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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, March 4th, 2021 - 7:00AM
Expires
Fri, March 5th, 2021 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger remains MODERATE today.  Triggering an avalanche on a persistent weak layer, buried 1-3’ deep, is still possible on slopes 35° and steeper. Use safe travel protocol, identify areas of concern and evaluate terrain consequences. Be on the lookout for old lingering wind slabs in the Alpine, give cornices a wide berth and watch your sluff.

 

Special Announcements

Heading to Hatcher Pass? Be sure to check the Thursday Conditions Summary at hpavalanche.org.

Thu, March 4th, 2021
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. Dangerous avalanche conditions. 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

Yesterday many people were out enjoying the soft surface conditions and nice weather. We have more sunshine on tap today.  If you decide to partake in some fun in the sun, keep in mind the lingering possibility of triggering an avalanche on the persistent weak layers that are lurking 1-3’ deep in snowpack. These are layers of surface hoar and/or facets that formed during late January and early February. These layers are showing signs that they are gaining strength but observers continue to find them in the snowpack across the advisory area at all elevations. In some locations they remain reactive in snowpack instability tests. The last reported avalanches that failed on these layers occurred about a week ago in the Girdwood Valley and on Tincan in terrain below 2000′. If choosing to push into steeper terrain today, remember you could still hit the wrong spot on your skis or machine and trigger an avalanche that fails on one of the buried weak layers. As always, it is really important to follow safe travel protocol and evaluate terrain consequences. This means avoiding terrain exposed to terrain traps, only exposing one person at a time to steep slopes, and watching your skiing and riding partners from safe spots.

As we anticipate new snow tomorrow keeping track of surface conditions is important. A new batch of surface hoar is growing on the surface and a sun crust was observed in steep southerly low elevation terrain yesterday. These two pieces of data will both be factors that could determine how well any new snow will bond to the existing surfaces. Let us know what you find today!

Wind Slabs: You may still find small and isolated pockets of wind-loaded snow that are capable of avalanching. These are most commonly located below ridgetops, convexities, and in cross-loaded gullies, and will be more likely at higher elevations. Steep terrain with smooth pillows of wind-drifted snow should still be treated with caution today. As winds increase this afternoon watch for blowing snow and any new wind slab development.

Sluffs: Steep slopes that have been protected from the winds and the sun have 2-6” of soft snow on the surface and triggering a loose snow avalanche (sluff) is possible.  Remember sluffs can gain volume and speed in steep terrain and can be dangerous if they knock you off your feet and take you for a ride in high consequence spot.

A layer of buried surface hoar was reactive in an Extended Column Test on Center Ridge yesterday, 3.3.21. 

Any day is a good day to practice avalanche rescue. CNFAIC director Wendy Wagner practicing in the sun, 3.3.21. When was the last time you and your skiing or riding partners took the time to practice? 

 

Weather
Thu, March 4th, 2021

Yesterday: Skies were mostly clear at upper elevations with a layer of fog/low stratus in the valleys that dissipated in the afternoon. There were a few fog snow flakes falling below the clouds. High temperatures were in the upper teens to mid 20°Fs. Winds were light and variable. Overnight skies were mostly clear with an inversion setting up. Low temperatures in the valley bottoms were in the single digits to a few degrees below 0°F while ridgetop temperatures were in the low teens. Winds remained light.

Today: Skies will be mostly clear again with some patchy valley fog. Highs are expected in the mid teens to mid 20°Fs. Winds will start off light and then increase in the afternoon becoming easterly 5-15 mph, with gusts into the 20s.  Overnight skies will become cloudy and snow showers are forecast to start early Friday morning. Lows will be in the teens.

Tomorrow: Cloudy skies with snowfall throughout the day, accumulation of 2-4″ possible. Highs in the mid 20°Fs to low 30°Fs. Winds will be easterly 10-15 mph with gusts into the 20s. Snow showers continue into the evening with a chance of snow overnight.  Winds shift the northwest around 10 pm and are forecast to blow 10-20 mph.  Temperatures will be in the mid teens to mid 20°Fs.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 11 0 0 113
Summit Lake (1400′) 5 0 0 46
Alyeska Mid (1700′) 14 0 0 116

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 12 SW 5 15
Seattle Ridge (2400′) 14 N 2 10
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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.