Fri, March 01, 2013 Turnagain Pass Avalanche Forecast

Fri, March 1st, 2013

Alpine

Above 2,500'

2 - Moderate

Treeline

1,000'-2,500'

2 - Moderate

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

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

The recent storm had peak intensity 2 days ago, and dropped more than 2 feet of snow in some areas including Turnagain Pass. It was deep enough on Wednesday to make travel downhill quite difficult. After yesterday’s relatively clear weather that snow should be settled and consolidated to the point that travel is easier.

A handful of large avalanches were caused by that storm, including a large natural that crossed the Portage road. All the major avalanche programs in our region were doing explosive avalanche reduction work. After the storm ended yesterday, explosive triggers were only causing small to medium avalanches occasionally, with nothing deeper than the recent storm snow. The picture below shows one of the larger avalanches produced by Seward highway work yesterday.

Colder temperatures and calmer weather will be acting to bring stability to the recent storm snow, but it still deserves a cautious approach today.

Avalanche Problem 2

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

The larger avalanches that we’ve seen this week are suspected to be a result of deeper weak layers at the late January crusts. The good news is it remains a less likely problem to encounter. The bad news is it remains a tough problem to predict and avoid 100% of the time, even for the experts.

The buried crusts can be found in some areas at mid elevations between roughly 1900 and 3000 feet. As those layers get deeper with each storm, the likely places to trigger deeper layers will be from shallow points that have been partially scoured by wind.

Weather

Fri, March 1st, 2013

Storm totals yesterday morning reached near 2 feet of snow and up to 2 inches of water equivalent in some areas. Areas such as Girdwood, Grandview, and Portage got more snow. Summit lake got less, but still enough to cause concern.

In the last 24 hours temperatures have cooled off a few degrees, wind has dropped, and precipitation ended by the afternoon.

Today, a weak low pressure is spinning in Prince William Sound which may bring a few inches of snow. Snowfall will end tonight and the weekend will bring a clearing trend. Expect temperatures in the 30s and light wind.

Fitz will issue the next advisory Saturday morning, March 2nd.

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
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
04/15/24	Turnagain	Observation: Taylor Pass	John Sykes Forecaster
04/15/24	Turnagain	Observation: Seward Highway – Tern Lake to Portage	Joel Curtis
04/14/24	Turnagain	Observation: Sunburst	T. Olsen-Lund, R. Sullivan I. Adams
04/14/24	Turnagain	Observation: Turnagain Pass Snomo	K. Gallaher
04/13/24	Turnagain	Observation: Spokane Creek	Everet Megli
04/11/24	Turnagain	Observation: Tincan	John Sykes Forecaster
04/10/24	Turnagain	Avalanche: Seattle Ridge	Schauer/ Keeler Forecaster

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