Sun, December 11, 2022 Turnagain Pass Avalanche Forecast

Sun, December 11th, 2022

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

And the next round in a series of storms begins, this time the Kenai Mtns will be favored rather than Anchorage. Snowfall has already started and should peak this evening along with a rapid rise in avalanche danger. From the National Weather Service’s Avalanche Weather Guidance page:

“Two to four foot snow totals are looking likely throughout the mountain ranges of Southcentral …., most of which will fall between Sunday afternoon and Monday afternoon.”

Along the with the feet of new snow, ridgetop winds are going to be howling from the east. Weather models are showing 40-50 mph with gusts near 80, or more by 3pm. It is the textbook case of ‘heavy snowfall plus strong wind = avalanches’. How big the avalanches will be depends on the amount of new snow (1-2′ deep wind slabs by midday) and 2-4′ deep slabs in the overnight hours. The snow will be falling on a mixed bag of surfaces, hard wind packed snow, wind crusts, scoured surfaces to the Thanksgiving crust, and soft loose faceted snow in protected areas. Even if the snow wants to bond to some degree it won’t happen right away with such rapid loading.

For those getting out today, remember the storm will start peaking in the afternoon and know your exit routes. Natural storm slabs and more likely new wind slabs and sluffs should start releasing in the higher elevations by this afternoon. It may not be till after sunset that we see HIGH danger develop in the mid and lower elevations. Nonetheless, this should be quite an intense 24 hours and we don’t want to be caught off guard.

Graphic shows snowfall totals from 6am today through 6am tomorrow – wow!

Wind effect above the Repeat Offender slide path on Seattle Ridge. Photo by Carolyn Spencer, 12.10.22.

In the protected areas (including in the trees) soft faceted snow exists on the surface and by tomorrow morning, there could be some very touchy storm slabs in the trees. There is also a chance older wind slabs and even deeper layers in the snowpack could be overloaded by the storm snow, creating a larger slide. We are watching the old crust formed just before Thanksgiving that in some areas has facets around it.

Snow pit at 2,400′ in the Repeat Offender slide path on Seattle Ridge. Note the softer snow above the melt freeze crusts. We are watching these older faceted layers as they could get weaker as time goes on and become more of an issue – hopefully not. 12.10.22

Weather

Sun, December 11th, 2022

Yesterday: Overcast skies were over the region with light easterly ridgetop winds (~5mph gusting 10). Temperatures warmed into in the teens at most locations and elevations and have continued to warm near 20F overnight. Light snow flurries developed after sunset.

Today: Light snowfall this morning should intensity by this afternoon and peak tonight. Up to 6″ is expected by noon and another 6+” by 6pm. Overnight we could see a foot, brining snowfall totals to 2+’ by tomorrow morning. Temperatures look to stay near 32F at sea level and 20F in the mid elevations – so snow should fall to sea level through the storm. Ridgetop winds are expected to increase to 40-50mph with gusts near 80 by 6pm.

Tomorrow: Snowfall is expected to taper tomorrow morning as skies begin to clear for the afternoon. Ridgetop winds look to decrease as well and shift northwesterly, blowing 20-30mph.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	18	1	0.1	29
Summit Lake (1400′)	13	1	0.1	21
Alyeska Mid (1700′)	16	1	0.05	28
Bear Valley – Portage (132′)	16	1	0.1	–

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	13	NE	9	30
Seattle Ridge (2400′)	16	SE	7	20

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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