Tue, April 16, 2019 Turnagain Pass Avalanche Forecast

Tue, April 16th, 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

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

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

On shaded aspects in the alpine above 3,000′, a weak layer of facets and buried surface hoar exists 1-2′ below the surface. A layer of buried surface hoar was the culprit in two human triggered avalanches over the weekend in Seattle Creek. Both of these avalanches occurred at 3200’ on North aspects and released on a layer of buried surface hoar sitting on a melt freeze crust. One of these avalanches was triggered at the top of a chute. The other avalanche we suspect was remote triggered from 5-10’ away. In both incidents no one was caught. Yesterday an observer reported whumping and found a weak layer of facets in a hand pit in higher elevations above Girdwood. They also reported a recent large natural avalanche on a West aspect of Raggedtop. Listening for whumpfing and digging test pits are good ways to gather information. Sticking to safe travel protocol is also wise; evaluate terrain for consequences, expose one person a time, watch partners and have an escape route planned.

SPRINGTIME CAUTION:

WET LOOSE: The snowpack has gone through a few days of hot sunny weather and freezing overnight. Today looks like our last sunny day this week. Pay attention to surface snow melting later in the day and avoid steep South facing slopes if surfaces become saturated and punchy. The forecast tomorrow has overcast skies and scatter rain showers and a possibility of no re-freeze overnight. It will be key to monitor surface conditions as you travel – even if we don’t see any sun in the next few days.

CORNICES: There are lots of large cornices on ridge tops. These features can heat up with daily warming making them easier to trigger. Give them lots of space.

An avalanche in Seattle Creek in -2 Bowl that occurred sometime over the weekend on a NW aspect at 3200′. We suspect this avalanche remotely triggered due to the nearby tracks on the slope. The weak layer was buried surface on a melt freeze crust.

An avalanche triggered on Sunday (4/14/19) on a NE aspect at 3200′. On the back side of this ridge is the -2 Bowl avalanche. Although we couldn’t access this crown we suspect the weak layer was buried surface hoar.

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

It’s been 10 days since our last known glide avalanche, but keep in mind glide cracks are continuing to creep down-hill. As always identify cracks and avoid traveling under their runout. They are unpredictable and can avalanche at any time.

Weather

Tue, April 16th, 2019

Yesterday: Skies were clear and sunny with a few clouds. Daytime temperatures reached the mid 50Fs at sea level, mid-40Fs at 1000′ and mid-30Fs along ridgetops. Overnight temperatures were in the upper-20Fs along ridgetops and in the mid to low-30Fs in the mid and lower elevations. Ridgetops winds were 5-15mph from the East. No precipitation fell.

Today: Partly cloudy skies becoming mostly cloudy tonight. Scattered rain showers are possible overnight. Temperature in the mid and lower elevations will be in the mid 40Fs to low 50Fs. Upper elevations will range from the upper 20Fs overnight to mid 30Fs during the day. Winds will be 5-15mph from the East. Up to 0.15″ rain is possible overnight, 1-2″ of snow above 2000′.

Tomorrow: Expect overcast skies with scattered rain showers most of the day, 0.15″ of rain possible, 1-2″ of snow above 2500′. Temperatures at sea level will be in the upper-30Fs to upper-40Fs. Upper elevation temps in upper 20Fs to low-30Fs. Rain/snow line may will be between 2000′ and 2500′. Easterly winds 5-15mph will remain.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	37	0	0	63
Summit Lake (1400′)	35	0	0	17
Alyeska Mid (1700′)	36	0	0	56

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	28	ENE	5	14
Seattle Ridge (2400′)	33	ESE	4	14

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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