Wed, December 30, 2015 Turnagain Pass Avalanche Forecast

Wed, December 30th, 2015

Alpine

Above 2,500'

4 - High

Treeline

1,000'-2,500'

4 - High

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

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.

More info at Avalanche.org

The second powerful storm of the week arrived overnight bringing heavy precipitation and sustained high winds to the region. Sunburst has been averaging 70 mph this morning with peak gusts of 117 mph. Turnagain Pass recieved over 10″ of snow with an inch of water overnight. The forecast is for the region to get another inch of water throughout the day. This will come as a mixture of snow and rain depending on elevation. Rain/snow line is forecasted to be around 1500′ today. This all equates to rapid loading of the snowpack. A variety of storm related issues are likely today.

1) Natural wind slab avalanches are likely on slopes being loaded by the winds. These will be 1-3′ thick and have the potential to break into older snow.

2) If slides break into older weak layers, we could see very large avalanches, up to 6′ thick (more on this below).

3) Cornices: With such strong winds and warm snow at the high elevations, we can expect cornices to be forming and breaking off. These ‘backcountry bombs’ are likely triggering avalanches below. Even if the winds subside today, these can still break naturally.

4) Wet loose avalanches: Rain on snow has the potential to saturate to upper layers of the snowpack and cause loose wet avalanches that may gouge into the snow below.

Wind effect from Monday’s storm. Note the new cornice formation and the scouring. Expect more of this today.

Avalanche Problem 2

Deep Persistent Slabs

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.

More info at Avalanche.org

Avalanches today have the potential to step down to the old weak snow that is sitting above the Thanksgiving Rain Crust to around 3000′ or old hard snow at higher elevations. This loose faceted snow that formed in early December is a persistent weak layer and it rests below a slab 3-6′ thick (and growing). Due to the depth of the slab these are a Deep Slab concern. Some of the avalanches triggered by avalanche hazard reduction by the Alaska Railroad ran on this layer as well as some natural avalanches observed from the hwy south of Turnagain Pass. Rapid loading today may cause deep destructive avalanches or they may go during the next storm later in the week or not at all. These avalanches are hard to predict but the current stucture of the snowpack makes them a continued concern.

Deep Slab Avalanche:
Avalanches that break deeply into old weak layers of snow that formed some time ago.

Weather

Wed, December 30th, 2015

Yesterday was generally overcast with a few minor breaks in the clouds throughout the day. Winds predominately Easterly blowing in the 20s became gusty in the afternoon. Temperatures were in the mid 30Fs in the valleys to high 20Fs towards the ridge tops. Light rain fell in the afternoon below 900′.

Overnight a strong storm moved into the area bringing heavy precipitation, rain/snow mix up to approximately 2000′ and snow above. Winds have been averaging in the 70s at ridge tops and gusting over 100 mph. There is a High Wind Warning in effect for Portage Valley and Turnagain Arm until 11 AM this morning with the winds decreasing in the afternoon. Winds are forecasted to be 50-70 mph from the East decreasing to 10-30 mph in the afternoon. Rain and snow should continue through the day. After receiving an inch of water overnight on Turnagain Pass almost an additional inch of water is forecasted to fall today into tonight. Precipitation intensity should decrease in the afternoon. Rain/snow line should be around 1500′ with temps ranging from the high 20s to high 30s depending on elevation.

This unsettled weather pattern will continue into the New Year with rain and snow likely throughout the week as a series of lows and a strong jet continue to push moisture into the region.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	33	12	1.2	79
Summit Lake (1400′)	35	0	.5	19
Alyeska Mid (1700′)	33	3	.7	56

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	25	ENE	60	117
Seattle Ridge (2400′)	27	n/a*	n/a*	n/a*

*Seattle Ridge started recording winds at 9 pm last night but does not reflect current wind conditions, the data is suspect.

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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Turnagain Pass

Deep Slab Avalanche:Avalanches that break deeply into old weak layers of snow that formed some time ago.

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

RIDGETOP 24-hour data (6am – 6am)

Deep Slab Avalanche:
Avalanches that break deeply into old weak layers of snow that formed some time ago.