Wed, March 15, 2023 Turnagain Pass Avalanche Forecast

Wed, March 15th, 2023

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

Wind 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

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.

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

After a two week dry spell, a pattern shift is here bringing a few inches of snowfall today and possibly heavier snowfall on Friday. As the graphic below from the National Weather Service suggests, varying degrees of snow is expected (3-6″ ish) in general with the highest amounts in Portage and Placer Valleys (6-10″). Ridgetop winds have already ramped up overnight and are blowing in the 15-25mph range with stronger gusts from the east.

Avalanche danger will depend on 1) how much new snow falls and 2) how strong the wind blows. With such meager snow amounts, the most likely avalanche type will be wind slabs where winds are blowing the new snow, and possibly some old soft snow, into slabs. These are likely to be on the smaller side, maybe up to a foot thick. The old snow surfaces were a mix of sun crusts on aspects facing ESE through WSW and soft old faceted snow and harder wind affected snow on shadier aspects. Wind slabs forming on either one of these surfaces should be touchy, even if they are smaller, the sun crust especially. Watch for stiffer wind deposited snow and shooting cracks. Wind slabs should be easy to identify if we look for them.

Sluffs in the new snow should be easy to trigger on steep slopes with over 4″ of new snow. These could run further than expected if a sun crust is underneath.

If a zone sees snowfall in the 6-10″ range, such as forecast for Portage Valley or Placer, then shallow storm slabs may form in areas out of the winds. This means triggering a slab avalanche composed of the storm snow is possible.

Travel Advice: Paying attention to new snow amounts and wind loading will be key. Many areas may not see enough snow/wind to even elevate the danger. For example, if only 2-3″ of new snow falls with no wind loading, the danger will remain LOW.

Snowfall totals through today. There is high uncertainty, but it looks promising for a little refresh. Graphic courtesy of NWS Anchorage. 03.14.2023

Southerly slopes, and a few east and west aspects too, sport a sun crust of varying thicknesses. This will provide a slick surface for the new snow to fall on. 3.14.23.

Example of a low angle northerly facing slope with soft old faceted snow. This was an old wind crust that has been slowly weakening (faceting) with the cold temps over the past week. 3.14.23.

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

We are still concerned about a layer of weak facets buried about a foot deep in the southern edge of our forecast area. This layer is most problematic in a small area between Pete’s North and Johnson Pass, The layer was the culprit in a wide propagating skier-triggered avalanche last Monday (3/6). These stubborn persistent weak layers can be difficult to predict and there could still be a chance a person could trigger another slab, as occurred just over a week ago. Extra caution is recommended in this area.

Weather

Wed, March 15th, 2023

Yesterday: High clouds were over the region with light ridgetop winds from the SE (5-10mph). Temperatures were in the single digits to teens and climbed into the 20’sF at the lower elevations with daytime warming.

Today: Light snow showers are expected through today with 4-6″ of total accumulation. Ridgetop winds should be easterly in the 15-25mph range with some stronger gusts. Temperatures look to hover in the teens at most locations with lower elevations climbing into the 20’sF this afternoon.

Tomorrow: A break between storms is expected tomorrow. Partly cloudy skies, moderate east winds (10-15mph), and maybe a lingering flurry or two. Another system pushes in Friday that could bring higher snowfall amounts. Stay tuned.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	14	4	0.2	67
Summit Lake (1400′)	10	0	0	37
Alyeska Mid (1700′)	12	1	0.05	65
Bear Valley – Portage (132′)	10	2	0.15	–

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	10	NE	10	36
Seattle Ridge (2400′)	10	SE	3	10

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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