Sat, December 16, 2017 Turnagain Pass Avalanche Forecast

Sat, December 16th, 2017

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

3 - Considerable

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

Storm Slabs

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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 strong Easterly winds combined with 13” of new snow have created storm slabs in the mid and upper elevations of Turnagain Pass. These newly formed slabs could vary in size from 1-2 feet thick and could be easy to trigger today. Due to strong winds yesterday these slabs may be wind loaded further downslope than expected and could catch you by surprise. Expect slabs to be thicker on leeward features and along gullies where the snow has been cross loaded. Cornices are also likely to be tender. In the alpine triggering a storm slab or cornice could also wake up a much larger and more dangerous avalanche deeper in the snowpack. Don’t forget we have had continued stormy weather and elevated avalanche danger for the last two weeks including periods of heavy precipitation (both rain and snow), strong winds, and lots of recent avalanche activity. These are all red flags warning signs the snowpack needs more time to adjust to it’s new load. Maintaining a conservative mindset and avoiding slopes steeper than 35 degrees is recommended.

Active wind loading was observed yesterday on Tincan early in the storm.

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

Above 2,500′ very little information is known about the snowpack, but we do know that 5-8′ feet of snow has fallen over the last two weeks and landed on weak faceted snow with crust(s) near the ground. Last week this set-up was very reactive and there were multiple remote triggered avalanches in Turnagain Pass. More time and information is needed to determine the sensitivity and distribution of the deep persistent slab problem. Likely trigger spots will be thinner area of the snowpack wear winds have stripped snow. Unless you have x-ray vision these thinner areas will be impossible to know. Finding a shallow spot could have devastating results. This type of avalanche would be unsurvivable. There may be no obvious clues to indicate instability and digging to find the weak layer could be challenging. Conservative route-finding will be essential due to the potential for large avalanches.

A snow pit at 2250′ yesterday found weak faceted snow near the ground to be reactive. This pit was intentially dug in a thinner area of the snowpack, a place where triggering a deep slab will be more likley. At this elevation (2250′) several crusts are forming within the snowpack which is helping to strengthen the slab and make triggering at this lower elevation less likely. In the higher elevations where drier snow has fallen little is known about how the weak layer and slab are adusting.

Weather

Sat, December 16th, 2017

Yesterday a storm moved through our region dumping 13 € of snow (1.0 € H2O) at Turnagain Pass and similar amounts in Girdwood. Temperatures remained in the upper 30F’s at seal level and precip fell as rain to about 700′ until early this morning when temps cooled and about an inch of wet snow fell in the lower elevations. Strong Easterly ridge top winds averaged in the 20’s (mph) with some gusts reaching the 70’s mph. Winds decreased early this morning to 10-20mph.

Today expected scattered snow showers with up to 3 € possible. Precip may be in the form of light rain up to 800′. Temperatures could reach the mid 30F’s at 1000′. Southeast ridgetop winds may be gusty this morning, but are expected to depreciate mid-morning to 5-15mph today.

Tomorrow scattered snow showers are likely and winds are expected to be light to moderate from the East as a weak low moves over the Kenai Peninsula. A window of clear weather is forecasted for Monday before another round of rain and snow moves through our region. At this point there is a lot of uncertainty as to what type and how much precipitation we will see next week.

* 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′)	31	13	1.0	38
Summit Lake (1400′)	30	3	0.3	11
Alyeska Mid (1700′)	30	13	1.16	31

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′)	26	SE	20	76

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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