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ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, February 22nd, 2023 - 7:00AM
Expires
Thu, February 23rd, 2023 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger is MODERATE on all aspects above 1000′. Triggering a lingering wind slab 1-2′ deep from Monday’s strong NW winds remains possible at the upper elevations and in exposed areas in the trees. Triggering sluffs in the loose surface snow on steep slopes also remains possible. There are several weaker layers in the top 3′ of the snowpack that could still be a concern. Triggering one of these layers is not out of the question and will be more likely with daytime heating. Below 1000′ the avalanche danger is LOW. 

SUMMIT LAKE / LOST LAKE / SNUG HARBOR:   On Monday strong NW winds impacted these regions and formed hard wind slabs at the upper elevations. Triggering one of these should still be possible and they could step down into older buried weak layers, creating a larger and more dangerous avalanche. Avoiding steep wind loaded slopes is recommended.

*Avalanche danger is expected to rise tomorrow due to a storm bringing snowfall (6-12″) along with strong easterly winds.

Wed, February 22nd, 2023
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
Avalanche risk
0 - No Rating
1 - Low
2 - Moderate
3 - Considerable
4 - High
5 - Extreme
Avalanche risk Avalanche risk Avalanche risk Avalanche risk Avalanche risk
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 known avalanches occurred yesterday, Tuesday. The last avalanches were natural small wind slabs and sluffs triggered by the strong outflow winds on Monday. The last human triggered avalanche was Sunday when two people were caught in a slab they triggered in very steep terrain in the Tincan Library (report here).

Avalanche Problem 1
  • Wind Slabs
    Wind Slabs
  • Aspect/Elevation
  • 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.

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

With high clouds moving in today, ahead of Thursday’s storm, and only light westerly winds, we can expect our avalanche issues to be fairly status quo. These are:

Lingering wind slabs:  Keep an eye out for any slopes with wind deposited snow. Monday’s NW outflow wind event moved a lot of snow around in many areas, but left others somewhat untouched. Older wind slabs could be sitting on a weaker layer and may not have totally bonded yet. If we find stiffer snow over softer snow, or cracking in the snow around us, that’s a tell-tale sign of a wind slab.

Sluffs:  Triggering a loose snow avalanche (sluff) on steep slopes harboring soft surface snow should be expected. Hence, we should watch our sluffs.

Daytime warming?:  If the sun penetrates the high clouds, it could cause warming of the snow surface enough to initiate sluffs on southerly aspects in steep rocky terrain. Warming of the snow surface can also change the character of the top layer and make a slab avalanche failing in any questionable weak layers more likely. We believe this scenario played a large role in the Tincan Library slab avalanche on Sunday mentioned above.

Cornices:  As always, give cornices a wide berth. We have not heard of any cornice falls during the past several days of nice weather, but that doesn’t mean they can’t be triggered if we accidentally get onto them.

 



Example of cracking in a wind slab by Elliot Gaddy.



Another example of cracking in a wind slab by Megan Guinn.


Avalanche Problem 2
  • Persistent Slabs
    Persistent Slabs
  • Aspect/Elevation
  • 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.

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

There are various layers in the top 3′ or so of the snowpack that are still giving us pause. The most prominent are a buried surface hoar layer (the 2/15 layer) that sits around 6-12″ deep and a faceted layer (2/5 layer) between 1.5 and 2.5′ deep. Despite a high degree of travel on steep slopes in our forecast zone, every now and then a person finds just the right setup and triggers a slab. The most recent was the scary avalanche in the Tincan Library that injured a person on Sunday, three days ago.

These types of buried weak layers are really tricky and can catch all of us off guard. The best way to look for them is paying close attention to any red flags (collapsing/whumpfing, or cracking in the snow), and using small test slopes to jump on and see if any cracks can form. Using hand pits can be great for the top foot or so, but they are hard to do for any layer 2′ deep. This is when digging a snow pit to test buried layers is the best way to assess any weaknessess in them. Unfortunately, one characteristic is there could be no clues the slope is unstable before it avalanches – that’s ultimately the tricky part.

Additional Concern
  • Deep Persistent Slabs
    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

For those just tuning into the forecast, in areas with a thin snowpack (i.e. Silvertip and the southern end of the forecast region to Summit Lake) there are various weak layers near the base of the snowpack that remain a concern. For these areas, triggering a larger avalanche is not totally out of the question and a more cautious mindset is recommended.

Weather
Wed, February 22nd, 2023

Yesterday:  Mostly clear skies were over the region yesterday. Ridgetop winds were light and variable, strongest gusts were in the teens from a southerly direction. Temperatures were chilly (single digits in some valley bottoms) but have climbed into the 20’sF at most locations overnight with cloud cover that moved in.

Today:  Generally overcast skies and light westerly winds are forecast today (~5mph). Temperatures are in the 20’sF at most locations and should warm to near 30F at the lower elevations with daytime heating. Light snow could begin to fall by this evening and pick up overnight as a storm moves in.

Tomorrow:  Light to moderate snowfall is expected tomorrow as a storm moving through peaks tomorrow. Weather models are showing around 6-12″ of snow by tomorrow evening. Temperatures warm with the southerly flow to near 32F at sea level, bringing a rain/snow mix. Ridgetop winds look to be 25-35mph with stronger gusts from the southeast.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 23 0 0 67
Summit Lake (1400′) 16 0 0 35
Alyeska Mid (1700′) 24 0 0 70
Bear Valley – Portage (132′) 22 0 0

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 17 var 5 16
Seattle Ridge (2400′) 20 var 4 13
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This is a general backcountry avalanche advisory issued for Turnagain Arm with Turnagain Pass as the core advisory area. This advisory does not apply to highways, railroads or operating ski areas.