Thu, March 21, 2013 Turnagain Pass Avalanche Forecast

Thu, March 21st, 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

The wind picked up 2 nights ago with ridgetop stations reading sustained northwest wind to the 20s and gusts over 50mph in some places. This was enough to produce specific areas of wind slab that can be triggered by a person. Yesterday we found a small mid slope pocket that collapsed a ~30 foot section with shooting cracks within the 2-5 inch thick stiff wind slab. Other people reported more serious occurances yesterday including one person who took a 250-300 foot ride.

Keep in mind that this problem is confined to the surface, meaning it will not step down into deeper layers given the current stability of the snowpack. However, in terrain with consequences it could be enough to be a hazard if caught off guard.

Avalanche Problem 2

Cornice

Cornice

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

More info at Avalanche.org

With increased wind we also get increased stress placed on the cornices as they gain mass. Below is a picture from the Summit Lake region that shows a good example of a large cornice failure. Also note the pocket of wind slab to the right that doesn’t appear to be directly connected to the section of cornice failure.

Weather

Thu, March 21st, 2013

Cold temperatures and clear skies continue this morning. Wind has been moderate to strong for the past 36 hours from the northwest.

Today is the last day of sun in the short-term forecast. We can expect clouds to build throughout the day. By tonight snow is back in the forecast, with 2-5 inches predicted. The storm track is from the west, which is not the typical direction for Turnagain Arm to receive a lot of precipitation. At this time the weather service is predicting more snow for the Matanuska and Susitna valleys.

For long term stability, we can expect a dramatic decrease in stability with new snow in the forecast. Our current snow surface is mostly near surface facets or sun crust, both of which do not allow for quick bonding of new snow.

Fitz will issue the next advisory tomorrow morning, March 22nd.

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
04/15/24	Turnagain	Observation: Taylor Pass	John Sykes Forecaster
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
04/10/24	Turnagain	Observation: Cornbiscuit South Face	Todd Kelsey
04/10/24	Turnagain	Avalanche: Tincan	N. Dumont J. Carlberg
04/10/24	Turnagain	Avalanche: Cornbiscuit	Tyler Grundstrom
04/07/24	Turnagain	Observation: Seattle Creek Headwall	Troy Tempel

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