Tue, March 29, 2016 Turnagain Pass Avalanche Forecast

Tue, March 29th, 2016

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

3 - Considerable

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

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

Deep southerly flow yesterday brought .4- 1.1” of water (4-12” of heavy, wet snow above 1,500’) across the eastern Turnagain Arm region with the Girdwood and Portage valleys being favored. Any new snow fell on a stout melt-freeze crust and the big question today is ‘how well is the new snow bonding to this pre-storm surface’? It’ll be prudent to ease in to terrain today and seek out small no-consequence test slopes to investigate the storm/ crust interface. Quick hand pits will also help distinguish how this interface is acting. With an increase in elevation expect to move from more of a wet snow avalanche problem to a wind slab avalanche problem in the upper elevations.

Heavy, wet snow was plastering to all windward surfaces yesterday including this vehicle in the motorized parking lot at Turnagain pass. photo: Ryan Lewthwaite

Wet avalanches: These can be easy to initiate in steep terrain (greater than 35 degrees) and will have the potential to entrain a significant amount of storm snow while gaining mass and momentum. These can be particularly dangerous when paired with a terrain trap. Unseasonably warm temperatures today, rain on snow or direct sun will further act to promote wet avalanches failing naturally.

Wind slabs: A brief period of wind toward the end of the storm yesterday likely built tender wind slabs in the alpine. These may be touchy, particularly on leeward (south and west), wind-loaded slopes. Expect wind slabs to be in the 16-24” range, again deposited on a stout melt-freeze crust that was found up to ridgetops before the storm.

Avalanche Problem 2

Glide Avalanches

Glide Avalanches

Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.

More info at Avalanche.org

With warming temperatures today and through the week, we could see another spike in glide avalanche activity. Glide cracks have been avalanching, several per day over the last couple of weeks with at least a couple caught on video HERE and HERE. Avoidance of the crack is best; but easier said than done at this point with glide cracks threatening very popular and well-traveled backcountry routes on both sides of the highway thru Turnagain pass. Continue to maintain vigilance and limit your exposure to slopes with glide cracks.

Additional Concern

Cornice

Cornice

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

More info at Avalanche.org

Cornices warrant mention today as well. We haven’t seen a real active day of widespread cornice failure yet this season. This just means they continue to grow and creep closer to failure. Warming temperatures this week could act as a catalyst for cornices to fall. Keep a wide berth both on ridges and when moving below corniced terrain.

Weather

Tue, March 29th, 2016

Moisture-rich southerly flow moved into the region Sunday night bringing storm clouds and consistent rain/ snow showers throughout the day yesterday. Girdwood and Portage saw the lion-share of moisture with over 2 € of water since Sunday evening. The rain/ snow line was generally between 1,000 €“ 1,500′ with a bit of wet snow falling as low as a couple hundred feet yesterday morning. Temperatures at Turnagain Pass averaged in the mid-30’s F at 1,000′ with winds generally light from the northeast before picking up yesterday in the late afternoon. Gusts late last night hit 64 mph at the Sunburst weather station (3880′) before dying off after midnight.

The moisture looks like it’ll mostly shut off today as the main band of precip moved just east of us overnight. Temperatures will continue to warm into the mid-30’s at 3,000; some of the warmest temperatures we’ve seen all winter are forecasted this week. Winds will be light (10-20mph) from the SE with perhaps just a few light rain showers this morning.

Another wave of moisture moves in late tonight and tomorrow before we see a day of drying on Thursday.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	33	4-6	.4	129
Summit Lake (1400′)	35	1-2	.1	42
Alyeska Mid (1700′)	32	7-12	1.1	119

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	24	ENE	18	64
Seattle Ridge (2400′)	26	SE	28	61

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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