Sat, April 20, 2019 Turnagain Pass Avalanche Forecast

Sat, April 20th, 2019

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

Storm Slabs
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

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.

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

Anywhere between 6-16″ of new dry snow has fallen over the past 36 hours across the region. Girdwood Valley seemed to have picked up the most snow, with over a foot accumulating at the high elevations. Turnagain Pass and Summit Lake have both seen around 6-8″ of new snow. Easterly ridgetop winds have been 15-25mph, which is enough to load leeward slopes. That said, avalanche concerns will be relegated to the new snow. How much snow fell in the area you travel, how much wind loading occurred and how the new snow is bonding to the underlying surfaces are the questions today. High elevation northerly aspects are the most suspect for poor bonding as the new snow fell onto weak older snow and possibly surface hoar. If you are headed out today in hopes of the clouds breaking, keep the following in mind:

Dry slab avalanches up to a foot thick could be triggered on steep northerly slopes
Wind loaded slopes are more concerning as slab depths could be greater (1-2′ feet)
Listen/feel for collapsing in the new snow
Watch for cracking in the new snow
Quick hand pits are great ways to assess the new snow/old snow bonding

Avalanche Problem 2

Wet Loose
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

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.

Likelihood of Avalanches

Size of Avalanches

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

On all Southerly facing slopes and slopes below 2,500′ we can expect the new snow to stick rather well to the old soft/warm crusts. The concern here will be daytime warming and sunshine. Roller balls and wet loose sluffs should be expected as the new snow heats up and begins to roll and sluff off the mountainsides. The size of potential sluffs will be determined by the amount of new snow. Wet sluffs could become quite large and dangerous on slopes with up to a foot or more of new snow.

CORNICES: There are still large cornices along ridge tops. Give them lots of space as they can break farther back than expected.

GLIDE AVALANCHES: It’s been close to two weeks since our last known glide avalanche, but keep in mind glide cracks are continuing to creep down-hill. As always, limit traveling under their runout. They are unpredictable and can avalanche at any time.

Weather

Sat, April 20th, 2019

Yesterday: Roughly 6-10″ of new snow fell over the region at all elevations late Thursday and early Friday morning, favoring Girdwood Valley. Light showers continued through the day adding another 1-3″. Ridgetop winds over the past 24-hours have been easterly 15-25mph with gusts to 40mph. Temperatures have been in the 20’sF along ridgetops and near 30F at 1,000′.

Today: Clearing skies and lingering clouds are expected today before a third round of precipitation pushes in tonight. Ridgetop winds should be in the 10-20mph range from the east today and bumping up to the 30’s tonight. Temperatures are on a warming trend and may reach 40F at 1,000′ today with ridgetops in the upper 20’sF.

Tomorrow: Stormy wet weather is on tap. Rain up to 1,500 – 2,000′ is expected tomorrow with 6-12″ of snow above this. Up to 16″ of snow could fall in favored areas such as Portage Valley and Girdwood. Ridgetop easterly winds will be in the 30-40mph range with gusts over 50. Unsettled and wet weather looks continue into the workweek.

Thanks to our NWS partners for their graphic of QPF (Quantitative Precipitation Forecast), which is the water amount expected through Sunday. A general rule of thumb is 1″ of water = 10″ of snow. Of course it has to cold enough for snow and this event is expected to bring rain up to 1,500′ with snow falling at the higher elevations.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	34	1	0.2	65
Summit Lake (1400′)	32	3	0.3	20
Alyeska Mid (1700′)	30	3	0.4	62

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	20	SE	15	40
Seattle Ridge (2400′)	25	*N/A	*N/A	*N/A

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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