Sun, December 17, 2017 Turnagain Pass Avalanche Forecast

Sun, December 17th, 2017

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

2 - Moderate

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

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

After two weeks of rain and warm temperatures, a welcome cooler storm arrived Friday bringing 12 – 20″ of new snow across the region. The wet snowpack below 2,500-3,000′ is slowly freezing as it’s now capped by this new snow. No avalanche activity was seen or heard of yesterday, including any small avalanches relegated to the new snow. However, very few people have been in the mountains, save for the most popular areas.

Our main concern are deep persistent slab avalanches at the higher elevations in thin snowpack areas. This means avalanches breaking deep in the pack above 2,500′, and in our case in a weak layer of facets near the ground. We are uncertain as to the extent and likelihood of triggering a large slab, but until we gather more information we recommend following the travel advice for CONSIDERABLE avalanche danger, which includes careful snowpack evaluation and conservative decision-making. Snow pit results at 3,300′ on Tincan yesterday pointed to good stability and this is a good sign, but remember this is one pit in a place that received significant snow compressing the weak layers. Areas on the Southern end of the pass and extending toward Summit Lake received less snow and are suspect for harboring reactive facets near the ground. A thinner snowpack also means triggering is more likely.

Keep in mind that no Red Flags are likely to be seen with this avalanche problem. In order to assess the weak layer in a snow pit you must dig to the ground. This makes it more difficult to know how touchy this problem is. Any help from you is much appreciated! Please let us know if you hear any whumpfing or collapsing, if you dig to the ground let us know what you find. Last, mid-elevations, roughly below 2,500′ and even up to 3,000′ in places, saw enough rain that the pack is now freezing into thick crusts, limiting avalanche activity.

The snowpack is still thin in area South of Turnagain Pass, pictured below is the Seattle Creek headwall and Big Chief on the right. (photo Sam Galoob)

Avalanche Problem 2

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

Anywhere from 12 – 20″ of new light snow from Friday’s storm sits on either a moist crust or dense snow. We found good bonding with the new and old snow interface yesterday. What we did find were stubborn wind slabs and storm slab potential as the storm snow had yet to completely bond. Today, watch for lingering wind slabs and areas where the storm snow feels stiffer over lighter. Quick hand pits and using your pole to cut small blocks are great ways to assess these ‘surface instabilities’ along your route. Look for blocks you cut to easily slide off and watch for cracks that shoot out from you. There is plenty low density snow that a little bit of wind and/or warming could form a slab.

New snow from Dec 16th showing signs of bonding and stabilizing.
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Weather

Sun, December 17th, 2017

Yesterday saw overcast skies with a few sunny holes poking through. There was no precipitation recorded during the past 24-hours, except for a trace in the Girdwood Valley late morning. Winds were light from the East with moderate gusts. Temperatures were mid 30’sF at sea level, 30F at 1,000′ and mid 20’sF along ridgetops.

Today, partly cloudy skies and cooler temperatures are expected with no precipitation. Ridgetop winds are forecast to shift Northerly today and remain light 5-10mph with stronger gusts. Temperatures are cooler in valleys this morning, 15-20F, as an inversion has set in while ridgetops are in the low to mid 20’sF. Expect valley temperatures to climb during the day and ridgetops continue cooling with cold air moving in from the North.

Monday, skies are forecast to finally clear and a sunny day is on tap, again with cooler temperatures and light winds. Tuesday another system heads in. This one looks to bring around 4-8″ of snow with the rain/snow ~1,000′. Stay tuned!

* 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	33
Summit Lake (1400′)	21	0	0	11
Alyeska Mid (1700′)	28	trace	0.03	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′)	24	SE	6	12

Snowpack at the Motorized Parking Lot: 18″ totoal: bottom 8″ is wet snow beginning to freeze, top 10″ is new snow from Dec 15.

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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