Sat, March 21, 2015 Turnagain Pass Avalanche Forecast

Sat, March 21st, 2015

Alpine

Above 2,500'

2 - Moderate

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

Slabs between 2-3’ thick were deposited over the last week. The bulk of that slab development happened 5 days ago. Those slabs built on top of a wide variety of surfaces. Our field day yesterday showed generally good bonding between these slabs and the underlying surfaces. However, there is a high degree of variability in the makeup of these interfaces. Many areas lack a persistent weak layer between the St Patty’s day slab and bed surfaces, but some areas have a thin layer of weak faceted snow. It is in these areas harboring a thin layer of facets where you will be most likely to trigger a slab avalanche 2-3’ deep. Last week we found this weak layer more widespread in the mid elevations and in sheltered areas in the Alpine.

Sounds tricky? That’s because It is. When the variability of the slab/weak layer interface is this high, it is challenging to accurately assess the stability of the snow below your feet. Tracks on a slope are not a reliable indicator of stability when we have this set up. The best ways to manage this problem are to dig in the snow and pick terrain that allows a margin of error should your assessment be off. Avoiding steep sustained slopes that do not offer escape options will be important today.

Photo of a persistent slab that released yesterday in the afternoon on Tincan. Trigger is uncertain, but is likely natural or remotely triggered by the party pictured at the top of the photo.

Tincan remote? 3/20/15

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

Another day of mild temperatures and occasionally intense sun will bring with it an increasing chance of triggering wet loose avalanches as the day progresses. Expect wet loose avalanches to release naturally and be generally low in volume. Human triggered wet loose avalanches could be larger. For that reason it will be important to get off of steep sunlit terrain as the surface snow becomes damp. The possibility also exists for the persistent slab problem (see above) to come in the form of a wet slab on steep sunlit terrain in isolated pockets, as pictured below.

Wet loosie triggers slab!

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 have grown significantly this week. As always it is important to steer away from cornices and know where the cornice begins and the terrain below you ends. With the onset of Spring also comes long days with potentially intense sunshine. Spikes in temperature as a result of sun and warm air can help to destabilize these behemoths.

Weather

Sat, March 21st, 2015

Over the past 24 hours temperatures have been mild. Winds have been generally light out of the East and no new precipitation fell. Clear skies allowed the sun to impact solar aspects during the day and the surface snow to re freeze overnight.

Today expect mostly clear and mild conditions with high clouds streaming in from the East later in the day. Winds will be light out of the Southeast at 5mph. Temperatures at 1,000′ will reach into the mid to high 30s F.

We look to be in a generally mild and dry pattern through the weekend as weak Low pressure in the Gulf is held at bay by high pressure building over mainland Alaska.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	36	0	0	56
Summit Lake (1400′)	31	0	0	12
Alyeska Mid (1700′)	35	0	0	35

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	29	ENE	6	28
Seattle Ridge (2400′)	30	n/a	6	16

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
05/13/24	Turnagain	Observation: Eddie’s, Sunburst, Seattle, Cornbiscuit, Pete’s South	H Thamm
05/13/24	Turnagain	Observation: Turnagain Pass non-motorized side	Amy Holman
05/12/24	Turnagain	Observation: Warm up Bowl	Tony Naciuk
05/07/24	Turnagain	Observation: Turnagain Pass Wet Slabs	A S
04/29/24	Turnagain	Avalanche: Turnagain aerial obs	Tully Hamer
04/27/24	Turnagain	Observation: Johnson Pass	Noah Mery
04/23/24	Turnagain	Observation: Turnagain Sunny Side	Travis SMITH
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

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