Fri, December 23, 2016 Turnagain Pass Avalanche Forecast

Fri, December 23rd, 2016

Alpine

Above 2,500'

1 - Low

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

Dry Loose

Dry Loose

Dry Loose avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.

More info at Avalanche.org

One more sunny day is on tap (24 seconds more than yesterday) before clouds fill in tomorrow and we wait in anticipation for how the ‘Christmas Storm’ develops. The winds that were forecast yesterday did not produce at Turnagain, leaving the majority of the snowpack capped with 6-12+” of very soft and loose faceting snow. This has made for good riding conditions, but it has also created a ‘base-less’ pack in thin areas and early season hazards such as rocks still remain.

Fresh shallow wind slabs: We are missing data from two of our key wind sensors from last night. Winds may have picked up just enough to form shallow wind slabs/crusts in certain areas. This will be something to look out for today. (The Seattle Ridge sensor is rimed up and the Sunburst station has a battery issue we are working on).

Sluffs: Watch your sluff if heading for the steep terrain. With such weakly bonded snow on the surface, triggering a loose snow avalanche on slopes 40 degrees and steeper is likely. These sluffs could be quick enough and carry enough momentum/volume to be a concern on the longer sustained slopes.

Avalanche Problem 2

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

With all the cold weather we have seen in the past 6 weeks, we have developed several weak layers in our thin snowpack. These are composed of two of the ‘classic’ persistent grain types: facets and buried surface hoar. These weak layers are still present, but the ‘slab’ that sits on them has decomposed and become faceted in most places – without a slab, you can’t have a slab avalanche. The exception are steep rocky slopes where old wind slabs, still stiff enough to be a slab, sit unsupported on weak layers. If you venture to these areas, watch for old stiff wind slabs and if one does release and knock you off your feet, where would you go?

Photo: Buried surface hoar found under a thick old wind slab. Pit results showed that with a big force, the layer could fail and propagate; unlikely but not impossible in the very steep zones.

Surface Conditions?? We are set up again with a very weak foundation once the next snowfall hits.

Additional Concern

Cornice

Cornice

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

More info at Avalanche.org

Although cornices are pretty frozen in place, breaking off one of these, or a chunk, is still possible while on ridgelines.

Weather

Fri, December 23rd, 2016

Yesterday’s first day with gaining daylight (11 seconds) was bluebird with light and variable winds. Temperatures were in the single digits in the parking lots at 1,000′ and near 10F on the ridgetops.

Today, we should see sunny skies with winds bumping up slightly from the North and West to the 5-10mph range. Temperatures are forecast to be slightly warmer, in the teens at all elevations.

Looking ahead to the Christmas weekend: A large low pressure is headed our way. Southerly winds should pick up along with high clouds tomorrow morning before the precipitation. The system is bringing warmer air and we could see rain at sea level but snow at 1,000′.

NWS forecast discussion:

“Strong winds coinciding with moderate to heavy
snowfall could produce blizzard conditions in turnagain Pass and
Portage Valley Sat afternoon through Sat night. There is still
some uncertainty as to the degree of warming that will occur in
Portage Valley. Temps in this area could rise into the mid 30s Sat
evening and produce a fairly wet snow or mix and mitigate the
potential for blowing snow. Turnagain pass should safely stay all
snow through Sat night. That said, issued a blizzard watch for
both areas.”

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	11	0	0	25
Summit Lake (1400′)	1	0	0	8
Alyeska Mid (1700′)	13	0	0	17

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′)	12	Rimed	Rimed	Rimed

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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