Sat, December 07, 2019 Turnagain Pass Avalanche Forecast

Sat, December 7th, 2019

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
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

New snow, wind and rain at sea level is on its way. This morning the storm is just moving in. Girdwood has picked up 3″ of new snow at the mid-elevations (rain at sea level). Turnagain Pass is still waiting for precipitation to start and is 34 degrees along the road at the Pass itself. There is a chance wet snow, and not rain, will fall at this elevation as our rain/snow line is forecast for 1,000′-1,500′. Avalanche danger will be related to how much snow falls. If the forecast verifies, there should be close to a foot of new snow above 1,500′ by this evening and another 5-8″ by tomorrow morning. (Don’t forget to click the ‘weather’ tab next to the ‘Avalanche Forecast’ for more on mountain weather!)

Storm snow avalanches can be expected today, either releasing on their own in areas with high snowfall rates or easily triggered by people. These will most likely be in the form of wind slabs just off ridgelines and/or in gullies that have been cross-loaded. Slabs could be anywhere from 6″ to 2′ thick. Cracking in the snow around you will be a sure bet you have found a slab and a sign to stick to low angle slopes, 30 degrees and less with nothing steeper above you.

Also keep in mind, soft slab avalanches on slopes out of the wind will be possible to trigger where a foot or more of new snow falls. Quick hand pits to test how the new snow is bonding with the old snow are a great tool on a day like today. Cornices will be growing with warm temperatures, wind and snow and should be very easy to break off.

*We hope it doesn’t, but if the rain line creeps higher than expected, wet avalanches will be possible in the mid-elevations. For example, along the east face of Seattle Ridge.

Natural sluffs along Sunburst’s SW face just under the weather station yesterday. How well will the new snow bond to this old snow surface is the question moving forward. (Photo: Billy Finley).

Avalanche Problem 2

Persistent Slabs
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

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.

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

The snowpack on high elevation slopes over 3,000′ remains a concern due to a layer of faceted weak snow over a hard crust near the bottom of the snowpack. With a new load of snow being added today/tonight and another round of heavier snow Monday, we will be watching to see if this weak layer becomes overloaded and results in larger avalanches that break near the ground. This is also something to pay attention to if traveling up to these higher elevations.

Weather

Sat, December 7th, 2019

Yesterday: Partly cloudy skies with moderate ridgetop easterly winds were over the region as our next storm system slowly moved in. A trace of snow fell at Turnagain Pass overnight and 2-3″ in the upper Girdwood Valley. Temperatures rose to the upper 30’s F at sea level and the mid 20’s F along ridgelines yesterday and that is where they are sitting this morning.

Today: Cloudy skies, strong ridgetop winds and snowfall are expected as the first round of several warm storms hits us. Today, we are expecting 1″ of rain at sea level (2″ in Portage area) with a rain line up to 1,000-1,500′. This equates to ~10-12″ of snow above 1,500′. Tonight, another .75″ of rain (1″ in Portage) is forecast with a rain line close to 1,500′, which should bring another 7-10″ of snow in the higher elevations. Ridgetop winds will be easterly in the 35-50mph range and temperatures will remain warm, near 40F at sea level and upper 20’s F along ridgetops.

Tomorrow: This first storm moves out tomorrow morning and models are showing a brief break in weather before a warmer and wetter system moves in for Sunday evening into Monday. Right now it’s looking like Girdwood and Turnagain Pass will see 2-3″ of rain at sea level (2-3 feet of snow in the Alpine) with Portage area seeing almost double this amount. Stay tuned!

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	30	1	0.1	15
Summit Lake (1400′)	27	1	0.1	10
Alyeska Mid (1700′)	28	3	0.25	18

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	20	NE	15	49
Seattle Ridge (2400′)	N/A*	N/A*	N/A*	N/A*

*Seattle Ridge anemometer (wind sensor) is rimed over and the temperature sensor is not functioning. A new temperature sensor is arriving soon and we hope to get it up on the next clear day.

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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