Wed, January 13, 2021 Turnagain Pass Avalanche Forecast

Wed, January 13th, 2021

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.

Recent Avalanches

With a break in the precipitation yesterday afternoon, there was an opportunity to see some of the avalanche activity from the storm. Evidence of large natural avalanches was observed across the forecast zone. A few notable slides listed below:

Seattle Ridge: one above Bertha campground and one just south of the moto lot.
The south end of the west face of Magnum stopping before the power lines
A 10′ crown on a lower slope of Twin Peaks.

Avalanche on Seattle Ridge observed from the moto lot. 1.11.21.

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

We finally have a quick break in the onslaught of heavy precipitation before the next storm starts early tomorrow morning. There is SO much new snow (6-10′) which is incredibly appealing but still potentially very dangerous. It’s a day to carefully evaluate terrain and consequences if a large avalanche does release. Low angle slopes without steep slopes above are great ways to enjoy the new snow without worry. As the snow settles it may become easier to travel but may also mean the slab is becoming more consolidated and may still be sensitive to human triggers. The old snow surface before all the snow fell had a thin layer of weak snow. In addition, there were a few temperature fluctuations throughout the storm cycle that could have also created weak interfaces. There have been large natural and explosive triggered avalanches during the storm with crowns ranging from 2-15′ deep. These are not the type of avalanches you want to mess with. We are hopeful that the new snow will eventually bond well and that we can enjoy it. However, we need more data after the storm and the snowpack needs more time to stabilize. As said many times before the snowpack is guilty until proven innocent and it is still likely that a person could trigger an unsurvivable avalanche today. We are in the 48 hour window after a storm where this is most common.

Signs of instability to watch for today:
– Recent avalanches
– Whumpfing (collapsing) of the snowpack, sure sign to avoid avalanche terrain period.
– Shooting cracks, likely to be seen near ridgelines and on slopes where the wind has formed wind slabs.

Avoid travel on or under cornices as they may be quite tender today and often break farther back than expected.

Turnagain Pass DOT snow stake, 1.12.21. The mountains have received a huge snow load in the past week. Please be patient and let the snowpack adjust.

Another view of the avalanche shown above in recent avalanches. Note the deep crown on the looker’s left. Seattle Ridge, 1.12.21.

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

There is another tricky element to this avalanche puzzle. We have been tracking a layer of faceted snow associated with the 12/1 rain crust, which is now buried around 8’ deep, and exists at elevations up to around 2500’. We are not sure if any of the large natural avalanches released at this layer. The majority seemed to be storm snow only but the 10′ crown on lower Twin Peaks is suspect. This end of the Pass gets less snow and a lower elevation crown fits into the elevation band where we have found this crust facet/combo. It is unlikely a person could trigger an avalanche on this weak layer at the depths it is buried in Turnagain Pass. However, as we head south towards Summit Lake the storm totals taper dramatically and this could be possible. This is another unknown that needs more data after this storm cycle.

Weather

Wed, January 13th, 2021

Yesterday: Skies were mostly cloudy in the morning with snow showers and became partly cloudy in the afternoon. Winds were easterly 5-15 mph with gusts into the 20s. Temperatures were in the mid 30°s at sea level and high teens to low 20°Fs at ridgetops during the day and overnight. Skies became mostly cloudy again overnight and easterly winds increased t0 10-20 mph with gusts into the 30s.

Today: Mostly cloudy skies with snow showers, potentially becoming partly cloudy again in the afternoon. Easterly winds 10-20 mph with gusts into the 30s and 40s decreasing in the afternoon. Temperatures in the 30°Fs at sea level and mid t0 high 20°Fs at ridgetops. Overnight snow showers with easterly winds 5-15 mph increasing early Thursday morning. Temperatures in the mid 30°Fs at sea level and mid 20°Fs at ridgetops.

Tomorrow: There is Winter Storm Watch from Thursday morning through Thursday afternoon for Portage Valley and Turnagain Pass. Blizzard conditions possible. Total snow accumulations of 6-10″possible with up to 15″ through the Pass. East winds 15-25 mph with gusts into the 40s. Temperatures in the high to mid 30°Fs at sea level and the mid 20°Fs at ridgetops.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	29	3	0.4	129
Summit Lake (1400′)	25	1	0.2	24
Alyeska Mid (1700′)	29	3	0.4	119

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	20	NE	13	39
Seattle Ridge (2400′)	25	N/A*	N/A*	N/A*

*Seattle Ridge anemometer is rimed over.

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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