Mon, December 18, 2017 Turnagain Pass Avalanche Forecast

Mon, December 18th, 2017

Alpine

Above 2,500'

2 - Moderate

Treeline

1,000'-2,500'

2 - Moderate

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

Deep Persistent Slabs

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.

More info at Avalanche.org

It has been four days since the temperatures began cooling off and the extended wet storm cycles ended. It has been two days since the last snowfall. We are slowly getting a handle on the stability of the snowpack and a big thank you to all the folks that wrote in last night. We are most concerned with a known layer of faceted snow near and/or on the ground. This layer produced widespread human and natural avalanche activity during the beginning of December. The wet storms saturated the pack (and many of the facets) from 2,500′ in elevation and below, which is now locked up into stable crusts. From 2,500′ – ~3,000′ the facets remain but several thick crusts in the slab are helping to stabilize the pack. The big question is, above 3,000′ where the crusts disappear and facets under a hard slab may remain. Stability tests so far have been done up to 3,300′ and all signs point to a strong snowpack and no reactivity in the facets. See video below. This is promising. But, we must be vigilant and conservative this week as we explore the higher elevations.

Things to keep in mind:
– Triggering a dangerous deep slab avalanche is possible above 3,000′
– Shallow snowpack areas are most concerning (more trigger points and possibly more reactive facets). For instance, South end of Turnagain Pass, Crow Pass, Summit Lake
– No red flags likely to be present to indicate an unstable pack

Pictured below is the North side of Magnum Ridge showing wind scouring and a thin snowpack. The top portion pokes into the elevation band we are concerned about.

–

Avalanche Problem 2

Wind Slabs

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.

More info at Avalanche.org

The other set of avalanches problems we are still assessing are old wind slabs, cornices and sluffing. These are surface instabilities and much easier to see and assess. So far we have found good bonding with Friday’s new snow as wind slabs seem quite stubborn. However, we have only had information for a small amount of terrain. Keep an eye out for cracking in the snow around you, hollow feeling snow and stiff snow over softer snow. In most places under 2,500′ there is crust underneath, watch for area where the Friday’s snow is not bonding to this crust.

The SW face of Sunburst. Old avalanche debris is covered by 8″ or so of new snow from Friday, Dec 15th.

–

Weather

Mon, December 18th, 2017

Partly cloudy skies were over the forecast area yesterday. During the past 48-hours there has been no precipitation. During the past 24-hours winds have been light from the East and North and temperatures in the mid 20’s at most locations. Overnight, an inversion has set in and valley bottoms are in the low teens while ridgelines are in the upper teens.

Today, we can expect a mostly sunny day. Ridgetop winds are forecast to remain light, 5-10mph from a Northerly direction and switch to Easterly tonight, remaining light. Temperatures in valley bottoms should climb during the day to the mid 20’sF while ridgetops remain in the low teens.

Tuesday, another round of snow is headed our way as a bearing low pushes a front over Southcentral. This system looks to be cold enough for snow close to sea level, it also should favor Anchorage, Hatcher Pass and bring a several inch refresh to our neighborhood in Turnagain Pass. Make sure and check tomorrow’s advisory!

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	24	0	0	33
Summit Lake (1400′)	25	0	0	27
Alyeska Mid (1700′)	18	0	0	10

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	*n/a	*n/a	*n/a	*n/a
Seattle Ridge (2400′)	22	SE	4	8

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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