Tue, December 22, 2020 Turnagain Pass Avalanche Forecast

Tue, December 22nd, 2020

Alpine

Above 2,500'

4 - High

Treeline

1,000'-2,500'

4 - High

Below Treeline

Below 1,000'

4 - High

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

The weather has gone into overdrive, and we are expecting a lot of avalanche activity today. With heavy snowfall, strong winds, and the rain level creeping up higher through the day, we are anticipating large natural avalanches anywhere from 3-6′ deep, and human-triggered avalanches will be very likely. Ridgetop winds have been blowing 49 to 66 mph since 1:00 this morning, with gusts as high as 104 mph, and the strongest period is expected to continue through the middle of the day. By the time this system passes tomorrow, we could see over 4′ of snow in higher elevation start zones, equaling 4-5″ snow water equivalent (SWE). Temperatures will continue to rise through the day, bringing the rain level as high as 1700′, with as much as 4″ rain on snow at lower elevations below 1000′ before temperatures cool down early tomorrow morning. As heavy snowfall and strong winds will be loading up start zones, rain will be increasing the likelihood of wet avalanches at lower elevations. With all of this active weather, we are anticipating large avalanches that could run far into lower elevation runout zones.

Gusts have reached 104 mph at Sunburst this morning.

NWS graphic showing predicted water content for this storm- over 5″ water at Turnagain Pass!

While we will likely see some impressive avalanches failing within the new snow, there is also the smaller possibility of deeper weak layers in the snowpack failing under the weight of all of this new snow (and rain), which could make for some really, really big avalanches. These weak layers have gained strength, and they have not been reactive in over two weeks. But 4” of SWE in less than 24 hours will sure be a good test to see just how strong they have become. The weather is wild, but that actually makes the avalanche situation quite simple. Today, avalanche conditions will be very dangerous, and travel in avalanche terrain is not recommended.

Glide Avalanches: As if we didn’t already have enough action, we saw our first reported glide avalanche of the season yesterday on Gilpatrick Mountain in the Summit Lake area. This is going to be another issue we will be keeping an eye on as the dust settles from this storm.

Weather

Tue, December 22nd, 2020

Yesterday: After a calm morning, ridgetop winds started picking up at noon, blowing 15-20 mph with gusts into the 30s out of the east. Temperatures climbed from the mid-teens to the low 30’s F at upper elevations, reaching the high 30’s F and even 40 F at lower elevations. Light flurries moved in later in the afternoon.

Today: Easterly winds continued to pick up overnight, blowing 49 – 66 mph since 1:00 this morning with gusts as high as 104 mph. The strongest period of winds is expected to continue through late this morning. We are anticipating up to 4′ of snow at upper elevations, equaling 4″ water. Rain level is expected to creep up to 1700′, and we could see as much as 4″ of rain on snow at lower elevations. High elevation temperatures will reach the low 30’s F this evening, and lower elevations could see temperatures as high as 40 F.

Tomorrow: Snowfall will taper off tomorrow morning, with winds shifting to the south/southwest and dropping to 15-30 mph. Temperatures are expected to be in the mid- to upper 20’s F, and skies may start to clear later tomorrow afternoon and early tomorrow evening.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	28	4	0.6	63
Summit Lake (1400′)	27	3	0.3	29
Alyeska Mid (1700′)	26	6	0.5	64

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	18	ENE	30	104
Seattle Ridge (2400′)	20	SE	18	51

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
10/27/24	Turnagain	Observation: Tincan	Michael Kerst
10/21/24	Turnagain	Observation: Turnagain Pass Road Observation	Trevor Clayton
10/19/24	Turnagain	Observation: Tincan – Below Todds Run	Andy Moderow
10/18/24	Turnagain	Observation: Taylor Pass	Eli Neuffer
10/15/24	Turnagain	Observation: Tincan Common	John Sykes Forecaster
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

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