Sat, April 03, 2021 Turnagain Pass Avalanche Forecast

Sat, April 3rd, 2021

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

No avalanche activity was seen or reported yesterday. The last known avalanche in our forecast zone was a natural 1-2′ slab Thursday on Raggedtop near Girdwood.

Avalanche Problem 1

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

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

Although a potent weather system is entering central Alaska from the Bering today, we will be far enough to the south that it looks to just brush us by. Only a trace to 1″ of snow is expected by sunset (8:49pm!) with a chance for 2-4″ tomorrow. The mountains north of Girdwood may see larger amounts this evening through tomorrow (6-12″). Ridgetop winds will be on a slow rise from the south today and may hit averages near 20mph by 6pm and continue rising into tomorrow with averages near 30mph.

Until we receive enough snow/wind to create a new avalanche issue, we remain concerned with old buried weak layers in the top 3′ of the snowpack. These continue to show enough signs of instability that we can’t forget about them. Hence, there is still a chance a person could trigger a persistent slab avalanche. Here are some things to keep in mind:

ASPECT: Sun crusts are present all the way to ridgetops on southerly aspects, including many east and some west aspects. It’s no surprise those slopes are being avoided and instead the northerly shaded slopes, the ones with the soft surface snow, are getting the attention. Crusts can change the slab character and make triggering a slab more difficult. Today, winds and cloud cover should keep the sun crusts from softening and so it will be those shaded aspects which are most concerning.
Of those shaded aspects, watch for steep slopes/terrain that has been previously wind loaded. Prior wind loading over these weak layers has been been the culprit in the Raggedtop avalanches near Girdwood and the Summit Lake avalanches from last week.
Listen/feel for whumpfing in the snowpack and watch for any other signs of instability. Chances are these will be slim to none and simply knowing there is some funny business is the main point. Girdwood Valley, south of Turnagain Pass and Summit Lake have been the regions showing the most unstable snow.
As always, use good travel protocol, and consider the consequences if an avalanche does occur. Watch your partners, expose only one person at a time and know your save zones.

Wind Slabs: With a bump in winds later today, watch for any snow that might get blown into small shallow wind slabs in the high terrain. With the winds from the south and surface crusts on south aspects, it might be difficult for the wind to pick up much snow. However, terrain channeling can cause winds to shift directions and so be sure to watch for any active loading.

Sluffs: As always, watch your sluff on steep slopes with soft surface snow (shaded aspects today).

Tincan seen from Seattle Ridge yesterday. It’s a bit difficult to see, but there are many recent tracks in the shade while the sunny slopes sport mostly older tracks. It’s that time of year that we are all playing the aspect. 4.2.21.

Snow pit on the east face of Seattle Ridge, just north of Repeat Offender slide path. Pit shows a thin weak layer of buried surface hoar and facets 16″ down that took a lot of force to fail, but when it did it propagated. This means the layer is difficult to trigger but could produce an avalanche if it failed. 4.2.21.

Weather

Sat, April 3rd, 2021

Yesterday: Sunny skies were over the region till around 4pm when high clouds began to filter in ahead of the next weather system. Ridgetop winds were light (5-10mph) from the west and southwest. Temperatures were in the 20’s at the mid elevations and teens in the higher elevations.

Today: Cloud cover overnight has keep valley bottoms in the 20’sF and cloudy skies will remain through the day. Light snowfall may begin this afternoon with 1-2″ of snow possible by Sunday morning. Ridgetop winds will remain southerly and pick up through the day from ~5mph to ~20mph by 6pm with stronger gusts. Temperatures should remain near the 20’sF at the mid elevations and the teens along the ridgelines.

Tomorrow: Light snowfall tomorrow may add another 1-2″ around Turnagain Pass and up to 6″ or more in the mountains north of Girdwood. This storm is favoring areas to the north and up to Hatcher Pass. Ridgetop winds will swing southeast and forecast to increase to 20-30mph. Temperatures will warm slightly but snow should fall close to sea level.

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

	Temp Avg (F)	Snow Depth (in)
Center Ridge (1880′)	24	110
Summit Lake (1400′)	20	49
Alyeska Mid (1700′)	24	116

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	14	SW	7	20
Seattle Ridge (2400′)	21	W>N	2	5

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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