Sun, February 23, 2020 Turnagain Pass Avalanche Forecast

Sun, February 23rd, 2020

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

3 - Considerable

Below Treeline

Below 1,000'

3 - Considerable

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

Wind Slabs
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

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

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

Cold temperatures and moderate to strong northwest winds are expected today from 20-25+mph and will be capable of transporting surface snow. With all this fresh snow, we can expect to find new touchy wind slabs around 1-2′ thick. In areas seeing snow pluming off the peaks and ridges, natural wind slab avalanches are possible. This is most likely south and north of Turnagain Pass in areas such as Summit Lake, Lost Lake and Girdwood. Additionally, there still could be older wind slabs or storm slabs lingering from last week’s storm. If a wind slab or other slab releases in the top few feet of the snowpack, it could ‘step down’ and trigger a much larger slide that breaks in deep weak layers; more on that below.

Cornices: These have grown substantially over the past week and could easily be triggered. If making it to the ridgelines, be sure to give these features a very wide berth.

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

3-5 feet of new snow accumulated from 2/18-20 and has added to forming a thick deep slab over weak faceted snow from January. The storm cycle this week naturally triggered many steeper slopes to avalanche. Many steep slopes didn’t avalanche, and may not take much load to initiate. If todays clear skies draw us into the mountains, it’s easiest to avoid these issues by remaining in the flats or on slopes 30 degrees or less with nothing steeper above.

The natural cycle we had a glimpse of Thursday afternoon proves that the snowpack isn’t stable. Many of the avalanches had crowns 5-8′ thick, were rated ‘very large’ and stepped down into buried weak snow. In shallower snowpack zones such as Lynx Creek or Summit Lake, it could be much easier to initiate a large slab. Yesterday a party felt many large whumpfs in Summit Lake.

If you are headed out into avalanche terrain today, things to remember:

Between 3-7′ below your snowmachine or feet sits various layers of weak sugary faceted snow
Very large avalanches could be triggered that take out the entire slope
Yes, many slopes have already slid, but these could have portions still intact and waiting for a trigger
There is no way to know how close a slope is to avalanching. It could be the first person or the 15th person that hits the trigger point.
Remote triggering is possible (initiating a collapse in the weak layer from the top, side or bottom on the slope resulting in an avalanche)

This slab avalanche was naturally triggered sometime in the range of 2/18-20 on a southwest facing aspect of Sunburst. This slope is frequented by skiers. What will it take to initiate the adjacent slopes? 2/22/2020 . Photo: Kellie Okonek

Pictured here is one slab of many naturally triggered avalanches that occurred earlier in the week. Many steep areas that haven’t avalanched could still be near the tipping point. 2/22/2020 . Photo: E. Roberts

Weather

Sun, February 23rd, 2020

Yesterday: Skies were partly cloudy with a few snow flurries but no accumulation. Ridgetop winds were out of the east in the 10-15mph range with gusts to 25mph. Temperatures were mostly in the teens °F to single digits in the evening.

Today: Isolated snow showers are expected in the morning but mostly sunny today, with a high near 12°F and a low of -11°F. Winds will be out of the northwest and expected to be 20 to 25 mph.

Tomorrow: Sunny skies are forecast with a high near 16°F and low near 0°F. Winds are expected to be from the Southwest at 5 mph, then shifting to easterly in the evening as clouds roll into the region.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	15	1	0.1	83
Summit Lake (1400′)	12	0	0	30
Alyeska Mid (1700′)	15	1	0.14	90

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	7	VAR	5	14
Seattle Ridge (2400′)	13	*N/A	*N/A	*N/A

*Seattle Ridge anemometer (wind sensor) is rimed over and not reporting.

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
10/27/24	Turnagain	Observation: Tincan	Michael Kerst
10/21/24	Turnagain	Observation: Turnagain Pass Road Observation	Trevor Clayton
10/19/24	Turnagain	Observation: Tincan – Below Todds Run	Andy Moderow
10/18/24	Turnagain	Observation: Taylor Pass	Eli Neuffer
10/15/24	Turnagain	Observation: Tincan Common	John Sykes Forecaster
10/14/24	Turnagain	Avalanche: Tincan	CNFAC Staff
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

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