Mon, December 12, 2022 Turnagain Pass Avalanche Forecast

Mon, December 12th, 2022

Alpine

Above 2,500'

4 - High

Treeline

1,000'-2,500'

4 - High

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

Skiers reported small natural avalanches on Eddie’s and Tincan as the storm started to ramp up yesterday afternoon. With peak intensity overnight, it is likely there have been larger avalanches since then.

This natural storm slab avalanche buried the Tincan skin track yesterday afternoon. Photo: Collin Atkinson. 12.11.2022

Avalanche Problem 1

Storm Slabs
N E S W NW NE SE SW

Aspect/Elevation
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.

Aspect/Elevation of the Avalanche Problem

Specialists develop a graphic representation of the potential distribution of a particular avalanche problem across the topography. This aspect/elevation rose is used to indicate where the particular avalanche problem is thought to exist on all elevation aspects. Areas where the avalanche problem is thought to exist are colored grey, and it is less likely to be encountered in areas colored white.

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

Avalanche conditions will remain very dangerous today. We’ve gotten 1-2′ snow in the middle elevations over the past 24 hours, equaling 1.5-2″ snow water equivalent (SWE). At upper elevations storm totals may be approaching 2.5-3′ snow. Along with the heavy snowfall, very strong easterly winds were blowing 30-60 mph with gusts up to 90 mph at the Sunburst weather station last night. The snowfall is tapering off this morning, but strong northwest winds are expected to blow 15-30 mph through the day. This is the biggest loading event so far this season, and it will make large to very large natural avalanches likely, with human-triggered avalanches very likely. With the combination of heavy snowfall and strong winds, we could see avalanches failing 3-5′ deep in the new snow alone– and this doesn’t include the deeper weak layers that may be pushed to their breaking point by this heavy load (see additional concerns for more).

Travel advice for days like this is simple: very dangerous conditions exist, so travel in avalanche terrain is not recommended. This includes traveling on steep slopes or spending any time below avalanche paths. Keep in mind that larger avalanches failing in upper elevation start zones have the potential to run long distances into lower runout zones. If you are trying to get out to enjoy the fresh snow, keep your slope angles very low (below 30 degrees), and be aware of what is above you.

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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

As mentioned above, this is the biggest loading event we’ve seen this season. There are multiple weak layers buried anywhere from 2-6′ deep that may or may not be reactive today. One layer we are tracking is the Thanksgiving crust, which has developed facets above and below. It has not seen any major loading event until now. The other, and less likely, problem is the October facet layer right at the ground. We have been thinking this layer is no longer a concern, but with 1.5-2″ water weight added since yesterday afternoon, we might see otherwise. Given the potential for very large avalanches with the new snow alone, these layers are of secondary concern, but just another reason to give steep terrain a wide berth today.

This snowpit from Eddie’s yesterday shows the Thanksgiving crust/facet combo, which may produce avalanches today. Photo: Andy Moderow. 12.12.2022

Weather

Mon, December 12th, 2022

Yesterday: Snowfall picked up during the day, with peak intensity overnight. As of this morning weather stations are showing 1.5-2″ SWE, which is equal to around 1-2′ snow. Strong easterly winds were blowing 30-60 mph with gusts to 90 mph along the ridgetops overnight. Temperatures have slowly increased from the mid teens to upper 20’s F since yesterday morning. The rain line has stayed down at sea level for this storm.

Today: We may see another inch or two of snow before the storm passes this morning. The northwest wind will be the big story for today, with sustained speeds of 15-30 mph and gusts of 40-50 expected today. These winds usually hit Summit Lake and Crow Pass harder than Turnagain Pass, but all of these zones should see significant wind loading today. High temperatures are expected in the low to upper 20’s F, with lows dropping into the single digits F starting this afternoon. Skies are overcast this morning, but clouds are expected to break up during the day with partly cloudy skies by this afternoon.

Tomorrow: There is a chance of 1-3″ snow tomorrow for our advisory area, as another system approaches that looks to favor Anchorage and Hatcher Pass. Skies will be overcast with easterly winds blowing 15-25 mph out of the east-southeast. The snowline is expected to stay near sea level. We are looking at another strong system Wednesday night into Thursday, so be sure to stay tuned to see how it develops.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	26	17	1.5	47
Summit Lake (1400′)	25	11	1.0	33
Alyeska Mid (1700′)	25	20	1.8	44
Bear Valley (132′)	32	17	1.7

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	17	ENE	30	90
Seattle Ridge (2400′)	21	SE	15	37

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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