Mon, March 26, 2018 Turnagain Pass Avalanche Forecast

Mon, March 26th, 2018

Alpine

Above 2,500'

2 - Moderate

Treeline

1,000'-2,500'

2 - Moderate

Below Treeline

Below 1,000'

1 - Low

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
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

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.

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

As we enter the last week of Marvelous March, we are in a holding pattern of sorts avalanche-wise. The snowpack structure remains poor. Various weak layers in the middle and the base of the pack are persisting, keeping the avalanche danger at MODERATE for triggering a hard slab avalanche 2-4 feet thick. As time passes, the likelihood of triggering one of these dangerous avalanches is decreasing, but with the high consequences conservative terrain choices continue to be recommended. Many folks are taking advantage of the hard-pack snow conditions by long tours and exploring. With this however, keep in mind this avalanche problem is tricky and triggering a slab from the flats below a slope is possible. Being aware of runout zones and considering how far an avalanche could send debris should be in the forefront of our minds this spring.

In case you are just tuning in, our problem layers are from January. They are facets sitting on a slick melt/freeze crust at the mid elevations and facets mixed with buried surface hoar at the upper elevations. The slab on top is 2-4′ thick and very hard due to a strong Northwest wind event that ended last Thursday. This wind event caused unusual loading patterns and created a situation where the typical windward slopes (thinner weaker snow) are more loaded and avalanches could be triggered in unexpected places. To show this, observers have found poor structure along scoured ridges and under sastrugi. The last avalanche we know of was Friday (three days ago) in the Girdwood Valley and was remotely triggered from 200 yards away.

*Keep in mind that the snowpack may not show any obvious signs before the slope releases. It may be the 10th skier or snowmachiner onto a slope that finds a thin part of the snowpack (a trigger point). It is always good for us to take a moment and visualize the consequences if the slope are exposed to does slide.

This photo is looking up at the Southwest face of Sunburst. This old avalanche was triggered exactly two weeks ago today after the last snowfall event. On that Monday, a storm ended that brought 2-3′ of new snow to Turnagain. This snow has now settled to around a foot or less and experienced much wind and sun damage over the past two weeks.

Pastoral Peak on the left and Kickstep on the right. The mountains look deceptively stable with a hard wind and sun affected surface.

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Additional Concern

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

Wind Slabs: Old hard wind slabs may be lurking on a variety of aspects due to prior unusual loading patterns. Steep rocky terrain, where slabs sit on unsupported slopes, are the most suspect for triggering a wind slab. This type of terrain is also suspect for triggering a deeper slab mentioned above.

Cornices: Cornices are large in places and the sun and above freezing temperatures can make them more unstable. Give cornices plenty of space and limit exposure underneath them.

Weather

Mon, March 26th, 2018

Sunny skies again filled the region yesterday before cloud cover moved in during sunset. The anticipated snow flurries never developed. Ridgetop winds remained Easterly in the 10-20mph range with the strongest recordings early this morning, gusting to the mid 30’s mph. Temperatures have remained in the teens along ridgetops during the past 24-hours but cloud cover has kept valley bottoms warm where stations are reporting 25-30F temperatures this morning.

Today, Monday, expect overcast skies, cool temperatures and moderate ridgetop Easterly winds. There is a chance a few snow flurries could make it down to the ground, but this is more likely on the Southern and Eastern most part of the Kenai Peninsula. Temperatures look to remain in the teens along ridgetops today with valley bottoms hovering in the mid 30’sF. Ridgetop winds are expected to remain in the 10-20mph range from the East.

For Tuesday and into Wednesday, a front moves through Western Alaska and may bring a light snow to Southcentral. There is much uncertainty as to this system, yet it doesn’t look like the Turnagain Pass area will see much snowfall, mainly cloud cover at this point.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	26	0	0	79
Summit Lake (1400′)	25	0	0	32
Alyeska Mid (1700′)	27	0	0	73

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	17	NE	12	34
Seattle Ridge (2400′)	23	ESE	17	34

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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