Thu, December 14, 2017 Turnagain Pass Avalanche Forecast

Thu, December 14th, 2017

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

Wet Slab
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood

Wet Slab

Wet Slab avalanches are the release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slabs can be very unpredictable and destructive.

Likelihood of Avalanches

Terms such as "unlikely", "likely", and "certain" are used to define the scale, with the chance of triggering or observing avalanches increasing as we move up the scale. For our purposes, "Unlikely" means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. "Certain" means that humans will be able to trigger avalanches on many slopes, and natural avalanches are expected.

More info at Avalanche.org

After two weeks of warm storms, heavy rainfall to 2,000-2,500′ and an extended natural wet avalanche cycle, cooler air has finally arrived overnight. As of yesterday, the snowpack was still undergoing a wet avalanche cycle below 3,000′. One of the most notable avalanches was a large slab the released naturally just under the CFR ridge on Tincan, pictured below. Other smaller wet slab avalanches occurred yesterday in the Tincan area along with some on Seattle Ridge. Although natural avalanches are still possible, the natural cycle was decreasing yesterday and is on a continued downward trend today. This is due to cooler temperatures beginning to freeze the surface of the wet snow and only light precipitation expected today (1-2″ snow).

Recent wet/moist slab avalanche on Seattle Ridge

Today’s avalanche problems:
Wet slab avalanches below 3,000′ and dry slab avalanches above 3,000′ are our two main concerns, and they are big ones. We have very little information other than visual observations of the snowpack. What we know is very wet and saturated snow exists below 2,500′. Even though a surface crust may be forming with cooler temperatures last night, that will not rule out the possibility of a wet slab releasing today. These wet avalanches are extremely dangerous and even a small one can be unsurvivable. There are no tests that can be done for determining wet slab potential other than a stout refreezing of the pack stabilizing it.

Avalanche Problem 2

Deep Persistent Slabs
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood

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.

Likelihood of Avalanches

More info at Avalanche.org

Above 3,000′ moist to dry snow has fallen for two weeks now. Again, we have little information for these upper elevations at this time, but we do know prior to the storm cycle weak faceted snow with crust(s) sat near the ground. The slab on top of this weak foundation has now grown to several feet and much more in places. This presents a deep slab problem and will be guilty until proven innocent. In addition to this, storm slabs, wind slabs and cornices are all concerns in the recent storm snow.

As you can see, for today, conservative route-finding will be essential considering the myriad of avalanche problems and the potential for large avalanches.

Weather

Thu, December 14th, 2017

Yesterday was a break in the heavy rain and skies were generally overcast. During the morning hours, .6″ of rain fell in the Girdwood Valley and around .3″ at Turnagain Pass before skies cleared. Ridgetop winds were moderate to strong from the East and have diminished overnight, although both the Sunburst and Seattle Ridge weather stations are not reporting we can look to the Max’s mtn wind data to give us an idea. Temperatures that were very warm during the day (mid 40’s at sea level and 30F at 2,500′) have decreased overnight and are sitting near 32F at sea level and the upper 20’s at 2,500′.

For today, Thursday, light precipitation is on the way as a weak low pressure system spins to our East. Forecasted snow amounts are small – an inch for today and another inch for tonight. The good news is temperatures stay cool and the snow line should fall to 1,000′ and possibly lower! Winds should be light to moderate from the North and West today before picking up tonight from the same direction tonight.

Looking ahead, a larger low pressure will bring another round of precipitation Friday afternoon through Saturday. This system looks to wrap in colder arctic air and snow should fall to 1,000′, and possibly sea level. This system may favor East Prince William Sound and snow amounts in our area may be small. Stay tuned on tomorrow’s forecast!

Glimpse of the Tincan meadows

* Sunburst weather station is down due to loss of battery power.
** Seattle Ridge anemometer is rimed and not reporting.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	33	1	0.3	26
Summit Lake (1400′)	30	0	0.1	9
Alyeska Mid (1700′)	33	3	0.6	19

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′)	29	**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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