Sat, April 27, 2013 Turnagain Pass Avalanche Forecast

Sat, April 27th, 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

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

Pockets of collapseable and reactive windslab have been reported to us in Turnagain Pass and the Summit areas. We got two reports that areas were whoomphing large sections, to “200 feet away”. The observers reported that the weak faceted layer responsible for the collapse was only about 6 inches deep, but it is still cause for concern. Check the observations page to read the full details.

The last couple days had strong wind at the ridge tops with visible plumes between Anchorage and Seward. The snow storm earlier in the week stayed relatively cold and dry at higher elevations, then that snow was transported by the wind. We may still find some of that wind slab this weekend, and it’s worth some careful thought if traveling in steep terrain.

The character of this type of avalanche will be relatively small and shallow, more likely at higher elevation, and consist of fairly stiff snow that may support a skier before it fails. The plumes that I saw were oriented north to south, meaning that south facing slopes are more likely to be loaded.

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

When I was out yesterday it just seemed too cold for wet avalanches to be a concern. That trend may continue this weekend. The wet avalanche concern is still relegated to lower elevation, direct south facing slopes, and late in the day. We got a hard freeze last night, meaning crusts will be thick this morning. It will take a lot of solar energy to break down that crust and penetrate deeper into the snowpack before any real loss of snowpack strength is realized today.

Most of the wet debris piles that you might see today happened just after the last storm from early in the week. This is when we had the most significant warming and soft surface snow was available before it went through multiple melt/freeze cycles.

Additional Concern

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

Cornices are still big! At elevations where cornices form, generally well above 2000 feet, we haven’t lost much of our snowpack to melting. Remember, cornices lose strength when it gets warm and the snow begins to melt. If the structures are already hanging in an unstable balance, that loss of strength can be the trigger to send them on their way. Or they could be close to falling, just needing another 100+ pounds of stress to tip the balance.

Avoid cornice exposure, especially late in the day when they will be more likely to collapse.

Weather

Sat, April 27th, 2013

We have had no precipitation in over 48 hours. Wind was mostly calm yesterday, with a northwest wind into the mid teens on Sunburst. Temperatures got to above freezing at sea level, but the freezing line did not extend all the way to the top of the ridges. Above 3000 feet stayed fairly cold.

Last night, temperatures dropped to about 26 degrees at sea level and to the mid teens at 4000 feet. Highs today will hit the low 40s. Wind should be light from the northwest. Mostly cloudy to partly cloudy skies are expected with no precipitation.

Wendy will issue the next advisory tomorrow, April 28th.

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
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
04/10/24	Turnagain	Observation: Cornbiscuit South Face	Todd Kelsey
04/10/24	Turnagain	Avalanche: Tincan	N. Dumont J. Carlberg

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