Fri, December 06, 2019 Turnagain Pass Avalanche Forecast

Fri, December 6th, 2019

Alpine

Above 2,500'

2 - Moderate

Treeline

1,000'-2,500'

1 - Low

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

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

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

Today should be our last day of cold temperatures before our weather does a complete 180 on us. A large scale system with strong southerly flow is headed in this evening and will bring considerably rising temperatures, gale force winds and heavy precipitation through the weekend. Rain could make it above 2,500′ by Sunday and another pulse looks to pump more warm air and moisture in for Monday and Tuesday. After today, expect rising avalanche danger, wet soggy snow along Turnagain Pass, and if we are lucky, the rain line will not reach our ridgetops. Stay tuned for more details on precip numbers tomorrow morning.

For today however, we are still in a similar avalanche pattern. Our main concern is at the upper elevations above 3,000′ where a person could trigger an avalanche 2-3′ thick breaking near the ground. If skies are clear enough to travel above 3,000′, keep in mind there is a potential weak layer of facets lurking under you. There could be no signs of instability before a slope cracks and releases. Unfortunately, snowpack tests continue to show reactivity in a weak layer of facets just above hard crust. This is keeping our hackles up on high alpine slopes.

High elevation terrain, pictured above, is where our avalanche concerns exist.

Snow pit at 3,200′ on Sunburst shows a thinner section of the snowpack (2.5 feet total depth) and thinner section of the slab (1.5 feet). This pit had the most reactive and concerning stability test results and tells us this layer of faceted snow could create an avalanche.

Weather

Fri, December 6th, 2019

Yesterday: Partly cloudy skies were over the region with a trace of snow overnight in areas near Turnagain Arm. Winds shifted from the NW to the east in the morning and have been averaging near 5mph with gusts in the mid-teens. The wind shift brought slightly warmer temperatures, mid-teens along the ridgelines and the low 20’s at the lower elevations.

Today: Partly cloudy skies, slowly warming temperatures and scattered light snow showers are forecast (to sea level). Only a trace to 2″ of snow is expected today along with a chance for 2-4″ of snow tonight. Winds will continue from an easterly direction and pick up into the 15-25mph range along ridgetops before increasing tonight to the 30-40 mph. Temperatures should rise to the 20’s F at the high elevations (4,000′) and close to 30 F at the low elevations (1,000′).

Tomorrow: A strong weather system and significant warming trend is on tap for the weekend and into early next week. Heavy snow and rain at sea level is expected along the eastern Kenai and western Prince William Sound. Seward is expected to see several inches of rainfall and Girdwood up to an inch tomorrow. Snow should make it to 1,000′ tomorrow but then turn to rain on Sunday with the snow line rising to 2,000′ or higher by Monday. We’ll be keeping close tabs on this storm so stay tuned!

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

	Temp Avg (F)	Snow Depth (in)
Center Ridge (1880′)	18	15
Summit Lake (1400′)	15	9
Alyeska Mid (1700′)	17	18

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	14	NE	5	12
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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