Sat, December 24, 2016 Turnagain Pass Avalanche Forecast

Sat, December 24th, 2016

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

3 - Considerable

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.

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

Soft settled snow will easily move as winds pick up in advance of the approaching storm. Wind slabs may be tender and easily triggered in the Alpine. As the snow starts today this will only become more likely. Wind speeds are forecasted to be easterly 25-45 mph gusting to 60 mph. Look for cracking, drifting and blowing snow. Cornices may also grow and become more tender with increasing wind, new snow and rising temperatures.

Avalanche Problem 2

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

As the snow starts to fall today it will land on very weak surface snow and is not expected to bond well. It is important to remember there is widespread surface hoar as well as small facets on almost surfaces from road level high into the alpine. Expect new snow to act as a slab fairly quickly due to this interface. As temperatures warm throughout the storm and the wind blows this may become even more pronounced. Rain or rain/snow mix falling on the old snow may produce avalanches as well at lower elevations. Look for red flags i.e. recent avalanches, cracking and collapsing (whumpfing). Pay attention to changing conditions!

Rimed surface hoar over soft settled (faceting) snow on Sunburst ridge. Photo: Kevan Dee

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

We have been tracking a couple of weak layers this season. Buried surface hoar from November 16th and December 15th have both produced avalanches. These layers are still buried in the snowpack in some terrain. In addition there is faceted surface snow and new surface hoar waiting to get buried. New snow load may cause avalanches that step down into old weak layers. This may not happen initially as the storm starts but should be a factor that is remembered as it progresses. We have a weak foundation/snowpack structure. Patience will be key in the next couple of days. We could see a large avalanche cycle if this storm really does it’s thing.

Weather

Sat, December 24th, 2016

Yesterday was clear with some valley fog. Temperatures were inverted with single digits in the valleys and high teens at ridge tops. Winds were light. Last night the temperatures rose into the 20s and winds picked up this morning gusting into the 30s.

Today there is a Winter Weather Advisory for the forecast area due to a warm wet storm that is forecasted to move into the region late this morning. Easterly winds will increase with gusting into the 60s and 3-7 inches of snow is expected today. Snow may be heavy at times and there is uncertainty with how warm it will be. Rain may fall below 1000′. The storm is forecasted to continue tonight into tomorrow with an additional 6-10 inches of snow overnight and 4-6″ tomorrow. The brunt of the storm is forecasted to be this evening.

...WINTER WEATHER ADVISORY REMAINS IN EFFECT FROM NOON TODAY TO
9 AM AKST SUNDAY FOR TURNAGAIN PASS AND PORTAGE VALLEY...

* LOCATION...TURNAGAIN PASS AND PORTAGE VALLEY.

* SNOW...12 TO 19 INCHES FOR TURNAGAIN PASS. 8 TO 15 INCHES FOR
  PORTAGE VALLEY.

* VISIBILITY...ONE HALF MILE OR LESS AT TIMES.

* WIND...EAST WINDS 25 TO 45 MPH WITH GUSTS TO 60 MPH.

Snow showers will continue into early next week with a clearing and cooling trend towards the weekend.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	15	0	0	24
Summit Lake (1400′)	7	1	.1	8
Alyeska Mid (1700′)	16	0	0	16

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	Under repair	Under repair	Under repair	Under repair
Seattle Ridge (2400′)	15	SE	21	27

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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