Sat, February 29, 2020 Turnagain Pass Avalanche Forecast

Sat, February 29th, 2020

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

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

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

This Leap Day morning, clouds are oozing in ahead of a low-pressure system that will skirt by to the south tonight. Snowfall is slated for this afternoon and should pick up overnight giving us a shot for 4-8+” of new snow by tomorrow morning. Ridgetop winds are on the same schedule, blowing light from the east today until increasing to the strong category after the sun sets. With new weather coming in, it will not only be important to pay attention to this, but also not to forget about the deep slab problem lurking below.

Today’s main concern still centers around the weak faceted snow from January hidden 3-6 feet below our feet. It has been almost 5 days now since the last known deep slab avalanche was triggered. That said, for a ‘persistent’ deep slab problem, that really isn’t that long. The message remains: a person could find just the wrong spot and trigger one of these deadly slabs. Things to keep in mind:

The snowpack can feel ‘stable’ and no signs of instability may be present before a deep slab releases.
Areas with a shallower snowpack are more concerning for triggering a large slab.
Areas with little traffic so far this year are also more likely for triggering a deep slab.
It can be the 10th or 15th person on a slope before it avalanches.
These slabs can be triggered remotely, from the bottom, top or side.
Sticking to the low angle terrain (slopes 30° and less) with nothing steep above us are ways to avoid this hazard.

Great photo from Magnum ridge of the SW face of Sunburst and the large slab avalanche that naturally occurred 9 days ago (Feb 19/20th). Roughly 1.5 to 2 feet of snow has fallen since the avalanche yet the crown line is still quite remarkable. Photo taken 2.28.20 by Billy Finley.

More avalanche eye candy from Billy Finley. This shot is of the north face of Cornbiscuit and shows the propagation of a very large slab avalanche that occurred naturally also 9 days ago, Feb 19/20th during the end of the Feb 18-20 storm cycle.

Avalanche Problem 2

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

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

The northwest wind yesterday escaped much of Turnagain Pass, but looked to have impacted portions to the south, such as Summit Lake and areas closer to Seward. As always, keep an eye out for areas with wind drifted snow and loaded slopes. Feel for stiff snow over softer snow and hollow feeling snow, all signs of a wind slab. With the January weak layer below, a wind slab or cornice fall has the ability to trigger a deep slab avalanche.

As new snow and wind picks up this evening, new fresh wind slabs will likely greet us tomorrow morning and pending on how much snow falls, these could be 1-2′ thick and quite touchy. Sluffs and shallow slab avalanches composed of the new snow are also likely and will depend on the new snow amounts. A layer of surface hoar sits on the snow surface currently, which can keep the new snow from bonding easily. Something we’ll be paying attention to as the storm progresses.

Cornices: As always, give these features extra room and limit exposure under them.

Surface hoar coats the top of the snowpack from peaks to valleys at the moment.

Weather

Sat, February 29th, 2020

Yesterday: Sunny skies with light to moderate northwesterly winds were over the region. Temperatures were chilly, in the 0-15°F range and have dropped to the minus single digits in low-lying valleys overnight.

Today: Partly cloudy skies becoming cloudy ahead of a storm system moving in this evening. Snowfall is expected to start in the afternoon and continue into tomorrow. Between 4-8″ of snow is expected with favored areas (such as Portage Valley) seeing closer to 10″ by tomorrow morning. Ridgetop winds will be 10-15mph from the east today before increasing to the 20-25mph range overnight. Temperatures will stay in the teens and rise to the 20’s°F at most locations.

Tomorrow: Snowfall should slowly tapper off tomorrow as the system moves out. An additional 2-4″ is expected through the day. Ridgetop winds switch more southwesterly and look to remain in the 15-20mph range. Temperatures are forecast to rise close to 32°F at sea level and the 20’s°F along ridgelines.

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

	Temp Avg (F)	Snow Depth (in)
Center Ridge (1880′)	15	76
Summit Lake (1400′)	8	28
Alyeska Mid (1700′)	13	82

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	9	SW	7	21
Seattle Ridge (2400′)	10	N	5*	10*

*Seattle Ridge wind speed and direction estimated from past 12 hours of data.

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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