Wed, December 11, 2019 Turnagain Pass Avalanche Forecast

Wed, December 11th, 2019

Alpine

Above 2,500'

3 - Considerable

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.

Recent Avalanches

A break in storms yesterday greeted us with some visibility to take a look around at the aftermath of Monday’s powerful storm. Between 2-4 feet of snow fell within 24-hours and large natural avalanches ran in many slide paths. Turnagain Pass however seemed to escape much of this natural activity. The only avalanches seen here were a slide in Todd’s Bowl on Tincan and the Elevator Shaft off the north side of Sunburst. Most of the avalanches began at elevations near 3,500 – 4,500′ and some ran all the way to sea level. Yesterday’s clear weather locked the snowpack into place below 2,500′ (ish) forming a surface crust where rain had fallen on snow.

Boston Bar slide path ran to sea level. This path sits above the 6-Mile river just north of the Hope Y and is a westerly facing slope.

Close up of the looker’s right crown on Moose Mtn just south of the Hope Y and north of Summit Pass (west facing slope). This is a classic leeward slope that often sees significant wind loading and storm snow avalanches such as occurred here on Monday.

Avalanche debris in the looker’s left side of Todd’s Bowl on Tincan- this can barely be seen through the flat light. The slide looks to be ‘relatively’ small in relation to the path size.

Avalanche Problem 1

Wind Slabs
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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

Ridgetop winds have been blowing from the east in the 20’s mph with gusts near 50mph overnight. These are forecast to pick up to the 40’s mph or higher today. Along with the wind, 2-3″ of snow is expected above 2,500′ with another 2-4″ tonight. That said, in upper elevation terrain harboring dry snow, wind loading is a sure bet. This could create some naturally occurring cornice falls and wind slab avalanches around a foot thick.

Although it’s one of those days where limited visibility, windy weather and a nasty breakable crust up to 2,500′ may deter many folks from heading to the high elevation dry snow, if you do- any fresh wind loaded slope should be viewed as easy to trigger a wind slab avalanche.

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

At the high elevations, above 3,000′, we are still tracking a layer of faceted snow sitting on a thick melt-freeze crust near the ground. This layer is now buried under anywhere from 3 to 6 feet or more of snow. The character of the natural avalanches from the 12/9 cycle did not point to this layer becoming overloaded and failing, which is a good sign. However, there is still much uncertainty at this point. Until proven otherwise, we will keep it on our radar.

Weather

Wed, December 11th, 2019

Yesterday: Partly cloudy skies were over the region with slightly cooling temperatures. Ridgetops have remained in the mid-upper 20’sF while trailheads at 1,000′ sit near 40F. Winds along the peaks dipped to the teens from an easterly direction before picking back up to the 20’s mph yesterday evening.

Today: Cloudy skies, strong easterly ridgetop winds and light precipitation are in store as a low pressure spinning in the Gulf of Alaska pushes a front our way. Around .25″ of rain up to 2,000′ is forecast today with another .4″ of rain overnight tonight. This equates to a total of 2-6″ of new snow above 2,500′ by tomorrow morning. The snow line should lower this evening possibly bringing snow to 1,500′. Ridgetop winds will continue to pick up today and average in the 30-40mph range with gusts double that. Temperatures will stay in the mid-20’s F along ridgelines and the upper 30’s at 1,000′.

Tomorrow: An active weather pattern is expected to remain over the region. Expect light snowfall over 2,000′ (rain below) and strong ridgetop easterly winds for the next several days.

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

	Temp Avg (F)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	36	0.1	18
Summit Lake (1400′)	34	0	7
Alyeska Mid (1700′)	35	0.1	16

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	27	NE	19	49
Seattle Ridge (2400′)	N/A*	N/A*	N/A*	N/A*

*Seattle Ridge weather station is down and as soon as the weather clears we will get it up and running!

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

Riding Areas

The riding areas page has moved. Please click here & update your bookmarks.