Thu, April 19, 2018 Turnagain Pass Avalanche Forecast

Thu, April 19th, 2018

Alpine

Above 2,500'

2 - Moderate

Treeline

1,000'-2,500'

2 - Moderate

Below Treeline

Below 1,000'

2 - Moderate

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

Human triggered fresh storm slabs or wind slabs 6″ to a foot or more thick are our main concerns today and tomorrow. We are in the midst of a springtime stormy weather pattern. Light rain has been falling at the lower elevations and snow above this. The rain/snow line has been fluctuating between 1,000-2,000′, in general. Strong Easterly winds accompanied the peak of the precipitation, which was yesterday afternoon.

Estimated Storm Totals from Tuesday night through 6am Thursday:

Girdwood Valley: ~4-8″ snow above 2,500′
Portage Valley: ~1-2′ snow above 2,500′ (by far the winner, areas closest to the Sound are the most favored for precip amounts)
Turnagain Pass: ~6-10″ snow above 2,500′
Summit Lake: 0″ to a trace

If you are headed out to catch the new snow at the higher elevations, caution is warranted. The new snow is falling on hard sun crusts on East, South and West aspects and on facets over a crust on high North aspects. Initial bonding should be poor in areas receiving over ~8″ of new snow. Keep in mind, wind drifted snow could form wind slabs as thick as a foot or two. This makes small avalanches much larger quickly. This hazard is a direct result of how much snow has fallen, if you only find a few inches of new snow, it’s not enough for a storm slab but watch for pockets of wind drifted snow on steep slopes.

PRE-STORM SURFACE – NORTHERLY ASPECT: 1cm of loose dry snow existed on hard melt/freeze crusts above 2500′ on shaded aspects

PRE-STORM: Slick melt-freeze surfaces on the South end of Turnagain Pass (Lynx Creek last week).

Avalanche Problem 2

Wet Loose
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

Wet Loose

Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

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

Light rain has been falling on an already saturated snowpack below 1,000′ and up to 2,000′ in places. Wet loose avalanches, and even a wet slab, are possible on any steep slope with a wet, soupy and non-cohesive snowpack.

These wet avalanches are most likely to threaten people that venture under avalanche terrain at the low elevations, such as in Portage Valley along the edges of Portage Lake and the Byron Glacier trail.

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 are starting to fall. New growth from the recent snow and wind on already teetering cornices will only add to their instability. Give them plenty of space and limit exposure time underneath.

Recent cornice fall and subsequent slab avalanche triggered below in the Skookum drainage last week before the wet weather.

–

Weather

Thu, April 19th, 2018

Yesterday, cloudy and wet weather was over the region. Light rain fell in Girdwood and Turnagain Pass (~.1-.4″) and heavier rain in Portage (1″). The rain/snow line fluctuated between 1,000-2,000′ and above this anywhere from 3-10+” could be found depending on favored zones mentioned above. Ridgetop winds during the peak of the storm were 20-30mph with gusts to 68 from the East. Winds have died off early this morning and sit in the 10-15mph range. Temperatures have been in the 40’sF at sea level and in the mid to lower 20’s along ridgelines.

Today, cloudy skies and light rain below 2,500′ is expected while a trace of snow may fall above this. Ridgetop winds will remain Easterly in the 10-15mph range. Temperatures should warm to 40F at 1,000′ and remain in the 20’s along ridgelines.

For tomorrow and even into the weekend, the active pattern we are in will continue. Cloudy skies, light rain, gusty Easterly gap winds along Turnagain Arm and incremental snow accumulation above 2,500′. Again, favored precipitation areas are those closest to the coast such as Whittier.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	33	4	.3	68
Summit Lake (1400′)	35	0	0	22
Alyeska Mid (1700′)	34	1	0.15	61

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	23	NE	23	51
Seattle Ridge (2400′)	28	E	12	31

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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