Sat, December 08, 2018 Turnagain Pass Avalanche Forecast

Sat, December 8th, 2018

Alpine

Above 2,500'

4 - High

Treeline

1,000'-2,500'

4 - High

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
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

Yesterday’s quick moving and powerful storm brought feet of snow to the Alpine, inches of rain to sea level and 90mph winds to the ridgetops. The rain snow line bounced between 1,000′ and 1,500′ in most areas. Mostly wet snow and rain fell along Turnagain Pass and as of this morning, there is what looks to be an inch or two of snow surviving along the Pass. Rain and snowfall have decreased overnight but are on tap to pick up slightly again this evening.

Storm totals at the mid-elevation snow stations (beginning Thursday night to 6am this morning)

Turnagain Pass at 1,880′: 1.5-2″ of water equivalent, roughly 1.5-2′ of snow above treeline
Girdwood Valley at 1,700′: 2.7″ of water equivalent, roughly 2-3′ above treeline
Summit Lake at 1,400′: 0.7″ of water equivalent (rain/snow mix along the road at Summit Pass), roughly 6-8″ of snow above treeline

Widespread avalanche activity was seen yesterday. Debris piles were observed running into snow free zones below 1,000′ in Girdwood and Portage Valleys as well as Seward. Although natural activity will be expected to decrease today, the mountains are still in shock and large avalanches are still likely in many areas. With additional snowfall tonight, natural avalanche likelihood could increase again. That said, today is another day to avoid avalanche terrain all together.

Moose Mountain avalanches, outline of debris marked in orange. This is near MP 52 along the Seward Highway (Summit zone). Photo: Alex McLain

Upper Girdwood Valley, North side of Crow Creek Valley. Debris from avalanches above running to valley floor.

Seward’s Mt. Benson with several large debris piles from avalanche activity higher on the mountain. Photo: Jamie Lyons

Just enough snow to make it white along Turnagain Pass yesterday. This is Rookie Hill in the foreground looking at the base of Seattle Ridge, seen from the motorized parking lot. Photo: Will Brennen

Weather

Sat, December 8th, 2018

Yesterday: Heavy rain below 1,000′ and heavy snowfall above 1,500′ – a wintery mix in between these elevations. Snow and water totals above and below. Winds were strong out of the East, averaging in the 60’s mph with gusts into the 90’s. Temperatures were in the upper 20’s along the ridgetops and mid 30’s at 1,000′.

Today: Rain and snowfall has decreased overnight. Scattered showers should be seen around the region today and could pick up slightly tonight. Between .2-.3″ of rain below 1,000′ is forecast with 2-4″ of additional snow at the higher elevations today. Tonight snow line should lower and an additional 3-5″ of snow could be seen near sea level by tomorrow morning. Ridgetop winds will continue from the east in the 25-30 mph range. Temperatures will be in the low 30’s at 1,000′ and mid 20’s along ridgelines through the day.

Tomorrow: Snowfall making its way to sea level will hopefully greet us in the morning as snow lines are forecast to lower. Snowfall should taper off by tomorrow morning bringing a break in storms before yet another wave of snow (and possibly rain at sea level) ushers in later on Sunday. Stay tuned!

*Seattle Ridge anemometer (wind sensor) reading uncommon wind direction during height of storm yesterday. Sensor is now rimed back over.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	33	14	1	30
Summit Lake (1400′)	34	0	0.4	4
Alyeska Mid (1700′)	33	2	1.92	4

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	26	NE	41	98!
Seattle Ridge (2400′)	30	*WNW	*25	66

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
10/14/24	Turnagain	Avalanche: Tincan	CNFAC Staff
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

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