Tue, February 12, 2013 Turnagain Pass Avalanche Forecast

Tue, February 12th, 2013

Alpine

Above 2,500'

2 - Moderate

Treeline

1,000'-2,500'

1 - Low

Below Treeline

Below 1,000'

1 - Low

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

It will be unlikely that we’ll see any natural avalanches today though it will be possible for a skier or snowmachiner to trigger a wind slab formed during our last storm day (Saturday). Pay attention to any red flags (recent avalanches, shooting cracks or collapsing in the snowpack) and continue to avoid cross-loaded gullies or steep wind-loaded terrain below ridges, as these are likely areas to trigger a wind slab. Stability tests from yesterday gave us some confidence in the Turnagain pass area that the new storm snow from over the weekend is bonding well to the older snow surface.

However, in areas such as the Girdwood Valley that received significantly more snow than Turnagain this past weekend, wind slabs will be deeper and easier to trigger. A report of a class 3 avalanche yesterday in the Girdwood valley showed significant propagation, likely on facets forming near a crust around 2700’.

One can sluice out several of these different crusts (rain, rime and melt-freeze) in our mid to upper pack with a hasty snow pit or probe poke. These distinct layers can act as a good bed surface or interface that a relatively shallow avalanche may step down to. Crusts are widespread throughout the forecast area, proving reactive in the mid elevation band at and just above treeline.

CNFAIC forecaster John Fitzgerald “feeling out the crusts” (~2600’) in addition to measuring spatial variability (depth) across a slope.

Avalanche Problem 2

Dry Loose

Dry Loose

Dry Loose avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.

More info at Avalanche.org

Loose, low-density surface snow will not be gaining any strength today as temps stay cool. Sluffs initiated in steep terrain greater than 35 degrees will have the tendency to run fast and far. Expect it, manage your terrain accordingly (avoiding terrain traps) and this should not prove a significant issue.

Cornice fall also warrants a brief discussion as we did find evidence of cornice failure on Seattle ridge yesterday. Cornices have a very nasty habit of breaking farther back than you expect so it’s a good idea to travel well away from the face of a cornice as these monsters continue to ripen into mid-February.

Weather

Tue, February 12th, 2013

Snow tapered off quickly yesterday morning giving way to mostly clear skies, calm winds and dropping temperatures.

2-5 € of snow and southeast winds in the 10-20 mph range are expected today under cloudy skies. Temperatures have rebounded somewhat this morning from a low of 16 degrees at 1800′ yesterday and look to land in the comfortable range of the high 20’s at 1000′ today. Snowfall is expected to become heavier this evening and overnight as a series of weak surface lows are lining up this week, keeping the weather active in south central Alaska for the near-term.

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Kevin will issue the next advisory tomorrow morning February 13th.

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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