Tue, December 29, 2015 Turnagain Pass Avalanche Forecast

Tue, December 29th, 2015

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

3 - Considerable

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

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

A 112mph gust on Sunburst yesterday and a flipped over tractor trailer on the Portage Valley Highway are both indicative of a wind slab issue today. Winds were primarily out of the ENE yesterday and overnight meaning slopes with a south and westerly tilt will be most suspect for triggering a wind slab. Strong winds such as this have a tendency to build wind slabs mid slope (further from the ridge crest). For skiers this means that mid-run you may find yourself on the slab; and for snowmachiners who generally attack a slope from the base, recognize that wind slab may be mid-hill climb, well below the ridge crest.

Watch out for any obvious red flags today such as whumphing, shooting crack or recent avalanches. Whumphing was present for several groups yesterday and is indicitive of a weak layer collapse. Sometimes this is harder to identify from your snowmachine but it’ll be worth the time today to slow down and keep a keen eye/ ear. As wind has a tendency to distribute snow unevenly across any given slope, a skier or snowmachiner is more likely to influence the weak layer in these areas where the slab is thinner. Shallow spots near rock outcrops or alder will be the more likely trigger points where your skis or snowmachine track is physiclly that much closer to the weak layer.

Folks were generally keeping terrain choices mellow yesterday, including these two kite skiers capitalizing on the sustained winds yesterday afternoon in the shadow of Sunburst.

Avalanche Problem 2

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

These warm, windy storms have a tendency to grow cornices rapidly here in the Chugach and Kenai mountains. These ‘backcountry bombs’ deserve an extra wide berth today and throughout this “New Year storm cycle”. A cornice breaking either naturally or from human traffic has the potential to trigger a large avalanche as it falls on the slope below. Limit time spent below a cornice and if travelling along a ridge, stay back waaaaaay further than you think necessary to give yourself that added margin of safety.

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

Given the current structure of our snowpack, we have reason for concern as these potent late-December storms are falling on top of a couple of older known persistent weak layers. With more weight in the form of snow and wind yesterday and today, these weak layers can edge closer to their point of failure, laying in wait for a trigger. Yesterday, Alaska Railroad avalanche reduction crews had several avalanche results using explosives to trigger these weak layers. They saw debris running to the valley floor in channelized terrain.

Weather

Tue, December 29th, 2015

Heavy snow and strong northeasterly winds on Turnagain pass subsided around 3pm yesterday after a 112mph gust on Sunburst and 4-6 € of heavy, wet snow fell at 1,000′. Skies broke apart allowing a brief glimpse into upper elevation start zones. Temperatures hovered right around 32F at 1,000′ and 35F at sea level. Rain and strong winds persisted throughout the day in the Girdwood and Portage valleys.

Today we can expect temperatures in the mid-30’s and ridgetop winds from the SE in the 20-40mph range. Cloud cover will be on the increase throughout the day and precipitation in the form of a rain/ snow mix at 1,000′ will be light during daylight hours and increasing tonight.

Late tonight and into tomorrow we’ll be feeling the full effect of another powerful and unseasonably warm and windy Gulf of Alaska storm. The NWS has issued another high wind warning that goes into effect tomorrow morning.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	31	4	.7	67
Summit Lake (1400′)	34	1	.3	20
Alyeska Mid (1700′)	31	4	.6	60

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	24	E	28	112
Seattle Ridge (2400′)	26	N/A	N/A	N/A

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
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
04/29/24	Turnagain	Avalanche: Turnagain aerial obs	Tully Hamer
04/27/24	Turnagain	Observation: Johnson Pass	Noah Mery
04/23/24	Turnagain	Observation: Turnagain Sunny Side	Travis SMITH
04/21/24	Turnagain	Observation: Bertha Creek	Anonymous
04/20/24	Turnagain	Avalanche: Spokane Creek	Schauer/ Mailly Forecaster
04/16/24	Turnagain	Observation: Cornbiscuit	Krueger / Matthys Forecaster

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