Mon, December 25, 2017 Turnagain Pass Avalanche Forecast

Mon, December 25th, 2017

Alpine

Above 2,500'

2 - Moderate

Treeline

1,000'-2,500'

2 - Moderate

Below Treeline

Below 1,000'

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

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.

More info at Avalanche.org

Strong Easterly winds have been changing the surface conditions of our forecast zone over the last 4 days creating hard wind slabs on leeward features. Many ridgelines have been exposed down to rocks, dirt and old crusts. Observations yesterday found hard supportable wind slabs as low as 1600’ on Eddies. These slabs averaged around 5” thick and were failing in hand pits on old weak snow. Any wind slabs today will be hard and supportable (to a skiers weight) and may lure a skier well onto a slope before it fails. Be suspect of unsupported slopes steeper than 30 degrees that have a fat, smooth, pillow-type shape to them. Listen for that hollow, drum-like sound below your skis or use a pole to probe for that hard snow over weak set up. These slabs could be as thick as 1-2’ deep in upper elevations and have the potential for initiating a much larger and more dangerous deep slab avalanche. More on this below.

A handpit that failed on isolation on weak snow below, near surface facets and buried surface hoar. Yesterday at Eddies at 1900′ on a West aspect.

Many windward slopes have eroded down to rocks, dirt, and crusts, but cross loading is still possible around gullies and terrain features on windward slopes. Photo of the SE face of Seattle Rigdge near Pyramid Peak.

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

The recipe for deep slab avalanches has been found in the upper elevations of our forecast zone, above 3000’ on slopes that did not avalanche during the early December storm cycle. The snowpack ranges from 3-5+’ thick and is sitting on weak basal facets. Observations over the last few weeks indicate this poor structure is widespread across our region in the alpine elevations. This is a high consequence avalanche problem that is impossible to outsmart.

When dealing with a deep slab avalanche problem, keep in mind:

Large snow covered slopes that do not have piles of old debris under them are all suspect
Thinner areas of the snowpack (1-2’ thick) are likely trigger spots as well as scoured areas near rocks
It is possible to trigger this avalanche from below and it could run further than expected
Due to strong winds over the last month the snow depths are highly variable and there may be more trigger spots than we realize
Thicker areas (3-5+’ thick) will be difficult to trigger and several tracks may be on a slope before someone finds a trigger point

This Deep Slab avalanche on the NW face of Pastoral was remotely triggered from below on Dec.20 by a party of two skiers who were able to run out of the way of the debris. This is an example of the potential that still lingers in upper elevations, above 3000′ on slopes that haven’t avalanched. For more details on the snowpack structure click HERE and for a description of what happened click HERE.

Weather

Mon, December 25th, 2017

Yesterday Strong Easterly winds decreased to Moderate, averaging 15mph with gusts in the 30’s. Temperatures at lower elevations were above freezing, mid to low 30F’s at 1000′ and low 40F’s at sea level. Temperatures at ridge tops remained in the low 20F’s. An inch of new snow fell yesterday.

Today a trace of snow is possible and winds are expected to be East, 5-20mph. Skies could range from overcast to partly sunny. Temperatures in the lower elevations should start cooling into the 20F’s today.

High pressure is expected to settle over mainland Alaska this week with several lows tracking South of the Gulf of Alaska. This should allow for some clear skies and cooler temperatures. Northerly gap winds are possible this week as well as an occasional snow shower.

*Center Ridge SNOTEL is reporting erroneous temperature data. See Turnagain Pass DOT weather station for accurate temperature at 1000′

**A max gust of 72mph occured at 7am on 12/24/17 at Sunburst Wx Staion before winds decreased and gusts were averaging in the 30’s mph most of the day.
PRECIPITATION 24-hour data (6am – 6am)

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	*34	1	0.1	30
Summit Lake (1400′)	31	1	0.1	11
Alyeska Mid (1700′)	31	2	0.2	27

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	22	ENE	15	**72
Seattle Ridge (2400′)	26	ESE	15	48

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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