Fri, March 17, 2023 Turnagain Pass Avalanche Forecast

Fri, March 17th, 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.

Recent Avalanches

There were several recent avalanches reported yesterday that were all on the small side (D1). Two avalanches were triggered in the Tincan Common area on south facing slopes where the new snow was not bonding well to the old icy snow surface. Similarly on Sunburst there was another southern aspect with a small surface snow avalanche. In areas that received consistent sunshine yesterday dry loose avalanches triggered by the sun on southern aspects were also observed.

Skier triggered avalanche on the south aspect of Tincan, the skier was not caught and was able to ski off the slope. Photo 3.16.23 from Elias Neuffer

Another skier triggered avalanche on Tincan Common on a southern aspect where no one was caught or buried. Photo from 3.16.23 by Catherine Rocchi

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

Heavy snowfall and strong winds will start again today which will rapidly increase avalanche danger. We are expecting 10-12″ of new snow during the day with periods of very heavy snowfall. The highest snowfall totals are expected to be near Girdwood and areas closer to Prince William Sound, like Portage. Wind speeds should be in the 40-60 mph range at upper elevations with stronger gusts. This rapid loading of the snowpack, which is still adjusting to the last foot or more of new snow that fell in many areas on Wednesday, will likely be enough to cause widespread natural avalanches 1-3′ deep in areas with active wind loading. In areas sheltered from the wind natural and human triggered avalanches avalanches 1-2′ deep are likely because the last round of snow did not bond very well to the old snow surface.

After the last round of snowfall we saw avalanche activity on southern aspects that had a sun crust on the old snow surface. With more stress added to the snowpack today it is possible that we could also start to see larger and more connected avalanches releasing on shaded slopes that have a buried weak layer of facets underneath the last round of snow. In general today is a good day to step back from avalanche terrain and give the mountains some space to adjust to the incoming storm. We recommend avoiding avalanche terrain, especially in areas with active wind loading, and being very aware of any avalanche paths overhead that could release naturally.

NWS forecast snow totals from Friday morning to Saturday morning. Graphic from NWS Anchorage 3.17.23

Additional Concern

Persistent Slabs

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.

More info at Avalanche.org

In areas of the forecast region with a thinner overall snowpack, like near Johnson Pass, there is the potential for avalanches on deeper buried weak layers that we are still concerned with. The rapid loading today with new snow and wind could be enough to reactivate these buried weak layers and larger avalanches with wide propagation are possible. We recommend an extra conservative mindset in these areas because there is the potential for a large avalanche to release at upper elevations and run into lower elevations.

Weather

Fri, March 17th, 2023

Yesterday: Mix of cloudy conditions and periods of broken skies with sun poking through. Turnagain Pass and the Portage/Placer area picked up another 0.2-0.3″ of water which should be roughly 2-3″ of snow. Winds were moderate to light at lower elevations and stronger at upper elevations. Sunburst had average wind speeds of 10-15 mph and gusts of 15-30 mph with occasional stronger gusts up to 50 mph. Temperatures remained in the teens to mid 20s F with snow falling down to sea level.

Today: Heavy snowfall and strong winds are expected to pick up again this morning. 10-12″ of new snowfall is forecast during the day today in Turnagain Pass and Girdwood with higher totals expected in areas closer to Prince William Sound. Over the next 24 hours Turnagain Pass could receive up to 12-18″ of new snow, with Girdwood estimated at 18-24″, and Portage possibly closer to 18-30″. Accompanying all the new snow will we wind speed averaging 40-60 mph at upper elevations with stronger gusts. Snow line could creep up to about 500′ today with some warmer temperatures accompanying this low pressure system.

Tomorrow: Light snowfall is expected to continue during the day tomorrow with mostly cloudy skies. An additional 2-3″ of snowfall is possible during the day and then another strong pulse of snowfall is expected to start late Saturday into Sunday. This second strong pulse of moisture could bring up to another 6-10″ of snow by Sunday morning. Even stronger winds will accompany the second pulse of moisture with wind speeds reaching 80 mph at upper elevations on Saturday evening through Sunday morning. Storm totals from Friday morning through Sunday morning could be as high as 20-25″ for Turnagain Pass and Girdwood, and closer to 30-36″ in Portage. These might be optimistic values, but the weather models seems to all agree that at least some heavy snowfall will impact the area this weekend.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	25	2	0.2	76
Summit Lake (1400′)	24	1	0.1	40
Alyeska Mid (1700′)	24	0	0	66
Bear Valley – Portage (132′)	27	2	0.23	–

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	13	NE	13	48
Seattle Ridge (2400′)	18	SE	4	12

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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