Sun, December 02, 2018 Turnagain Pass Avalanche Forecast

Sun, December 2nd, 2018

Alpine

Above 2,500'

2 - Moderate

Treeline

1,000'-2,500'

1 - Low

Below Treeline

Below 1,000'

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

Persistent Slabs
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.

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

Snowy skies are in the forecast for today and tonight. Although Turnagain Pass is not favored for this storm, up to 3″ could fall by this evening and an additional 3 to 6″ tonight. The Portage zone and areas on the southern Kenai should see rain at sea level but up to a foot of snow by tomorrow morning at the mid elevations. The rain line should hover near 1,000′ and possibly lower. Will this system start building back the mid elevation snowpack? It could be a start, but another warm system looks to move in Monday night.

At the higher elevations, even small amounts of new snow will add weight to our existing snowpack. Sitting anywhere from 1 to 3′ below the snow surface is a thin layer of weak snow (buried surface hoar) . Friday’s earthquake gave this layer a good shake and triggering a large slide may becoming less and less likely, however it is still a concern. We can’t forget this layer is there in terrain above 2,500′, which can be easy as any obvious signs of instability are not likely to be seen. What we can do is listen for whumpfing, use safe travel protocol and choose lower angle slopes if we wish to avoid any uncertainty.

Avalanche Problem 2

Wind Slabs
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.

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

Moderate to strong ridgetop winds from the south and east are again over the area. Wind slabs lurking in catchment zones from yesterday as well as new slabs formed today are possible to find and trigger in steep terrain. Keep an eye out for yesterday’s wind loading patterns, which could be obscured by last night’s one to two inches of snow. Snowfall should accompany the strongest winds today, which are forecast for the afternoon.

Weather

Sun, December 2nd, 2018

Yesterday: Mostly cloudy skies were over the area with light snow showers adding 1-2″ of snow in the evening. Winds were strong out of the northeast at Sunburst weather station, yet the main flow direction was more southerly. Temperatures were in the low 30’s F at most mid and low elevations and in the upper 20’s F along ridgelines.

Today: Another round of light snowfall is expected later today (1 to 3″) that could continue through tonight (an additional 3 to 6″). Snow/rain line should hover around 1,000′. This system is coming in from the SW and more favorable for snowfall in the Anchorage area and Hatcher Pass. Associated winds will again be southerly and easterly in the 20-30mph range with stronger gusts. Temperatures will remain in the low 30’s near 1,000′ and the mid to upper 20’s along ridgetops.

Tomorrow: A short break in weather systems will give us mostly sunny skies for Monday. Models are showing that Monday night clouds and precipitation move back in as another large low-pressure pushes a moist frontal band over Southcentral.

*Seattle Ridge wind sensor is rimed over.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	32	2	0.2	12
Summit Lake (1400′)	28	1	0.1	2
Alyeska Mid (1700′)	31	0	0.15	0

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	23	NE	21	56
Seattle Ridge (2400′)	27	*no data	*no data	*no data

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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