Fri, February 23, 2018 Turnagain Pass Avalanche Forecast

Fri, February 23rd, 2018

Alpine
Above 2,500'

Moderate (2)

Treeline
1,000'-2,500'

Moderate (2)

Below Treeline
Below 1,000'

Low (1)

Alpine Above 2,500'		Moderate (2)
Treeline 1,000'-2,500'		Moderate (2)
Below Treeline Below 1,000'		Low (1)

Danger Scale:

No Rating (0)

Low (1)

Moderate (2)

Considerable (3)

High (4)

Extreme (5)

Avalanche Problem 1

Persistent Slabs
Almost Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic

Very Large

Large

Small

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

Triggering a slab avalanche 1-2+ feet deep remains possible above 1000’ due several weak layers buried within the snowpack. A layer of buried surface hoar from Jan. 21st continues to show signs of reactivity in the upper elevations, and a layer of facets over a melt-freeze crust is suspect in the mid elevation band. The slab ranges from hard to soft depending on exposure to recent winds. Speaking of wind… yesterday we had 3-4” inches of new snow combined a strong Northwest winds overnight. This wind direction can funnel through Turnagain Pass from a variety of directions causing unusual wind loading patterns. Be suspect of hard snow over soft snow that feels upside down or where the slab is fully supportable or hollow sounding. Red flags like shooting cracks or “whumpfing” may not be present before a slope releases. Evaluate the terrain for consequences and be aware of places that haven’t seen much traffic. These less traveled places are more suspect for triggering a more connected slab.

Deep Persistent Slabs: Keep in mind that there are deeper persistent layers that could ‘wake up’ if you find the wrong spot above 3,000′ in the Alpine. At these high elevations, old weak layers of facets and buried surface hoar sit in the bottom half of the snowpack. This structure is most pronounced in places with a thin overall snow cover, such as the South end of Turnagain Pass, the Summit Lake area and Crow Pass.

Left picture shows the buried surface hoar about a 1′ below the surface, and the right hand picture is an example of the weak faceted snow sitting on a slick crust in the mid elevations.

Avalanche Problem 2

Wind Slabs
Almost Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic

Very Large

Large

Small

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

Triggering a small isolated wind slab is possible today on steep unsupported terrain features where winds have been transporting snow. These slab will likely be hard and supportable due to strong winds overnight. A slab could release from above once committed to a steep slope. Identify smooth pillow-shaped snow and cross loaded gullies and be aware that buried weak layers are widespread throughout our region. It may be difficult to distinguish between a small isolated wind slab versus a more connected persistent slab.

Sunshine: It’s that time of year when we need to pay attention to the sun. The sun can heat up Southerly aspects, and melt surface snow and cause small point releases in the loose new snow. This will be more of a concern if winds are calm today, which may be the case this afternoon. This heating can also cause a slab sitting on a weak layer to become more reactive. Avoid steep solar aspect if you notice the surface snow becoming moist or you see roller balls or point releases under rocks.

Moderate North winds loading a Southern aspect on Tincan yesterday afternoon.

Weather

Fri, February 23rd, 2018

A storm yesterday brought 3-4 € of new snow throughout our region followed by a strong Northwest wind event. These winds started late afternoon and have continued through the morning. Gap areas like Turnagain Arm have experienced stronger winds. Seattle Ridge weather station was averaging in the 30’s mph overnight with gusts into the 60’s and 70’s. Sunburst weather station was more protected and the strongest winds were reported early this morning with gust into the 30’s mph. Ridgetop temperatures dropped from the 20F’s into the low teens (F) overnight. Sea level temps remained in the 20F’s overnight.

Today expect clear and sunny skies most of the day. High clouds are expected by this evening as another low pressure system move into our region overnight. Northwest winds are expected to decrease this morning remaining light from the Northwest this afternoon. Temperatures will be in the mid 20F’s in upper elevations and may rise to low 30’s at sea level. Snow showers may start by late evening.

Several inches of new snow are forecasted by Saturday. An active Southwest pattern will bring a period of clear skies on Sunday followed by another round of snow on Monday. Southwest flow tends to favor Cook Inlet and the Matanuska Valley for snow, but precipitation is expected across our region. Temperatures should remain near normal averaging in the 20’s to mid 30’s with daily warming. Precipitation is expected to remain as snow.

*Center Ridge weather station has been producing erratic temperature data.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	*33	3	.2	65
Summit Lake (1400′)	25	2	.2	25
Alyeska Mid (1700′)	25	2	.2	57

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	18	NW	10	34
Seattle Ridge (2400′)	24	NW	23	73

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
05/28/22	Turnagain	Avalanche: Turnagain Pass – late May wet slab cycle	CNFAIC Staff
05/21/22	Turnagain	Avalanche: Magnum, Lipps and Tincan	CNFAIC Staff
05/17/22	Turnagain	Avalanche: Sunburst	CNFAIC Staff
05/17/22	Turnagain	Observation: Tincan Ridge	Joe Kurtak
05/11/22	Turnagain	Avalanche: Cornbiscuit and Magnum west faces	CNFAIC Staff Forecaster
05/07/22	Turnagain	Observation: Granddaddy	Kit Barton
04/29/22	Turnagain	Observation: Sunburst wx station	AS/ MM/ AM/ NH
04/28/22	Turnagain	Observation: More Turnagain Pass/Summit Lake wet slab activity	Alex Marienthal
04/27/22	Turnagain	Observation: Magnum	Sykes / Buttrick Forecaster
04/27/22	Turnagain	Observation: Girdwood/Summit/Turnagain Road obs	A S

Riding Areas

Updated Wed, June 01st, 2022

Status of riding areas across the Chugach NF is managed by the Glacier and Seward Ranger Districts, not avalanche center staff. Riding area information is posted as a public service to our users and updated based on snow depth and snow density to prevent resource damage at trailhead locations. Riding area questions contact: mailroom_r10_chugach@fs.fed.us

Area	Status	Weather & Riding Conditions
Glacier District
Johnson Pass	Closed	Closed as of May 1 per Forest Plan.
Placer River	Closed	Closed as of April 25th due to insufficient snow coverage.
Skookum Drainage	Closed	Closed as of April 1st per Forest Plan.
Turnagain Pass	Closed	Closed as of June 1st.
Twentymile	Closed	Closed as of April 6th due to insufficient snow coverage.
Seward District
Carter Lake	Closed	Closed as of May 1 per Forest Plan.
Lost Lake Trail	Closed	Closed as of May 1 per Forest Plan.
Primrose Trail	Closed	Closed as of May 1 per Forest Plan.
Resurrection Pass Trail	Closed	Closed as of May 1 per Forest Plan.
Snug Harbor	Closed	Closed as of May 1 per Forest Plan.
South Fork Snow River Corridor	Closed	Closed as of May 1 per Forest Plan.
Summit Lake	Closed	Closed as of May 1 per Forest Plan.