Thu, December 14, 2023 Turnagain Pass Avalanche Forecast

Thu, December 14th, 2023

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

3 - Considerable

Below Treeline

Below 1,000'

2 - Moderate

Fri, December 15th, 2023

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

3 - Considerable

Below Treeline

Below 1,000'

2 - Moderate

Fri, December 15th, 2023

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

3 - Considerable

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

Another sneaker snowfall event developed yesterday, doubling the expected snow amounts for Turnagain Pass, Girdwood, and Portage Valley. Snow fell to sea level and because the temperatures were just perfect (not too hot and not too cold…) snow densities recorded at the Turnagain Pass SNOTEL were 5.6%!

Snowfall totals from yesterday (Wednesday Dec 13):
Turnagain Pass SNOTEL: 18″ snow (1″ SWE)
Girdwood Valley: 14-18″ snow (0.9″ SWE)
Portage Valley: ~20″ snow (1.6″ SWE)
Summit Lake: 2-3″ snow (0.2″ SWE)
Seward Area: just a trace

*Over the past 7 days, Turnagain Pass has seen 42″ of snow at the SNOTEL station, which means around 50″ or so near and above treeline.

Wind Slab Avalanches: With skies clearing today, avalanche issues will be focused on the new low density snow from yesterday. Ridgetop winds have been 10-15mph from the east and after quieting overnight, are forecast to turn northwesterly and blow in the 5-15mph range with gusts near 20mph today. This is not all that much wind, but with such fluffy snow, it will not take much to move the new snow into soft touchy wind slabs. Keep a close eye out for active wind loading and any signs of wind effect on the snow surface. Wind slabs could be anywhere from 6″ to 2′ thick.

Note: When winds are northwesterly region-wide, terrain channeling often produces southerly winds along the high use non-moto side (east side) of Turnagain Pass.

Storm Slab Avalanches: At the end of John’s field day yesterday the top half of the new snow was failing easily on steep little terrain features. This is sign that storm slabs were forming. For today, we could see shallow storm slabs composed of light snow

Super small storm slab triggered along the Lynx Ck trail yesterday during heavy snowfall. This is a Red Flag that tells us the new snow is beginning to act like a slab in areas out of the wind. 12.13.23.

Loose Snow Avalanches: Slopes without wind effect not only are suspect for storm slabs but are sure bet for loose snow sluffing. Any steep slope today should sluff easily. Sluffs could entrain a significant amount of snow in the larger terrain.

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

As folks test out the new snow, it will be good to keep in mind there is a questionable weak layer several feet below us. This is that layer we have been talking about for a while now and is weak facets that have been developing over the buried Thanksgiving Crust. The layer is 3-5′ deep at this point.

So far, data is showing that triggering a huge avalanche on this layer may not be likely. Yet, with added snow each day that likelihood may be increasing. The problem is a lot of uncertainty exists and at the end of the day, snow does not play by the rules. If skies clear enough to get onto larger slopes this issue should be in our minds, along with all the new snow avalanche issues. To avoid the problem stick to lower angle slopes, smaller terrain, and be aware of runout zones below large and steep terrain.

Snowpack profile in Lynx Ck mid-storm yesterday. 12.13.23.

Weather

Thu, December 14th, 2023

Yesterday: Cloudy skies with periods of heavy snowfall was seen yesterday. Between 2-20″ of very low density snow feel across the region, highest amounts in Portage Valley and Turnagain Pass, lowest in Summit Lake. Ridgetop winds were 5-15mph with gusts near 25mph from the east. Temperatures were in the teens to 20F.

Today: Snowfall is tapering off this morning and skies should break up to some degree through the day. Ridgetop winds are forecast to turn northwesterly and increase into the 10-15mph range with gusts near 20mph. Temperatures look to drop into the low teens and single digits as cold air moves in from the north.

Tomorrow: For tomorrow, Friday, another round of snow showers heads in with increasing northeasterly winds. Only a few inches is forecast, but snowfall looks to pick up on Saturday/Sunday which could add another foot or more during the weekend. Temperatures should remain just cool enough for mostly snow to sea level.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	17	18	1	80
Summit Lake (1400′)	13	3	0.2	34
Alyeska Mid (1700′)	19	18	1	65
Bear Valley – Portage (132′)	21	20	1.6	–
Grouse Ck – Seward (700′)	20	0	0	32

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	14	NE	9	34
Seattle Ridge (2400′)	16	E*	-*	12*

*Seattle Ridge weather station is rimed over with only a few hours or data reporting for wind.

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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