Tue, December 27, 2022 Turnagain Pass Avalanche Forecast

Tue, December 27th, 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.

Recent Avalanches

Despite the limited ability to travel along the roads and no known reports from the backcountry, many avalanche paths seen from the highway had fresh debris in them. It is clear a natural avalanche cycle was occurring yesterday, we are just not sure of the extent yet.

Fresh avalanche debris in the 5 Sisters slide path in Portage Valley. This path sits next to the 5 Fingers ice climbs. 12.26.22.

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

The first storm in a series of storms is slowing down this morning. Light rain is expected up to 1,000′ with light snowfall above this. Ridgetop winds will continue from the east but have backed off to the 20-30mph range. Despite the slowing of the storm, avalanche danger remains HIGH. Road conditions could also remain dangerous. I attempted to drive to Turnagain Pass yesterday and was turned around due to incredibly slick roads.

How the snowpack is responding to the onslaught of rain, warm snow, and wind is pretty clear from debris in the few avalanche paths seen along the Seward Highway. Today, natural avalanche activity should slow down but probably won’t entirely stop until cooler temperatures settle in and winds become light. We can expect all the varieties of storm avalanches: wet loose avalanches due to rain on snow below 2,000′, wind slabs releasing on wind loaded slopes in the higher elevations, storm slabs releasing in the weak older snow that the new snow is falling on, and dry snow sluffs in steep 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

Hopefully we can get some breaks in cloud cover to assess the extent of the recent avalanche activity. We are especially interested to see if avalanches are breaking deeper in the snowpack, in faceted snow around the old Thanksgiving crust. This would create a larger avalanche that could propagate wider than your typical wind slab or storm slab. As we move forward in time, understanding if these old weak layers could be triggered by people will be one of our biggest questions. For now however, avoiding avalanche terrain and letting the mountains do their thing is recommended.

Weather

Tue, December 27th, 2022

Yesterday: Warm, wet, and windy… Moderate to heavy rain fell to 2,000′, possibly higher yesterday with heavy snowfall above this (.5 to 2″ of water in the past 24-hours, equating to 5-20″ of snowfall up high). Ridgetop east winds were 25-45mph with gusts near 70mph. Temperatures hovered near 35F from 2,000′ and below and were near 25-30F along the peaks.

Today: The stormy weather should continue through today although precipitation rates and winds will be less than yesterday. Between .25 and .5″ of rain is expected at 1,000′ and below while 3-5″ of snow should fall at the high elevations. Ridgetop winds look to be 15-25mph with gusts near 40mph from and easterly direction. Temperatures will slowly decrease through the the day (mid 30’s at sea level and ~32F at 2,000′).

Tomorrow: The active storm pattern should remain tomorrow with light precipitation, strong ridgetop easterly winds, and cooling temperatures. The rain/snow line could drop to close to sea level by tomorrow morning. Stronger pulses of moisture look to hit Friday and into the New Year.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	35	rain	0.3	41
Summit Lake (1400′)	35	rain	0.1	28
Alyeska Mid (1700′)	35	rain	0.7	38
Bear Valley – Portage (132′)	40	rain	2	–

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	26	NE	35	69
Seattle Ridge (2400′)	29	SE	12	24

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
11/27/23	Turnagain	Avalanche: Tincan Ridge	Schauer/ Stiassny Forecaster
11/26/23	Turnagain	Observation: Road report: Slide with dirt on Repeat offender	Anonymous
11/26/23	Turnagain	Observation: Pete’s North	Ben Sullender
11/25/23	Turnagain	Observation: Tincan trees	John Sykes Forecaster
11/21/23		Observation: Spokane Creek	John Sykes Forecaster
11/20/23	Turnagain	Observation: Sunburst	Anonymous
11/19/23	Turnagain	Observation: Magnum – PMS Bowl	Schauer/ Cullen/ Jonas Forecaster
11/19/23	Other Regions	Observation: Sunnyside/Penguin	Jose Ramos-Leon
11/19/23	Turnagain	Observation: Eddies	Andy Moderow
11/19/23	Turnagain	Observation: Cornbiscuit	Anonymous

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