Wed, April 12, 2023 Turnagain Pass Avalanche Forecast

Wed, April 12th, 2023

Alpine

Above 2,500'

3 - Considerable

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

No new avalanches were seen or reported yesterday. However, there was relatively little backcountry traffic and poor visibility. The last know avalanches were on Sunday and Monday when people found reactive storm slabs within the new snow (~10-14″ at Turnagain). The sun also induced some storm slabs on Monday afternoon due to warming.

Example of one of the smallish storm slabs triggered on Monday. This was on Tincan. Photo by Mike Records 4.10.23.

Avalanche Problem 1

Storm Slabs
N E S W NW NE SE SW

Aspect/Elevation
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.

Aspect/Elevation of the Avalanche Problem

Specialists develop a graphic representation of the potential distribution of a particular avalanche problem across the topography. This aspect/elevation rose is used to indicate where the particular avalanche problem is thought to exist on all elevation aspects. Areas where the avalanche problem is thought to exist are colored grey, and it is less likely to be encountered in areas colored white.

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

Snowfall overnight and this morning has added another 3-6″ of new snow, possibly more in favor areas like Portage and Placer Valleys. Turnagain Pass may have only received 1-2″. The snow has come in with relatively light easterly winds along the ridgetops and is making its way to sea level. Light snow showers may continue until around noon with little additional accumulation.

The avalanche problem ‘icon’ above may say storm slab, but really there are all kinds of avalanche problems to be on the lookout for. The most concerning will be higher elevations that saw wind loading yesterday. Wind slabs up to 2′ deep formed by yesterday’s bump in easterly winds, gusting in the 30’smph, could be found and triggered by us. These could be obscured by the light snow from overnight. There could also be some storm slabs lingering in areas out of the winds that are sitting on a crust from early April. Loose snow sluffs will probably be easy for us to trigger on steep slopes and cornices could be teetering close to failure. It is a day to really pay attention to the surface conditions and the Red Flags (recent avalanches, cracking in the snow around us and collapsing/whumpfing in the snow under us.

Daytime warming and avalanches possible in the afternoon: This afternoon if the clouds break and the sun peaks through, we could see some naturally triggered avalanches in the storm snow from last night as well as that from over the weekend. Look for loose snow sluffs and shallow slabs as the surface snow heats up later today. These could be triggered by us, or naturally. As is often the case, a smallish sluff can trigger a slab making for a larger avalanche.

Snowpack in the Placer Valley area. More snow fell here than at Turnagain. The storm snow in the lower elevations is stabilizing, but much more uncertainly exists in the higher elevations. 4.12.23.

Example of the surface snow heating up and becoming sticky – and what we call ‘slabby’. This is a red flag for creating larger sluffs and slabs in steep terrain due to warming of otherwise loose and dry storm snow. 4.11.23.

Additional Concern

Deep Persistent Slabs

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.

More info at Avalanche.org

In addition to the concerns related to the new snow mentioned above, we still want people to remember there is a small likelihood of triggering a very big avalanche. This would be on weak snow that was buried in mid March and is 3-6′ deep. It has now been over two weeks since we last saw the last deep slab avalanche. We have been tracking the weak layer in snowpits for nearly a month, and although it is showing signs of gaining strength we don’t totally trust it. To avoid the problem, limiting traveling on or below steep slopes. As time goes on this type of avalanche is becoming less and less likely, but we shouldn’t forget about it yet.

Weather

Wed, April 12th, 2023

Yesterday: Cloudy and obscured skies with some afternoon light snowfall was over the region. Ridgetop winds were blustery, 10-20mph with gusts over 30mph. Snowfall picked up overnight and 2-5″ looks have accumulated by 6am this morning, to see level.

Today: Light snowfall should continue until around noon with another 1-2″ of snow accumulation. Skies could break a bit this afternoon allowing for the daytime warming to bump close to 40F in the parking lots and near 30F along ridgelines. Ridgetop winds have quieted overnight and should be light and variable through the day.

Tomorrow: Partly to mostly cloudy skies are expected tomorrow, Thursday, before another round of snow hits on Friday. Winds are expected to be light and easterly before picking up tomorrow night as well. Temperatures should remain on the cooler side, generally in the 20’sF.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	28	2	0.1	96
Summit Lake (1400′)	25	3	0.2	50
Alyeska Mid (1700′)	27	6	0.4	94
Bear Valley – Portage (132′)	34	5	0.4	–

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	16	ENE	13	33
Seattle Ridge (2400′)	21	SE	12	20

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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