Tue, March 21, 2017 Turnagain Pass Avalanche Forecast

Tue, March 21st, 2017

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

3 - Considerable

Below Treeline

Below 1,000'

3 - Considerable

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

Ah, springtime in the mountains. Warm daytime temperatures, longer days (now longer than our friends in the lower 48) and a fresh dose of snow. What more could we ask for? Possibly, a more stable snowpack… Sunday’s refresher storm that brought over 2 feet of snow to the Placer Valley and around a foot to Turnagain Pass and Girdwood fell on a very weak old surface of facets and surface hoar. In areas with greater snow amounts, triggering a slab avalanche is still very possible and even likely. Essentially, the places that are the most enticing for powder lovers (areas with up to 2 feet of new snow) will be the most likely places to trigger a large slab avalanche. In areas with lower snow amounts (a foot or less) triggering a smaller shallow slab is still possible.

There was a report of natural avalanche activity in the Placer Valley yesterday along with one snowmachine triggered slab avalanche. In the Turnagain Pass area, there was one report of a skier triggered avalanche on Magnum. Along with the avalanche activity, we had several reports of whumphing and shooting cracks. Interestingly enough, on our field day over on Seattle Ridge, we had a hard time finding unstable snow. The new snow here was quite shallow (10-12″) and light – just not enough snow to overload the old weak snow surface.

For today, things to keep at the forefront of our minds:

How much new snow is there where you are traveling?
The more new snow the higher the likelihood of triggering a slab avalanche.
Is the day heating up? Warming can increase the chance for triggering an avalanche.
RED FLAGS? Recent avalanches, whumphing, shooting cracks? All these signs point to unstable snow.

Shallow soft slab avalanche triggered by a skier on Magnum in the Turnagain Pass area yesterday. Photo: Jordan Bancroft

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

Similar to the slab avalanche concerns above, expect the size of sluffs to be dependent upon the amount of new snow you find. That said, human triggered sluffs (loose snow avalanches) are likely in steeper terrain that were not affected by the sun yesterday. Natural damp or wet loose snow avalanches may still be possible with warming in the afternoon. Keep in mind, sluffs have the potential to initiate a slab lower on the slope.

Dry loose avalanches on the West face of Eddies. Photo: Conrad Chapman

Additional Concern

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

Persistent Slabs and Deep Slabs: There are a variety of very old weak layers in our thin snowpack. The February 9th buried surface hoar sits 2-4+’ below the surface and faceted snow sits in the mid and base of the pack. These weak layers (with varying degrees of strength) have been in a ‘dormant stage’. Although unlikely, an avalanche breaking deeper in the pack isn’t completely out of the question with the new snow load and warming temperatures. Areas such as Johnson Pass, Lynx Creek and on the Northern side of Girdwood Valley (near Crow Pass) are the most suspect and observers continue to find poor structure.

Weather

Tue, March 21st, 2017

Partly cloudy skies covered the region yesterday. Intermittent light snow showers and cloud cover added a few flakes here and there but no measurable precipitation was recorded at the snow stations. During the past 24-hours ridgetop winds have been light (5-15mph) with moderate gusts (10-25mph) from the East. Temperatures that warmed during the day to 30F at 1,000′ and 25F along ridgetops have dropped back down to the teens overnight.

For today, we can expect another day of in-and-out cloud cover (partly cloudy skies) and no precipitation. Ridgetop winds should remain light 5-10mph from the East and Northeast with a possible bump to the 15-20mph range later this afternoon. Daytime warming will again let temperatures climb into the 25-30F range by the afternoon.

Looking ahead, mostly clear skies with warm days and cold nights is in our future.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	18	0	0	65
Summit Lake (1400′)	19	0	0	30
Alyeska Mid (1700′)	23	0	0	61

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	15	8	NE	24
Seattle Ridge (2400′)	20	6	SE	19

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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