Sun, December 20, 2015 Turnagain Pass Avalanche Forecast

Sun, December 20th, 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

The storm continued on Turnagain pass yesterday depositing an additional 8+ inches during the day. There was a period where snow was falling over an inch/per hr. Easterly winds were strong enough to move the new snow around. There was very limited visibility and no opportunity to see if there was natural avalanche activity in the Alpine. Observers reported reactive storm slabs near treeline and shooting cracks in wind effected snow. Today the snowpack will need time to adjust after this new load. Look for signs of instability. Shooting cracks, whumpfing and recent avalanching all indicate that the snowpack is tender. Watch out for areas that have stiffer snow over soft snow and look for the patterns of wind loading. Leeward and cross loaded areas in steep terrain (35 degrees and steeper) have the most potential hazard today. Fresh cornices may break very easily and should be given a wide berth. Loose snow avalanches and soft storm slabs are also possible today in steep terrain. Watch out for terrain traps, gullies and depressions, where even a small slide can bury you very deeply. Don’t let clearing skies and fresh snow lure you into making poor decisions. Practice safe travel techniques: ride/ski slopes one at a time, stop in save zones, have escape routes planned and as always carry rescue gear!

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 continue to monitor the buried Surface Hoar and Near Surface Facets that sit over the Thanksgiving Rain Crust to 3000′ and over stiff old snow in the Alpine. These have been non-reactive in most tests on Turnagain Pass. Yesterday we couldn’t see into the upper elevation start zones to see if the new load triggered any deeper slabs (or storm slabs). We did find both grain types in our pit yesterday. This buried weak layer/bed surface combination is also a concern in the shallower Summit Lake snowpack. Signs are pointing to this becoming a non-issue but it is worth thinking about today. The snowpack has recieved a significant load and needs time to adjust. If the snowpack fails at this depth the slab could be deep and very dangerous.

Photo by Andy Moderow: Buried Surface Hoar intact and laid over.

Weather

Sun, December 20th, 2015

Yesterday’s storm brought an additional 8+ inches to Turnagain Pass, with high preciptation intensity for much of the day. The storm shut down much earlier in the Girdwood Valley and there was significantly less snow south of the Pass. Storm totals: Turnagain Pass 15-18″, Girdwood Valley 10-13″, Summit Lake 2-5.” Winds were Easterly and blew 20-30 mph gusting in the high 40s during the height of the storm. Temperatures ranged from the upper 20Fs to mid 30Fs with rain falling at sea level to approximately 800′.

Today will be partly cloudy to mostly sunny. Temperatures will be in the 20Fs and winds will be Northerly and light. Tonight will be mostly cloudy.

Tomorrow will be cloudy with a chance for scattered snow showers as a low in the Gulf pushes moisture into Southcentral. The Jet dips south later in the week and allows colder, drier air to move into the region over the holiday.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	28	8	1.1	63
Summit Lake (1400′)	30	1	.1	19
Alyeska Mid (1700′)	30	3	.2	39

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	22	ENE	18	35
Seattle Ridge (2400′)	22	ESE*	10*	20*

*Seattle Ridge was only collecting wind data from 5am-4pm 12/19/15.

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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