Wed, December 09, 2020 Turnagain Pass Avalanche Forecast

Wed, December 9th, 2020

Alpine

Above 2,500'

2 - Moderate

Treeline

1,000'-2,500'

2 - Moderate

Below Treeline

Below 1,000'

2 - Moderate

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

It’s been just over a day since the end of the last storm that hit Sunday into Monday, which dropped 2-3′ of new snow. With light snow showers forecast today, possibly adding 3-5 inches and even another 2-4″ tonight, this should not be quite enough to create new avalanche issues in itself and the main concern still lies with the snow that fell early this week.

Large storm slab avalanches are possible. These are in the 1-2 foot thick range depending on the amount of new snow in Turnagain Pass and up to 3′ thick in favored areas in the upper Girdwood Valley where more snow fell. Although we have little information from yesterday, we know these storm slabs were touchy on Monday. This is in large part due to the snow being warmer and heavier as well as falling onto 4-10″ of loose faceting snow with surface hoar on top. In areas up to 2,500′ that loose snow sat on a stout crust. Yesterday, observers in the 2,000-2,500′ elevation band on Seattle Ridge found the storm slabs to be bonding, a great sign, but this isn’t something to assume is happening everywhere just yet. It’s a day to really watch for RED FLAGS:

Any whumpfing in the snowpack?
Cracks that shoot out in the snow around you?
If yes, then the new snow is still unstable and sticking to low angle slopes and the flats is key.

Wind Slabs: Ridgetop winds have been steady on Sunburst right around 15mph with gusts into the 20’s mph, from the east. This is just enough to keep any snow that is still available for transport along ridgelines moving and fresh wind slabs may be found in the Alpine. Add to that, we should be on the lookout for any older wind slabs from the Sunday/Monday storm in the Treeline and Alpine elevation bands.

Cornices: Although visibility has been difficult, we know cornices have grown and could still be teetering on the brink. Give them a wide berth as they can now easily break further back than we might expect. As seen last week, they can also trigger an avalanche below, creating a bigger problem if a person is involved.

Seattle Ridge at 2,000′: There is around 20″ (50cm) of settled storm snow from the Sunday/Monday storm. This fell on top of another foot (30cm) of older snow that had become faceted with the cold and clear weather during the end of last week. No signs of instability were seen in this pit yesterday (12.8) and the crust, formed when rain fell up to 2,500′ on 12.1, sits ~32″ (80cm) below the surface. Photo: Brian Murphy

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

That old October snow near the ground is getting more and more buried with the snowy weather. This is great news! It helps to squash the layer and keep our weight far above it; in essence making it so triggering a deep persistent slab is much less likely. That said, hitting a thin area in the snowpack or traveling in areas with an overall shallower snowpack (around 3-4′) is still concerning for triggering a large slab. The south end of Turnagain Pass through Silvertip, Johnson Pass and Summit Lake all have slopes without as much snow as Turnagain or Girdwood. Heads up in these areas and remember to feel/listen for whumpfing and watch those shallow slopes.

An example of an area that is still suspect for triggering a deep slab. This is pit is from Tenderfoot Ridge in the Summit Lake area (2,500′ elevation) and shows the weak facets at the bottom of the snowpack. 12.8.20. Photo: Nick D’Alessio

Weather

Wed, December 9th, 2020

Yesterday: Light snow flurries were over the area until late morning then skies began to break slightly. Only 1-3″ of new snow fell in those morning hours. Ridgetop winds have been moderate (1-15mph) from the east with a 2 hour period at noon of 20’s mph gusting up to 49 on Sunburst. Temperatures were near 30F during the day before falling overnight and sit in the teens and 20’sF this morning.

Today: Light snow showers will pick up again today adding 3-5″ over the Girdwood and Turnagain Pass area before another 2-4″ is forecast to fall tonight. Ridgetop winds are expected to remain from an easterly direction in the 15-25mph range. Temperature should remain near 30 at sea level and in the 20’s along the ridgelines.

Tomorrow: Some lingering snow showers over the region before skies are expected to start clearing for a day or so. Ridgetop winds look to be light and variable while temperatures should continue to drop into the teens in most locations.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	28	0	0	66
Summit Lake (1400′)	18	1	o.1	26
Alyeska Mid (1700′)	29	3	0.2	64

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	20	E	15	49
Seattle Ridge (2400′)	25	SE	6	20

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
04/16/24	Turnagain	Observation: Cornbiscuit	Krueger / Matthys Forecaster
04/15/24	Turnagain	Observation: Taylor Pass	John Sykes Forecaster
04/15/24	Turnagain	Observation: Seward Highway – Tern Lake to Portage	Joel Curtis
04/14/24	Turnagain	Observation: Sunburst	T. Olsen-Lund, R. Sullivan I. Adams
04/14/24	Turnagain	Observation: Turnagain Pass Snomo	K. Gallaher
04/13/24	Turnagain	Observation: Spokane Creek	Everet Megli
04/11/24	Turnagain	Observation: Tincan	John Sykes Forecaster
04/10/24	Turnagain	Avalanche: Seattle Ridge	Schauer/ Keeler Forecaster
04/10/24	Turnagain	Observation: Cornbiscuit South Face	Todd Kelsey
04/10/24	Turnagain	Avalanche: Tincan	N. Dumont J. Carlberg

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