Sun, February 05, 2023 Turnagain Pass Avalanche Forecast

Sun, February 5th, 2023

Alpine

Above 2,500'

4 - High

Treeline

1,000'-2,500'

4 - High

Below Treeline

Below 1,000'

4 - High

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
N E S W NW NE SE SW

Aspect/Elevation
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.

Aspect/Elevation of the Avalanche Problem

Specialists develop a graphic representation of the potential distribution of a particular avalanche problem across the topography. This aspect/elevation rose is used to indicate where the particular avalanche problem is thought to exist on all elevation aspects. Areas where the avalanche problem is thought to exist are colored grey, and it is less likely to be encountered in areas colored white.

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

Finally, after an 11 day stretch of benign weather a storm is headed into the region. Ridgetop winds began picking up around 4am and snowfall should begin around 6am. The avalanche danger will rise through the day relative to the amount of new snow and wind. Turnagain Pass could see up to 18″ of snow by 6pm. Other areas could see more or less depending on where the storm favors. Ridgetop winds however should be widespread through the region blowing 45-55mph with gusts into the 70’s and 80’s mph.

If the storm verifies, we can expect a variety of storm snow avalanches to start occurring through the day. The first is storm slabs that form on slopes out of the wind. These are composed of the new snow and release when the new snow hasn’t bonded yet with the older snow surface. Second are wind slabs that develop on wind loaded slopes. These could be up to 2′ deep due to the strength of the winds. Third are loose snow avalanches that are common during storms in steep terrain. And last are fresh cornices. Chunks of cornices are likely to break off and could trigger avalanches below. Hence, we are expecting a HIGH avalanche danger by sunset.

For those headed out today, paying close attention to the changing weather will be key. Areas seeing less snowfall could still be quite dangerous due to the strong winds. If you are getting out early, know the danger will be rising rapidly. This new snow should be very light and with snow falling all the way to sea level, avalanches could be seen even in the lower elevations. Additionally, powder clouds and debris could run further than expected.

A quote from our friends at the National Weather Service this morning:
“There is quite a bit of uncertainty in snow amounts in the Portage and Turnagain Pass areas, where high snow ratios could bring up to 2 ft for Turnagain Pass and 3 ft for Portage Valley of snow through Monday morning.”

Snowfall amounts from 6am this morning through 6am Monday. Graphic by the NWS that can be found on their Avalanche Weather Guidance page HERE.

Avalanche Problem 2

Persistent Slabs
N E S W NW NE SE SW

Aspect/Elevation
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

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.

Aspect/Elevation of the Avalanche Problem

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

Today’s new snow and wind will be a good test for the layer of buried surface hoar that we’ve been worried about for the last two weeks. This layer was 2-3′ deep but should be 3-4′ deep after today. After being very reactive 2 weeks ago, it was healing and not showing signs of reactivity the past several days. We will be trying to assess if any storm snow avalanches will step down and cause this layer to fail. If that happens, a much larger avalanche would result.

Additional Concern

Deep Persistent Slabs

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.

More info at Avalanche.org

We are still tracking the Thanksgiving facet/crust combo at the base of the snowpack. Even with 1-1.5″ of water weight with today’s storm, it’s unlikely this is enough load to reactivate this very deep layer. This is something we are watching for in the future if we get into a more potent storm series.

Weather

Sun, February 5th, 2023

Yesterday: Partly cloudy to sunny skies were over the region with light southeasterly winds along the ridgetops. Temperatures were in the teens at the higher elevations and in the 20’s at the lower elevations.

Today: A storm is heading in today bringing strong easterly winds and snowfall. The peak of the storm should be this afternoon. Between 1-2′ of snow is expected. Ridgetop winds should pick up to 45-55mph with stronger gusts by midday. Temperatures will remain just cool enough for snow to fall to sea level.

Tomorrow: As the storm moves out tomorrow, light snow showers could add another 2-6″ to the storm total. Ridgetop winds shift more southerly and decrease to the 20-30mph range. Temperatures are forecast to rise slightly, creating some light rain at sea level.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	N/A*	N/A*	N/A*	N/A*
Summit Lake (1400′)	N/A*	N/A*	N/A*	N/A*
Alyeska Mid (1700′)	22	trace	0.05	64
Bear Valley – Portage (132′)	25	2	0.2	–

*The Snotel server is down. No data.

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	15	NE	5	26
Seattle Ridge (2400′)	18	SE	2	21

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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