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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, February 15th, 2020 - 7:00AM
Expires
Sun, February 16th, 2020 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger is a ‘scary MODERATE‘ at all elevations. Triggering a large slab avalanche, 2-4′ thick, is possible. Although the likelihood of triggering is slowly decreasing, the consequences could be deadly. This is due to buried weak layers 2-4’ below the snow surface. It is a tricky setup and a continued cautious mind set and conservative terrain choices are necessary to limit the danger. Other avalanche issues include, cornice breaks along ridgelines and wind slab avalanches composed of Wednesday’s storm snow.

Special Announcements

Headed to Hatcher Pass?  Be sure and check HPAC’s Saturday morning forecast!

A BIG THANK YOU to all those joining in the Snowball 2020 festivities last night!!

Sat, February 15th, 2020
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
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

We had a report of a 2-3 foot thick slab avalanche occurring in the Lost Lake area on Thursday (2/13). It was reported that it had “slow propagation but certainly could have buried somebody. No injuries.” This area is outside our forecast zone.

Also on 2/13 was a skier triggered shallow soft slab on Jr’s Run in 2nd Bowl on the backside of Seattle Ridge. Photo below.

We do not know of any avalanches that occurred yesterday.

Soft wind/storm slab avalanche with fairly wide propagation on Jr’s Run in 2nd Bowl on Seattle Ridge. 2.13.20. 

Avalanche Problem 1
  • Persistent Slabs
    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

With another mild weather day forecast (some clouds, light wind and cool temperatures) and some great powder conditions to play in, it could be easy to forget there is bad news in the basement… Simply put, between 2-4 feet below the snow surface lies various layers of weak faceted snow and buried surface hoar. These layers have been responsible for countless slab avalanches over the past 2 weeks, many of them human triggered. If you didn’t see it yesterday- check out this current state of the snowpack video.

The trick is, triggering a large slab avalanche is becoming harder for two reasons; (1) recent snowfall has increased the thickness of the slab making 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 slab avalanche, along with the lowering changes a person will trigger one, we are in a ‘scary MODERATE’ avalanche hazard.

Things to keep in mind this weekend are:

  • Areas that have seen little traffic this year are more likely to have a large slab release.
  • Signs of instability may not occur 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 natural avalanches from on the east side of the Placer Valley that sent debris into the common access point for the riding area some call ‘squirrel flats’. Avalanches likely occurred on Wed (2.11). Photo taken 2.13.20 by Travis Smith.

Avalanche Problem 2
  • Wind Slabs
    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

Shallow wind slabs, around a foot thick, formed from the snow and wind on Wednesday and Thursday could still be found and triggered on steeper leeward terrain. Keep an eye out for areas that saw prior windloading and any cracking in the snow around you. Hollow feeling snow and stiffer snow over softer snow are also clues you may have found a wind slab.

Cornices:  Give cornices a wide berth from above and limit time under them from below.

Weather
Sat, February 15th, 2020

Yesterday:  Mostly to partly sunny skies with high clouds were over the region. Winds were  calm to very light from the east. Temperatures rose to 20-25°F in the mid-elevations and below during the day and were otherwise in the teens along ridgelines.

Today:  Cloudy skies with a chance for some clearing is expected today. A few light snow flurries may fall in areas with no accumulation forecast. Ridgetop winds shifted around to the NW early this morning where they should remain light (5-10mph) through the day. Temperatures are expected to stay in the teens to 20°F.

Tomorrow:  Mostly sunny skies are now forecast for Sunday with a significant pattern shift in the weather for next week. A large low-pressure system is moving in for Monday and Tuesday. As of now this next storm looks to be warm, wet and windy. Models are showing up to 2-3″ of SWE (2-3′ snow) and a rain line that could approach 1,000′ and even 1,500′ by Tuesday evening. Stay tuned.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 19 0 0 64
Summit Lake (1400′) 12 0 0 25
Alyeska Mid (1700′) 16 0 0 62

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 17 SE 4 10
Seattle Ridge (2400′) 13 E 4 14
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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.
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