Thu, December 20, 2018 Turnagain Pass Avalanche Forecast

Thu, December 20th, 2018

Alpine
Above 2,500'

High (4)

Treeline
1,000'-2,500'

High (4)

Below Treeline
Below 1,000'

High (4)

Alpine Above 2,500'		High (4)
Treeline 1,000'-2,500'		High (4)
Below Treeline Below 1,000'		High (4)

Danger Scale:

No Rating (0)

Low (1)

Moderate (2)

Considerable (3)

High (4)

Extreme (5)

Avalanche Problem 1

Storm Slabs
Almost Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic

Very Large

Large

Small

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

Today we recommend avoiding the backcountry due to HIGH avalanche danger. Heavy snow has been falling over the last 24 hours and is expected to continue through early afternoon. The estimated snowfall for Turnagain Pass is 4-5’ and more snow is expected through early afternoon. Snowfall rates have been in the 6”/hour (0.2 – 0.3” SWE/hr) overnight at the Turnagain Pass SNOTEL and 1-2”/hour in Portage and Girdwood. This is an unprecedented amount of snow in a very short period of time and natural avalanches could run far distances and very fast. Avoid all avalanche terrain including slopes steeper than 30 degrees and stay far away from any runout zones. In Portage Valley where channeled terrain runs to sea level avoid any trails with steep slopes above, like Byron Glacier trail. As an alternative to the backcountry, consider going to Alyeska!!! Santa’s ski for free today.

24-hour Precip:

Turnagain Pass SNOTEL: estimated 4-5 feet (3.4” SWE)
Portage Valley: 22” (1.03” SWE)
Girdwood – Alyeska Midway: 20” (1.04” SWE)
Summit Lake: 10” (0.4” SWE)

The snow stake at Turnagain Pass DOT lot shows ~ 3-4 feet of new snow in the last 24 hours.

Crowns from natural storm snow avalanches were seen yesterday in the Girdwood Valley at around 2500′. In this zone there was 13″ new snow mid-afternoon.

Avalanche Problem 2

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

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

In thinner snowpack zones such as Summit Lake we are tracking buried layers of facets, crusts and buried surface hoar that sit 2-3′ under the snow surface. 10” of snow has fallen over night in Summit Lake and 6-12” is possible today. This new snow and moderate ridgetop winds may be adding more stress to an already weak snowpack. Triggering a slab near the ground is likely in avalanche terrain. The Lynx Creek and Johnson Pass zone have little to no snowpack data and often have a similar snowpack to Summit Lake. These zones are now open to snowmachining and this area likely received 2-3’ of snow in the last 24 hours. Today is not the day to push into Johnson and Lynx. HIGH avalanche danger is expected in this zone similar to Turnagain Pass.

Weather

Thu, December 20th, 2018

Yesterday: Heavy snowfall started around 8am and has continued through this morning. An estimated 4-5′ has fallen at Turnagain Pass (3.4 € SWE.) The Portage Rail Road Weather station is reporting 22 € at sea level (1.03 € SWE) and Alyeska midway is similar. Temperatures have remained in the mid to low 20F’s in the lower elevations and in the teens F at ridgetops. Ridgetop winds have been from the East 15- 30mph with some higher gust.

Today: A band of moisture and low pressure continue to pump snow through the Eastern Turnagain Arm region (North of Moose Pass to Girdwood.) The weather service has issued a winter storm warning with an additional 6-16″ is possible through early afternoon. Temperatures are expected to remain in the low 20F’s to low teens (F). Ridgetop winds will be light to moderate.

Tomorrow: An active weather pattern will continue tomorrow into the weekend with more snow showers in the forecast. Temperatures are expected to be in the 20F’s with light to moderate winds.

*Seattle Ridge weather station is rimed over and not recording any data.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	20	46″	3.4	85
Summit Lake (1400′)	16	10″	0.4	23
Alyeska Mid (1700′)	20	20″	1.04	40

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	13	ENE	14	37
Seattle Ridge (2400′)	*N/A	*N/A	*N/A	*N/A

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
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
04/24/22	Turnagain	Avalanche: Seattle Ridge – large glide avalanche on Repeat Offender path	CNFAIC Staff Forecaster
04/24/22	Turnagain	Avalanche: Seattle Ridge uptrack	Martin Schmidt

Riding Areas

Updated Fri, May 13th, 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	Open	Open. Extended opening through May 31.
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.