Tue, April 04, 2017 Turnagain Pass Avalanche Forecast

Tue, April 4th, 2017

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

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.

More info at Avalanche.org

Today we will see the brunt of the third storm to impact the area since March 27th. Winds are currently gale force from the East along ridgetops and .75-1″ of rain has fallen overnight up to 1,000′. For snow, this equates to the higher elevations accumulating between 1-2+ feet; the mountain landscape will have a much different look when skies finally clear. Another spike in natural avalanche activity is expected today and paths that have already released may be reloading and release again. Storm snow avalanches may also step down into weak older layers in the pack, creating an even larger slide. Add warm temperatures and rain to the mix and there is no question, avalanche terrain should be avoided.

How much load is on the old weak surface from March? Storm totals (March 27 – 6am April 4th):
Turnagain Pass: 3.0″ of H2O, 42+” of snow at upper elevations
Girdwood Valley: 4.2″ of H2O, 50+” of snow at upper elevations
Summit Lake: 1.1″ of H2O, 12-15″ of snow at upper elevations

Large amounts of debris are running to valley bottoms during this storm cycle. Thank you to Turner Pahl for sending in this photo from the north side of the Skookum Valley (just below the toe of the Skookum Glacier). We are unsure of when this specific avalanche released but the photo was taken yesterday.

Again, if choosing to brave the roads and parking lots and play in the flats, please remeber to stay well away from slopes and runout zones – debris can run further than you think during storms like this!

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Avalanche Problem 2

Wet Loose

Wet Loose

Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

More info at Avalanche.org

The rain/snow line has climbed from near sea level yesterday to 1,000′ overnight, possibly up to 1,500′ in places. Although we already saw a widespread natural wet slab cycle at 1,500′ on April 1st, we could see another one again today as another round of rain on snow is expected at the lower elevations. Hopefully the visibility will be good enough to see some of the action happening, but as is often the case, much goes unseen.

Snow pit at 2,000′ at Turnagain Pass on Sunburst, April 2nd. Note the wet layers up to 2,000′, they only become wetter at the lower elevations where we are expecting wet avalanches.

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Additional Concern

Persistent Slabs

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.

More info at Avalanche.org

Prior to this storm cycle the snowpack consisted of many weak layers of facets and buried surface hoar. As mentioned above, avalanches releasing in the storm snow have the potential to ‘step down’ into old layers where snowpack tests are still showing propagation potential. Today’s strong winds and precipitation are likely to tip in the balance in the upper elevations and we could see an avalanche breaking in the mid-pack or near the ground, with crown depths over 6′ and running the entire length of a slide path.

Weather

Tue, April 4th, 2017

Obscured skies filled the region yesterday with light flurries and rain to 100′ before the main portion of the storm moved in late in the day. Most of the action is from overnight when ridgetop winds picked up to the 40’s and 50’s mph with gusts in the 80’s from the East. Over 1″ of rain fell to 1,000′ at Turnagain Pass with .75 in the Girdwood Valley. This equates to 1-2+’ of snow at the upper elevations over the past 24-hours. Temperatures have been warm and fluctuating – sea level upper 30’sF, 2,000′ the lower 30’s F, ridgetops in the mid 20’sF.

Today, we should see continued precipitation with .6″ of rain below 1,000′ and 6-10″ of snow above 1,500′. Tonight another .25 of water is expected. The rain/snow line is tough to forecast and it could drop to 500′ and even lower if colder air slides in. Ridgetop winds will remain strong in the 30’s and up to 40mph from the East with higher gusts. Temperatures should remain warm, up to 40F at sea level, in the 30’s at the middle elevations and 20’s F along ridgelines.

This storm looks to stall out and churn through tomorrow and into Thursday bringing continued, but lighter, precipitation and strong Easterly winds.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	35	12	1.1	89
Summit Lake (1400′)	34	2	0.3	30
Alyeska Mid (1700′)	33	1	0.75	75

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	23	ENE	37	82
Seattle Ridge (2400′)	26	sensor rimed	sensor rimed	sensor rimed

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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