Wed, December 20, 2017 Turnagain Pass Avalanche Forecast

Wed, December 20th, 2017

Alpine

Above 2,500'

2 - Moderate

Treeline

1,000'-2,500'

1 - Low

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

Yesterday 3-5″ inches of snow fell at upper elevations and winds were southeasterly, 15-25 mph gusting into the 30s. Winds shifted to the west in the evening. Observers reported pockets of reactive wind affected snow in leeward terrain. Today wind slabs are possible on steep wind loaded slopes. Look for drifting, cracking and pay attention to stiff snow under foot. Even a small wind slab can be very dangerous in high consequence terrain. In addition, give cornices a wide berth. They have been growing with each new snow and loading event.

Cornices building and the leeward slopes of Hippy Bowl on Tincan. Photo: Sam Galoob, December 16th.

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

It has now been a week since the widespread natural avalanche cycle that was initiated by the warm wet storms ended. We have had no reports of human triggered avalanches since December 6th. We still do not have much snowpack data from above 3,300′ this season. What we do know is that the snowpack depth is variable and that there were large natural avalanches in upper elevation terrain during the storms in the first two weeks of December. The concern is that on slopes that did not slide in the Alpine, there is still weak snow underneath all that storm snow and that a deep slab avalanche could be triggered if you found the wrong spot. If you venture out into the higher elevations today there are a few things to keep in mind:

– Triggering a dangerous deep slab avalanche is still possible above 3,000′
– Shallow snowpack areas are most concerning (more trigger points and possibly more reactive facets). For example: the South end of Turnagain Pass, Crow Pass and Summit Lake
– No red flags are likely to be present to indicate an unstable snowpack. It might not be the first person on the slope that triggers the avalanche. It could be the 10th. That is why deep slabs are so spooky.

The bottom line is that we need more data on the snowpack structure above 3500′ to rule out deep slab avalanches. This picture shows the basal structure we are concerned may still be lurking under feet of snow at upper elevations.

Weather

Wed, December 20th, 2017

Yesterday was cloudy and there were snow showers on and off throughout the day. 3-5″ fell at upper elevations. There was brief sleet/drizzle later in the day below 500′. Winds were easterly during the day and shifted to the west in the evening. Wind speeds averaged 15-25 mph and gusted into the 30s. Temperatures were in the low 30Fs at sea level and the low 20Fs at ridge-tops.

Today is will be partly sunny with a slight chance of snow showers in the am. Westerly winds will be light. Temperatures will cool into the low 20Fs today and dip into the teens tonight.

Tomorrow will be sunny with temperatures in the low 20Fs and light southerly winds increasing in the late afternoon. Friday will be mostly cloudy with a chance of snow showers. The next storm system looks to impact the area over the weekend. There is some hope that it won’t be too warm but there is still some uncertainty. Here are the last few sentences of the NWS discussion this morning; “Behind this low is some significant cold air, which brings a chance for snow for all portions of the forecast area before Christmas Day. Either way, the forecast looks to be messy for the weekend before a return to more seasonable weather early next week.” Let it snow, let it snow, let it snow!

* Sunburst weather station is down due to loss of battery power.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	27	0	0	32
Summit Lake (1400′)	27	3	.3	12
Alyeska Mid (1700′)	28	3	.3	29

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	*n/a	*n/a	*n/a	*n/a
Seattle Ridge (2400′)	22	SE-W	16	37

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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