Mon, March 20, 2017 Turnagain Pass Avalanche Forecast

Mon, March 20th, 2017

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

3 - Considerable

Below Treeline

Below 1,000'

3 - Considerable

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

The forecasted scattered snow showers turned into a full-blown storm and 7-24 inches of snow fell across the advisory area in the past day and a half. Turnagain Pass received a foot. Girdwood Valley 7-10″, Portage Valley 18″ and Skookum/Placer was the winner with up to 24″. The new snow fell on weak surface snow that formed during the 3 weeks of high pressure (near surface facets and surface hoar). Yesterday observers reported natural loose snow avalanches and small slabs. Today as the snow settles and the slab sets up triggering a larger storm slab avalanche will be likely on steep slopes from sea level to the Alpine. Along ridgelines NE winds have stiffened snow and wind slabs will be likely in leeward terrain. Warming temperatures this afternoon and the possibility of clearing skies and sun will add to the likelihood of triggering and may cause natural avalanches. Whumpfing and shooting cracks were observed yesterday, both are obvious clues that the snowpack is unstable. Be on the lookout for these signs today.

New snow in Skookum over old faceted snow. Photo: Conrad Chapman

Cracking in the new snow. Photo: Conrad Chapman

Avalanche Problem 2

Dry Loose

Dry Loose

Dry Loose avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.

More info at Avalanche.org

Yesterday numerous natural loose snow avalanche (point releases) were observed as new snow accumulated over weak surface snow and skiers triggered sluffs in steep terrain. Natural damp or wet loose snow avalanches will be possible today with warming afternoon temperatures and sun. Triggering a loose snow avalanche will remain likely throughout the day. These avalanches could also initiate a slab on the slope below as the slab become more cohesive today. Remember loose snow avalanches/sluffs/point releases can be very hazardous if they knock you off your feet and/or push you into a terrain trap or slide onto you in a terrain trap from above.

Dry loose avalanches on West face of Magnum. Photo: Conrad Chapman

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

Persistent Slabs and Deep Slabs: In addition to the now buried weak surface snow (near surface facets and surface hoar) there are a variety weak layers in our thin snowpack. The February 9th buried surface hoar sits 2-4+’ below the surface and faceted snow sits in the mid and base of the pack. Due to the lack of changing weather over the past three weeks, these weak layers (with varying degrees of strength) have been in a ‘dormant stage’. Although unlikely today, an avalanche breaking deeper in the pack isn’t completely out of the question especially in areas such as Johnson Pass, Lynx Creek and on the Northern side of Girdwood Valley (near Crow Pass). Observers continue to find poor structure. The buried weak layers will require monitoring as the load on top increases incrementally and as temperatures rise. Yesterday was the warmest temperatures we have seen in the last month. An avalanche triggered in the upper snowpack could trigger a deeper avalanche.

Snow pit in Spokane Creek on March 18th. An Extended Column Test showed propagation potential on the Feb. 9th buried surface hoar layer. Photo: Chris McNeil

Weather

Mon, March 20th, 2017

Yesterday was mostly cloudy in the morning with snow falling until around 1pm. Snowfall was heavy at times. As noted above storm totals ranged from 7-24″. Skies became partly cloudy in the afternoon and temperatures rose. A few low to mid elevation weather stations hit just above 32F. The temperatures in the Alpine stayed in the teens to low 20Fs. Winds were predominately NE 10-20 mph gusting into the 30s. Overnight another couple of inches fell and skies were partly to mostly cloudy.

Today there is a chance of snow showers throughout the day with potential clearing this afternoon. Skies will be mostly to partly cloudy. Winds will be light and easterly. Temperatures will be in the 20Fs to 30Fs with the high forecasted to be above freezing.

Overnight there is a chance of showers and tomorrow is forecasted to be clear and sunny with highs in the 20Fs. Sunshine and mostly clear skies look to be the dominant pattern for most of the week.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	25	10	.2	69
Summit Lake (1400′)	21	2	.2	30
Alyeska Mid (1700′)	22	6	.4	66

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	14	NE	12	33
Seattle Ridge (2400′)	18	ESE-ENE	8	21

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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