Thu, February 27, 2020 Turnagain Pass Avalanche Forecast

Thu, February 27th, 2020

Alpine

Above 2,500'

2 - Moderate

Treeline

1,000'-2,500'

2 - Moderate

Below Treeline

Below 1,000'

2 - Moderate

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

The sun made a quick appearance yesterday afternoon and initiated some roller balls and small loose snow sluffs on steep southerly aspects.

Loose snow sluffs on the southerly face of Pk 4940 (south end of Seattle Ridge), viewed from near the Johnson Pass trail head.

Avalanche Problem 1

Deep Persistent Slabs
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.

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

With clearing skies, light winds, some great powder conditions to be had, and some smaller avalanche issues in the top of the snowpack, it might be easy to forget there is a very large and deadly avalanche problem to deal with. The facets from January, that are now buried 3-6′ deep and have been responsible for countless large slab avalanches, continue to be our main concern. It was three days ago two large slabs were remotely triggered from 1,000′ away in the Grandview area and this type of scenario is still possible today and into the weekend. Areas with a shallower snowpack, such as the Summit Lake area, have a higher likelihood for triggering.

Why is this so tricky? It’s because the snowpack can ‘appear generally stable’. In this situation, it’s easy to focus on the current surface instabilities, listed below, and forget about what’s several feet below us. Additionally, triggering these large slab avalanches is becoming more difficult because (1) the thickness of the slab makes it harder to affect the weak layers, and (2) time is allowing the snowpack to slowly adjust to the weight of the slab. Due to the size and deadly nature of a potential large avalanche, along with the lowering changes a person will trigger one, we are in a ‘scary MODERATE’ avalanche hazard.

Current surface instabilities:

Wind slabs: Lingering wind slabs are still possible to trigger on steep wind loaded slopes and in steep rocky terrain. These were formed either in Tuesday night’s 8″ of new snow or hidden below that snow and formed during Sunday’s strong outflow wind event. Either way, even a small wind slab triggered in the wrong place can send us somewhere unforgiving.

Sun Effect: Solar heating in the afternoon may cause loose snow sluffs again today on steep southerly aspects.

Cornices: As always, give cornices a very wide berth and limit exposure under them.

Keep in mind, any one of the above issues on the surface has the potential to ‘step down’ and trigger a large slab. It’s also good to remember there has been very little traffic since the big storm last week. As we venture further into the hills during this clear sky period we need to consider a number of things:

Areas that have seen little to no traffic this year are more likely to have a large slab release.
Signs of instability are not likely to be present before a slope releases
It could be the 10th person on the slope before it avalanches
Remote triggering a slab from below, the side or on top is possible
Consider the consequences if the slope slides, will debris take you into a terrain trap or fan out?
This is difficult low probability / high consequence situation and our guard has to stay up

*If you’d rather leave these issues behind, sticking to slopes 30° and less, with nothing steeper above you, is a great way to enjoy the excellent powder.

Large slab that was remotely triggered on Monday, Feb 24th, in the Grandview area.

Weather

Thu, February 27th, 2020

Yesterday: Cloudy skies gave way to large patches of sunshine in the afternoon. No precipitation was recorded in the past 24-hours. Ridgetop winds were light and easterly with some moderate gusts in the afternoon up to 22mph. Temperatures were generally in the 20’s°F, yet did warm during the daytime sun and hit the mid 30’s°F at mid and lower elevations briefly.

Today: Partly cloudy skies with valley fog in areas, inland skies could be fairly blue. An inversion has setup overnight with cold single digit temperatures in valley bottoms and 20’s°F in the higher terrain. Winds just shifted to the NW this morning and are expected to be light, in the 5-10mph range.

Tomorrow: Mostly sunny skies with some potential for valley fog. The inversion is likely to still be present with valley bottoms in the single digits and warmer air along ridgelines. Ridgetop winds look to remain northwesterly and could kick up to the 10-20mph range. A weak storm front moves in late Saturday and looks to give us another few inches of snow over the weekend.

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

	Temp Avg (F)	Snow Depth (in)
Center Ridge (1880′)	27	80
Summit Lake (1400′)	18	30
Alyeska Mid (1700′)	22	86

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	22	NE	8	22
Seattle Ridge (2400′)	26	*N/A	*N/A	*N/A

*Seattle Ridge anemometer (wind sensor) is heavily rimed and not reporting.

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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