Tue, December 11, 2018 Turnagain Pass Avalanche Forecast

Tue, December 11th, 2018

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

Wind Slabs
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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

Leeward, steep, unsupported slopes above 2500′ may still harbor wind slabs today from the winds Sunday night/Monday morning. Look and feel for stiff, pillowed snow and cracking and listen for hollow, drum-like sounds. Loading patterns can be very localized and it may be hard to tell where the wind effect is with the few inches that fell yesterday and overnight. Another 2-5″ is forecast to fall today.

Additionally, pay attention to how new snow falling today bonds to the snow below. As the inches slowly stack up over the hard wind-blown snow surface or over the melt-freeze crust at lower elevations loose snow avalanches may be a possibility in steep terrain. If the snow is cohesive enough shallow storm slabs may form. Hand pits and small tests slopes will be useful ways to assess bonding.

Wind effect in Tincan Common, 12-9-18

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 is often the case we are concerned with the snowpack in the periphery zones of our advisory area, Crow Pass and Summit Lake. Where the Alpine snowpack is shallower, the potential to trigger a persistent slab avalanche on weak snow near the ground increases. Observations prior to the storm showed weak facets near the ground in Summit Lake. We have limited information from north of Girdwood but suspect a weak set-up. If traveling to these areas the possibility of triggering a dangerous avalanche remains. Choose terrain carefully.

Red flags to watch for:

Recent avalanches
Whumpfing (collapsing) of the snowpack, a sign to avoid avalanche terrain.
Shooting cracks, likely to be seen near ridgelines where the wind has formed wind slabs.

Tenderfoot, 12-5-18

Weather

Tue, December 11th, 2018

Yesterday: Skies were cloudy with snow showers throughout the day. Temperatures were in the 20Fs to low 30Fs. Winds were easterly 5-15 mph with gusts into the 30s. Snow showers continued overnight, winds were light and temperatures dropped a few degrees.

Today: Snow showers continue with 2-5″ forecast to fall today with an additional 2-6″ tonight. Temperatures will start in the mid to high 20Fs and drop slowly throughout the day into the low 20Fs and teens tonight. Winds will be variable and light.

Tomorrow: Snow showers and cooler temperatures are the trend for the week. Thursday may see a brief window of clearing. The pattern remains active into the weekend. Stay tuned as we watch storm tracks and temperatures!

*Seattle Ridge anemometer (wind sensor) is rimed over and not reporting.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	29	4	0.4	29
Summit Lake (1400′)	27	1	0.1	5
Alyeska Mid (1700′)	28	3	0.3	12

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	21	NE	9	36
Seattle Ridge (2400′)	25	*no data	*no data	*no data

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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