Thu, December 19, 2019 Turnagain Pass Avalanche Forecast

Thu, December 19th, 2019

Alpine

Above 2,500'

2 - Moderate

Treeline

1,000'-2,500'

1 - Low

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

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

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

If traveling in the Alpine today, be on the lookout for wind effect from the almost two weeks of moderate to strong winds redistributing the snow that fell over the same time period. Windward areas are scoured to the tundra and catchment zones have drifts over 10 feet deep. After a day of benign weather yesterday, wind slabs may be more stubborn to trigger but should not be ruled out in steep wind-loaded terrain. Be especially cautious on unsupported slopes and in cross-loaded gullies. A wind slab avalanche could take you for a bad ride if triggered in high consequence terrain. You may be able to get well out onto the slope before it releases. Listen for hollow sounding snow, feel for stiffer snow over softer snow and pay close attention to loading patterns.

Cornices: The recent snow and wind have also created larger cornices that may be triggered along ridgelines. Give them a wide berth as they often fail farther back than expected. Be aware of other parties traveling above or below you in relation to the cornices.

Cornice development and wind effect along the Sunburst ridgeline, 12.17.19.

Looking at cross-loading on Tincan Proper, 12.17.19

Avalanche Problem 2

Persistent Slabs
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

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.

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

With sunshine, good visibility and improved snow cover at upper elevations, travel further into the Alpine is more appealing. Keep in mind overall there is limited snowpack data from above 3000′. Various layers of facets and crusts could still remain intact at the high elevations, especially in shallow snow cover areas like the southern end of Turnagain pass and Summit Lake. Where this is the case, the potential exists for a larger avalanche to be triggered that breaks deeper in the snowpack.

Observers on Tuesday dug a pit at 3,500′ on Sunburst and found the old November Veteran’s Day facets over the hard basal melt-freeze crust. The layer didn’t show signs of reactivity here, which is good news. However, it is still something we are keeping on our radar.

Additional Concern

Glide Avalanches

Glide Avalanches

Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.

More info at Avalanche.org

Keep your eyes peeled for glide cracks in the terrain that you are traveling in. Steer clear and avoid lingering in the runout of this unpredictable hazard. There is a chance we may see some more glide cracks open and release as temperatures drop or the glide activity may completely stop.

Weather

Thu, December 19th, 2019

Yesterday: Skies were partly cloudy and there was some patchy valley fog. Temperatures were in the low 30°Fs at sea level and in the mid 20°Fs at the ridgetops. Winds were mostly light and easterly. Overnight temperatures slowly cooled to the high 20°Fs at sea level and low 20°Fs at upper elevations. Winds were light and variable.

Today: Skies will be partly cloudy with a chance of isolated snow showers. Temperatures will continue to slowly drop with ridgetops in the low 20°Fs to high teens and low elevations in the 20°Fs. Winds will be light from the northwest. Tonight temperatures will get down into the teens and single digits.

Tomorrow: Sunshine and colder temperatures are on tap as high pressure moves over the region. The forecast is for teens and single digits. There is the potential for increasing northerly outflow winds as this weather pattern sets in. The sunny skies and cold temperatures look to continue through the weekend. The outlook for the early next week remains uncertain.

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

	Temp Avg (F)	Snow Depth (in)
Center Ridge (1880′)	28	30
Summit Lake (1400′)	27	7
Alyeska Mid (1700′)	29	15

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	23	E	6	28
Seattle Ridge (2400′)	NA*	NA*	NA*	NA*

*Seattle Ridge weather station is not reporting.

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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