Sun, February 15, 2015 Turnagain Pass Avalanche Forecast

Sun, February 15th, 2015

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

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

Strong Easterly winds yesterday, along with a powerful warm storm moving in this evening, will keep our tenuous snowpack on the brink. Although visibility was limited yesterday, we were able to see two natural slab avalanches (~12″ thick with wide propagation) that occurred sometime early in the morning. These were on the North side of the Magnum Ridge on Turnagain Pass where winds commonly cross-load the North facing gully features (photo below). With a well-documented layer of weak faceted snow underlying the recent snowfall from February 11th, it is not a surprise to see this natural activity.

Although there are variations in snowpack structure region-wide (Girdwood Valley to Turnagain Pass to Summit Lake), the set-up for today and coming into this evening’s storm is similar: Essentially, we have very weak faceted snow that sits under dense slabs anywhere from 4-20″ thick. Some of these slabs are older and some newer, but nonetheless we have weak snow under them. Along with that, below 3,500′ many of these slab-weak layer combos have a slick melt-freeze crust underneath; this can act as a bed surface and encourage propagation of a slab if one is triggered.

Before the addition of snow and wind later tonight for nature to start tipping the balance, human triggered avalanches will be likely. These areas are mainly confined to upper elevation steep slopes over 35 degrees harboring a slab 6″ thick or thicker. Watch for collapsing and cracking and triggering an avalanche remotely will be possible. With a bit less-than-desirable surface conditions below 3,000′, it may be best to let this next storm roll through before venturing down to Turnagain Pass.

Avalanche Problem 2

Wet Loose

Wet Loose

Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

More info at Avalanche.org

Warming temperatures and rain on snow up to 2,000′ later tonight will keep wet avalanche activity on the radar. Although a superficial re-freeze has likely occurred overnight and we only saw a few wet loose avalanches yesterday, a higher rain/snow line should re-activate and encourage wet slides. With a poor snowpack structure in the top 1-2′ of the snowpack, the potential is there to see shallow wet slab avalanches along with the typical wet loose variety.

Weather

Sun, February 15th, 2015

Yesterday’s weather boiled down to overcast skies, strong Easterly winds and little precipitation. See the tables below for exact numbers. Although little precipitation was seen on the Pass itself, it was raining moderately on the far Northern end and in Portage Valley. The rain/snow line was around 1,500′ and seemed to lower slightly through the day.

This morning we should see a brief lull between storms as winds and showery precipitation have died down. The Satellite image (seen below from 6am) is showing a large – and warm – plume of moisture headed our way later today. Expect winds to pick up to the 30’s mph from the East this afternoon, visibility to shut down and light rain begin to fall up to 2,000′. Temperatures should remain in the upper 30’s F at 1,000′ and warm to ~30F on ridgetops.

Beginning this evening and into tomorrow, gale force winds, rain up to 2,500′ (or higher) and around 2″ or more of water amounts are expected!! The good news with this system is that the models are showing colder air wrapping in for Monday evening; this will hopefully lower the rain/snow line before it stops precipitating. Stay tuned on tomorrow’s advisory.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	36	rain	0.1	35
Summit Lake (1400′)	35	0	0	9
Alyeska Mid (1700′)	35	rain	0.24	25

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	27	E	25	58
Seattle Ridge (2400′)	29	NE	20	40

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
11/27/23	Turnagain	Avalanche: Tincan Ridge	Schauer/ Stiassny Forecaster
11/26/23	Turnagain	Observation: Road report: Slide with dirt on Repeat offender	Anonymous
11/26/23	Turnagain	Observation: Pete’s North	Ben Sullender
11/25/23	Turnagain	Observation: Tincan trees	John Sykes Forecaster
11/21/23		Observation: Spokane Creek	John Sykes Forecaster
11/20/23	Turnagain	Observation: Sunburst	Anonymous
11/19/23	Turnagain	Observation: Magnum – PMS Bowl	Schauer/ Cullen/ Jonas Forecaster
11/19/23	Other Regions	Observation: Sunnyside/Penguin	Jose Ramos-Leon
11/19/23	Turnagain	Observation: Eddies	Andy Moderow
11/19/23	Turnagain	Observation: Cornbiscuit	Anonymous

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