Thu, January 18, 2018 Turnagain Pass Avalanche Forecast

Thu, January 18th, 2018

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

3 - Considerable

Below Treeline

Below 1,000'

1 - Low

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
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

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.

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

Clear skies following a big storm is a red flag warning for human triggered avalanches. Over the last three days rain, snow and strong winds impacted our region with an additional 2-3’ of snow in the upper elevations on top of 2-3’ that fell over the weekend. Rain/snow line reached around 3000’ on Tuesday and slowly dropped to ~1300’ yesterday morning as temps cooled near the end of the storm. There was a widespread natural wet avalanche cycle observed on Tuesday in steep channelled terrain from Girdwood to Summit Lake.

STORM TOTALS above 2,500′ (1/15-1/17)

Turnagain Pass 20-30″ (2.3” of snow water equivalent)
Girdwood Valley 20-30” (2.1” of snow water equivalent)
Summit Lake 6-10” (0.6” of snow water equivalent)

Don’t Forget!!! This was the second storm in one week. Basically 4-6+’ of snow is sitting on old weak surfaces (surface hoar and facets) from last week. In between these two storms, several human triggered avalanches occurred on Seattle Ridge and Eddies, without incident, and stability tests were showing propagation potential in older layers of the snowpack. More time and lots of patience is required for all of this new snow to adjust. If you are headed out – keep these points in mind

Slabs triggered will be deep (2-3+ feet) and could step down to a deeper layer. These are dangerous and unmanageable avalanches
Due to the depth of the storm snow, no signs of instability may be present before someone triggers a slab
Remote triggering an avalanche from below, or near a slope is possible
Cooling temperatures will be forming a crust at lower elevation and stabilizing the snowpack below 1500’- don’t let this fool you into thinking the upper elevations are stabilizing quickly

Dry snow is covering and insulating a wet layer of snow within the new storm snow. Until this wet layer freezes, triggering an avalnche will remain possible in mid elevations.

Most of the gullies along Seattle Ridge are full of new debris from a wet snow avalanche cycle that occured on Tuesday.

Avalanche Problem 2

Deep Persistent Slabs
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.

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

Weak layers within the snowpack have the potential to re-activate with the added load of this week’s new snow (5” of water weight). Additionally, avalanches in the storm snow, discussed above, could step down to these deeper layers and produce a very large avalanche depending on the size of the slope. In short, two layers of buried surface hoar and near surface facets sit roughly 4-8’ deep at this point and is a concern at elevations above 2,000′. Basal facets, near the ground, remain a concern at elevations above 3,000′. At this point very little is known about reactivity of these weak layers, and its going to take time and patience for the snowpack to adjust.

*Giving the snowpack time to heal from these storms is key. Sticking to low angle terrain with nothing steeper above is recommended. Remember, it’s the first 2 days after a storm where most avalanche fatalities occur. Although there is nice powder at the upper elevations that can lure us, now is not the time to be sampling it.

A natural avalanche near Bertha Cr on the Southern end of Seattle Ridge (SE aspect) that broke into older layers of the snowpack.

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 been growing and were unstable in between the two storm cycles this week. Triggering a cornice is likely if you get too close. A cornice fall could trigger a very large avalanche on the slope below. Give these features lots of space and remember they can break further back from a ridge than expected.

Weather

Thu, January 18th, 2018

Yesterday morning a potent storm ended with a cooling trend. Freezing temps (32F) creeped down in elevation, and rain/snow line moved from around 3000′ the night before to ~1300′ by late morning. The most intense precipitation ended by 6am yesterday, but rain and snow showers were observed throughout the region most of the day. No measurable amount was recorded at Center Ridge weather station, but .25 € was recorded at the Turngain Pass DOT Lot. Girdwood midway station picked up ~.3 € of water late morning, which fell as a few inches of heavy wet snow at 1700′. Easterly ridge top winds were moderate, averaging in the 20-40mph becoming light by early evening. Overnight Satellite images indicate clearing skies and temps dropped to mid 20F’s 1000′ at Turnagain Pass.

Today expect clear skies, temperatures in between 20-30F and light Northerly winds. No precipitation is expected.

Cold and clear weather is expected over the next two days as high pressure from Siberia settles in over interior Alaska.

**Seattle Ridge weather station stopped recording wind data yesterday evening due to rime forming on the anemometer

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	29	0	0	56
Summit Lake (1400′)	29	0	0	15
Alyeska Mid (1700′)	30	3	0.3	42

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	23	ENE	13	44
Seattle Ridge (2400′)	27	**SE	**7	**31

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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