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ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, March 10th, 2021 - 7:00AM
Expires
Thu, March 11th, 2021 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger will rise to CONSIDERABLE in the Alpine (above 2500′) today. With sustained easterly winds, increasing snowfall and weak surface snow, expect dangerous avalanche conditions to develop if the weather forecast verifies.  Human triggered wind slabs, 1-2′ deep, will be likely in steep wind-loaded terrain, especially on unsupported slopes and in cross-loaded gullies. Natural wind slabs will be possible. Below 2500′ the avalanche danger is MODERATE. Pay attention to what terrain is above you as conditions change. Watch for blowing snow, shooting cracks and recent avalanches. Cautious route-finding and conservative decision-making are essential.

The National Weather Service has updated the  Special Weather Statement  this morning.

Wed, March 10th, 2021
Alpine
Above 2,500'
3 - Considerable
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.
Avalanche Problem 1
  • 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

East winds increased overnight and a few inches of snow fell. The winds are forecast to remain strong enough to transport snow today and the snow will continue with precipitation intensity peaking in the afternoon. Expect this combination of snow and wind to form sensitive wind slabs on top of weak snow surfaces. During the past week, we have seen near-surface facets and surface hoar develop all the way up to ridgetops. Prior to this storm the weak snow was sitting on top of stiff old wind surfaces and sun crusts, and this has the potential to be very sensitive to triggers now that it is buried.  If the weather forecast verifies and we actually get a foot of snow by this afternoon, human triggered avalanches up to 2′ will be likely in wind-loaded terrain in the Alpine. Natural avalanches may also occur at upper elevations. It is really important to pay attention to changing conditions.

What to watch out for if you’re headed out today:

  • Areas with current wind-loading (winds should blow all day, with a wind direction shift this evening)
  • Slabs lower on slopes from cross-loading (Seattle Ridge is a good example)
  • Stiff snow over softer snow (punchy snow)
  • Cracks shooting from your skis or machine
  • Whumpfing (collapsing of the snowpack into the faceted snow underneath)
  • Out of the wind, you might see some shallow storm slabs forming.

Remember, expect any wind slab you find to be sitting on weak faceted snow and/or surface hoar and poor bonding is likely. There is also a chance a persistent slab avalanche could be triggered if an avalanche triggered near the surface overloads the older weak layers in the upper 1-3’ of the snowpack and steps down creating a larger avalanche.

Sluffs: Steep slopes that are sheltered from the wind have 2-6” poorly bonded snow on the surface, which makes it easier to trigger dry loose avalanches (sluffs). Be aware of the potential for sluffs to gain volume and speed in steep terrain, since these can be dangerous if they knock you off your feet or your machine.

Surface hoar on a wind crust below Kickstep, 3.7.21. Expect slabs forming over this set-up to be sensitive today.

 

 

Weather
Wed, March 10th, 2021

Yesterday: Skies were cloudy with very light snow showers and only a trace of accumulation during the day. Snowfall picked up around 8 pm and 2-4″ fell overnight. Winds were easterly 10-15 mph with gusts into the 20s during the daytime. East winds picked up overnight blowing 15-30 mph with gusts into the 40s and 50s.  Temperatures were in the low 20°Fs to mid 30°Fs.

Today: Skies will be cloudy with snow throughout the day, 6-8″ expected. Snowfall is forecast to be heaviest in the late afternoon and continue overnight with an additional, 2-4″. Winds will remain easterly 15-30 mph with gusts into the 40s and 50s. Winds will shift to the northwest in the evening, blowing 15-25 mph with gusts into the 30s. Temperatures will be in the mid 20°Fs to low 30°Fs during the day dropping overnight to the teens and single digits as colder air moves into the region.

Tomorrow: Snow showers showers in the morning tapering off by the afternoon. Temperatures in the teens and low 20°Fs. Winds will remain northwesterly 15-25 mph with gusts into the 30s. Overnight skies become partly cloudy and temperatures dip to around 0°F. Winds ease a bit overnight but remain northwesterly. There is some sunshine in the forecast for Friday into the weekend.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 29 3 0.3 110
Summit Lake (1400′) 28 1 0.1 45
Alyeska Mid (1700′) 28 3 0.2 114

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 19 NE 16 52
Seattle Ridge (2400′) 22 E 12 27
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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.