Thu, February 11, 2016 Turnagain Pass Avalanche Forecast

Thu, February 11th, 2016

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

Wet Slab
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

Wet Slab

Wet Slab avalanches are the release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slabs can be very unpredictable and destructive.

Likelihood of Avalanches

Terms such as "unlikely", "likely", and "certain" are used to define the scale, with the chance of triggering or observing avalanches increasing as we move up the scale. For our purposes, "Unlikely" means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. "Certain" means that humans will be able to trigger avalanches on many slopes, and natural avalanches are expected.

Size of Avalanches

Avalanche size is defined by the largest potential avalanche, or expected range of sizes related to the problem in question. Assigned size is a qualitative estimate based on the destructive classification system and requires specialists to estimate the harm avalanches may cause to hypothetical objects located in the avalanche track (AAA 2016, CAA 2014). Under this schema, "Small" avalanches are not large enough to bury humans and are relatively harmless unless they carry people over cliffs or through trees or rocks. Moving up the scale, avalanches become "Large" enough to bury, injure, or kill people. "Very Large" avalanches may bury or destroy vehicles or houses, and "Historic" avalanches are massive events capable of altering the landscape.

Signal Word	Size (D scale)	Simple Descriptor
Small	1	Unlikely to bury a person
Large	2	Can bury a person
Very Large	3	Can destroy a house
Historic	4 & 5	Can destroy part or all of a village

More info at Avalanche.org

Rain to 2,000′, or higher?, say it isn’t so…. We have squeaked through the season so far with good snow coverage at the road elevation at 1,000′, but today it will be raining. Our snow coverage should remain mostly intact, but things will be wet and soggy out there. We are expecting .5″ of rain up to 2,000-2,500′ with roughly 4-6″ of dense snow above 2,500′. Hence, wet avalanches and glide avalanches will be our primary concerns.

WET AVALANCHES:
At the mid-elevations wet loose avalanches should be expected today with rain-on-snow. These could release naturally or be triggered by a person or snowmachine on a slope. Even a small wet loose slide can entrain a significant amount of snow and become large and dangerous on long steep slopes/gullies. There is also the possibility we could see a few wet slabs, these are likely to be triggered by a wet loose slide running over from above. If you do head out today, keep an eye on steep slopes in the rain.

GLIDE AVALANCHES:
The warm temperatures and wet weather could initiate another glide cycle. Or at least, get a few more ‘looming cracks’ to avalanche. Anyone that has been in the Turnagain Pass area knows hundreds of glide cracks littler the slopes. Avoid being under these – today especially!

Avalanche Problem 2

Wind Slabs
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

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.

Likelihood of Avalanches

Size of Avalanches

Signal Word	Size (D scale)	Simple Descriptor
Small	1	Unlikely to bury a person
Large	2	Can bury a person
Very Large	3	Can destroy a house
Historic	4 & 5	Can destroy part or all of a village

More info at Avalanche.org

In the Alpine, where it is snowing and blowing, you can expect to see fresh wind slabs up to a foot deep. We have had 2-3″ of new snow overnight with another 4-6″ on the way today. This new snow is expected to be dense and quite sticky due to the warm temperatures. This set up usually allows these fresh wind slabs to stabilize relatively quickly. That said, if you find yourself in avalanche terrain today at the high elevations, watch for wind loaded areas, cracking in these spots and of course any recent avalanches. Wind slabs are most easily triggered while, and just after, forming.

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

In areas outside of our core forecast zone, including South of Turnagain Pass and the Summit Lake area, the snowpack is shallower and harbors various weak layers; the most notable weak layer is a layer of buried surface hoar 2-3′ deep in the pack. We have found these to be unreactive in the past week but with today’s warm weather, they could re-activate. For more information see this report from the Lynx Creek drainage and this avalanche triggered January 30th.

Weather

Thu, February 11th, 2016

Mostly cloudy skies covered the region yesterday as a weak low-pressure system is over us currently. We had only a few light snow/rain showers with a rain/snow line around 1,200′. Winds were blowing ~15mph with gusts in the 30’s from the East on the ridgetops. Temperatures were mild, in the mid 30’sF at 1,000′ and the upper 20’s at 3,500′.

Overnight, precipitation has increased slightly and we have picked up .25″ of rain at 1,000′ with 2-3″ of snow above 2,500′. We are expecting another .5″ of rain up to 2,000′ today and 4-6″ of snow at the higher elevations. This is one of the highest rain/snow lines we have seen this season. Winds are forecast to blow strong along the peaks and ridgelines, 30-35mph with gusts in the 60’s. Temperatures will be warm….up to 30F at 3,500′ and 40F at 1,000′.

For Friday and into the weekend this warm and unsettled weather pattern looks to remain over us. However, as is usually the case, we could see some breaks in cloud cover and sunshine poke through here and there.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	34	Slush	0.2	107
Summit Lake (1400′)	35	0	0	31
Alyeska Mid (1700′)	33	0.3 slush	0.45	90

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	27	NE	20	58
Seattle Ridge (2400′)	28	–	–	–

Observations

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