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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, February 22nd, 2018 - 7:00AM
Expires
Fri, February 23rd, 2018 - 7:00AM
Forecaster
Heather Thamm
Avalanche risk The Bottom Line

The avalanche danger is  MODERATE  above 1,000′ on all aspects. Human triggered slab avalanches 1-2′ thick remain possible.  Be aware of a few inches of new snow combined with wind that may form fresh isolated wind slabs in the Alpine. Additionally, weak layers deeper in the snowpack may still be triggered, creating a larger avalanche.  

Thu, February 22nd, 2018
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.
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

Triggering a slab avalanche 1-2+ feet thick continues to be our main concern. Today 3-5” of new of new snow and moderate winds (Easterly becoming Westerly) may add stress to the overall snowpack, but not enough to elevate the danger rating. Several weak layers are buried within our snowpack at varying depths and elevation bands. A layer of buried surface hoar from Jan. 21st continues to show signs of reactivity in test pits above 2000’, and we’re also tracking a layer of facets over a melt-freeze crust in the mid elevations. Persistent slabs are becoming more difficult to trigger with time, but a large and unmanageable avalanche is still possible. Today watch for active wind loading on all aspects and be aware that red flags such as “whumpfing” may not be present before a slope releases. The most likely place to find this avalanche problem are slopes that have not seen significant traffic this season. 

Deep Persistent Slabs: Keep in mind that there are deeper persistent layers that could ‘wake up’ if you find the wrong spot above 3,000′ in the Alpine. At these high elevations, old weak layers of facets and buried surface hoar sit in the bottom half of the snowpack. This structure is most pronounced in places with a thin overall snow cover, such as the South end of Turnagain Pass, the Summit Lake area and Crow Pass. 

 Propagation on buried surface hoar and facets was found in several location on Eddies earlier this week

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

New wind slabs will depend on how much snow we see today and if winds are activity moving snow around. Ridgetop winds from the East are expected to change to a Westerly direction this afternoon and remain in the 20’s mph. Not much old snow is available for transport, and only a few inches of new snow is forecasted. Expect wind slabs to be small, but they could be tender should you see any obvious clues like blowing snow or shooting cracks. If this is the case steep leeward aspects and cross loaded terrain features will be suspect.   

Sunshine:  Although the sun is unlikely today, be aware it’s that time of year when we need to pay attention to the sun. Following a storm the sun can heat up Southerly aspects and melt surface snow and cause point releases. This heating can also cause a slab sitting on a weak layer to become more reactive. Keep this in mind should a clearing trend behind today’s storm happen sooner than expected. 

Surface conditions before today’s weather consisted of an inch of loose surface snow on top of variable surfaces (wind affected snow and patches of loose settled snow.)

Weather
Thu, February 22nd, 2018

Yesterday high cirrus clouds moved into the area and skies were broken becoming mostly cloudy by the evening. Ridgetop temps were in the mid 30’s and overnight temps dropped back into upper 20F’s. Winds were light from the West and transitioned overnight to an Easterly direction. As of this morning both Seattle Ridge and Sunburst were averaging in the 20’s mph with gusts in the 30’s (East direction.)  As of 6am no precipitation has been recorded.  

Today a low pressure system is moving through our region with a Southwesterly direction. Snow showers are expected this morning through mid day and tapering off in the afternoon. Snow totals may reach 3-5 € (.3 € H20) in parts of our forecast zone. This storm is expected to favor Anchorage and the Mat-Su areas. Temperatures at sea level may be in the low to mid 30F’s and this precip may fall as rain to 500 ft.   Ridge top winds will remain in the 15-30mph range, starting out as Easterly and transitioning to a Westerly direction this afternoon.  

Active Southwest flow will bring several more low pressure systems into Southcentral, Alaska throughout the weekend and into early next week. Each system is followed by a period of clearing skies. Southwest flow tends to favor Cook Inlet and the Matanuska Valley for snow, but precipitation is expected across our region. Temperatures are expected to be near normal and range from the 20F’s into the mid 30s. Rain and/or snow is possible at lower elevations.  

 *Center Ridge weather station has been producing erratic temperature data.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) *34   0   0   62  
Summit Lake (1400′) 26   0   0   24  
Alyeska Mid (1700′) 30    0 0    55

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  30 W –> E   6   34  
Seattle Ridge (2400′) 32   W –> E     8   32  
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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.