabs
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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

With the active weather pattern continuing and the possibility of natural avalanches, today is a day to stick to low angle terrain and avoid avalanche runout areas. Yesterday morning brought a pulse of 3-6″ of light fluffy snow that was followed by rising temperatures, heavier snow and rain overnight (with rain/snowline around 1500′).

24 hr storm totals:

Turnagain Pass at 1,880′: 0.7″ of water equivalent, roughly 8-12″ of snow above treeline.
Girdwood Valley at 1,700′: 1.31″ of water equivalent, roughly 1-2′ above treeline.
Portage Valley at sea level: 1.23″ of water equivalent, roughly 1-2′ of snow above treeline.
Summit Lake at 1,400′: 0.3″ of water equivalent, roughly 5-8″ of snow above treeline.

With heavier snow falling on lighter snow and the old surface snow being weak, expect storm slabs to be sensitive today. In addition, below 2500′ this will all be resting on top of the rain crust. This could easily act as a bed surface with the weak snow sandwiched between it and the storm snow. Observers yesterday noted snow not bonding well to the crust. Steady ridgetop winds will have transported snow to leeward terrain and created touchy wind slabs. Cornices will also be larger and more sensitive to triggering today. Look for roller balls and loose snow avalanches at low elevations as rain falls onto dry snow. With all these storm related avalanche issues today use good travel protocol, avoid small terrain trap features and look for signs of instability.

Red flags to watch for:
– Recent avalanches, from yesterday or today?
– Whumpfing (collapsing) of the snowpack.
– Shooting cracks, likely to be seen near ridgelines where the wind has formed wind slabs.

Corniced ridgeline on Notch Mountain, 12.6.20. Expect cornices to be sensitive today.

Sparkling surface hoar and small near-surface facets are now buried at the new snow/old snow interface and could be quite reactive today. Photo: Manitoba. 12.5.20

The stout rain crust to around 2500′ has the potential to act as bed surface. Crust @ 2400′, Notch Mountain. 12.6.20

Avalanche Problem 2

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

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

Another reason to keep it mellow today is that once you travel into terrain where the stout rain crust ends (around 2500′), we cannot rule out the potential for large avalanches failing at the ground. Weak, faceted snow still exists at the bottom of the snowpack. Saturday’s snowboarder-triggered avalanche on Silvertip is reminder of this lingering deep slab avalanche problem. Snowfall and wind-loading will add stress to the snowpack and an avalanche failing in the in the upper snowpack or a cornice fall could make these more sensitive today.

Cornice triggered deep slab in upper Bertha Creek (Gold Pan). Photo from 12.4.20 by Alan Abel. Cornice falls today could trigger deep slab avalanches.

Weather

Mon, December 7th, 2020

Yesterday: Skies were overcast with snow falling throughout the day, turning to rain at lower elevations in the late afternoon. Winds were easterly 15-25 mph gusting into the 40-50s. Temperatures started in the teens and rose to the high 20°Fs to mid 30°Fs. Overnight snow and rain continued. Temperatures were in the high 30°Fs at sea level and mid to high 20°Fs in the alpine. Ridgetop winds remained easterly and steady, averaging 10-20 mph and gusting into the 40s.

Today: Rain and snow continue today with another 5-10″ of snow (0.5 inches of H2O) forecast to fall. Temperatures will be in the high 20°Fs to mid 30°Fs depending on elevation. Rain/snowline could be as high as 1500′ and should lower this evening. Winds remain easterly 10-20 mph gusting into the 40s. Rain and snow continue tonight becoming all snow with temperatures decreasing to the 20°Fs and winds easing off.

Tomorrow: Tuesday is forecast to be mostly cloudy with snow showers, light east winds and temperatures in the 20°Fs. The active pattern looks to persist this week but there is still some uncertainty in the details. As always think cold thoughts! #snowtosealevelplease

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	31	8	0.7	63
Summit Lake (1400′)	30	3	0.3	24
Alyeska Mid (1700′)	31	10	1.31	60

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	24	NE	24	56
Seattle Ridge (2400′)	26	E	9	23

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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